来自<C++编程艺术>.
Herbert Schildt已经把很多基本的程序解析的技巧表达了出来, 不过现在的源代码比较脆弱, 经过我改进的代码也很脆弱, 所以, 如果你感兴趣, 请使用简单的代码来测试它.
书中MiniCPP(理解为MiniCpp1.0)支持的特性如下:
具有局部变量的参数化函数.
嵌套的作用域.
递归.
if语句.
switch语句.
do-while, while及for循环.
break语句.
int, char类型的局部及全局变量.
整型和字符型的函数参数.
字符串常量(部分实现).
return语句, 可以带或不带返回值.
少量的标准函数.
基本的运算符.
返回整型的函数.
/*和//注释
使用cin和cout的控制台I/O
作者的源代码如下:
View Code
listing 1 // Recursive descent parser for integer expressions. // #include <iostream> #include <cstring> #include <cstdlib> #include <cctype> #include "mccommon.h" using namespace std; // Keyword lookup table. // Keywords must be entered lowercase. struct commands { char command[20]; token_ireps tok; } com_table[] = { "if", IF, "else", ELSE, "for", FOR, "do", DO, "while", WHILE, "char", CHAR, "int", INT, "return", RETURN, "switch", SWITCH, "break", BREAK, "case", CASE, "cout", COUT, "cin", CIN, "", END // mark end of table }; // This structure links a library function name // with a pointer to that function. struct intern_func_type { char *f_name; // function name int (*p)(); // pointer to the function } intern_func[] = { "getchar", call_getchar, "putchar", call_putchar, "abs", call_abs, "rand", call_rand, "", 0 // null terminate the list }; // Entry point into parser. void eval_exp(int &value) { get_token(); if(!*token) { throw InterpExc(NO_EXP); } if(*token == ';') { value = 0; // empty expression return; } eval_exp0(value); putback(); // return last token read to input stream } // Process an assignment expression. void eval_exp0(int &value) { // temp holds name of var receiving the assignment. char temp[MAX_ID_LEN+1]; tok_types temp_tok; if(token_type == IDENTIFIER) { if(is_var(token)) // if a var, see if assignment { strcpy(temp, token); temp_tok = token_type; get_token(); if(*token == '=') // is an assignment { get_token(); eval_exp0(value); // get value to assign assign_var(temp, value); // assign the value return; } else // not an assignment { putback(); // restore original token strcpy(token, temp); token_type = temp_tok; } } } eval_exp1(value); } // Process relational operators. void eval_exp1(int &value) { int partial_value; char op; char relops[] = { LT, LE, GT, GE, EQ, NE, 0 }; eval_exp2(value); op = *token; if(strchr(relops, op)) { get_token(); eval_exp2(partial_value); switch(op) // perform the relational operation { case LT: value = value < partial_value; break; case LE: value = value <= partial_value; break; case GT: value = value > partial_value; break; case GE: value = value >= partial_value; break; case EQ: value = value == partial_value; break; case NE: value = value != partial_value; break; } } } // Add or subtract two terms. void eval_exp2(int &value) { char op; int partial_value; char okops[] = { '(', INC, DEC, '-', '+', 0 }; eval_exp3(value); while((op = *token) == '+' || op == '-') { get_token(); if(token_type == DELIMITER && !strchr(okops, *token)) throw InterpExc(SYNTAX); eval_exp3(partial_value); switch(op) // add or subtract { case '-': value = value - partial_value; break; case '+': value = value + partial_value; break; } } } // Multiply or divide two factors. void eval_exp3(int &value) { char op; int partial_value, t; char okops[] = { '(', INC, DEC, '-', '+', 0 }; eval_exp4(value); while((op = *token) == '*' || op == '/' || op == '%') { get_token(); if(token_type == DELIMITER && !strchr(okops, *token)) throw InterpExc(SYNTAX); eval_exp4(partial_value); switch(op) // mul, div, or modulus { case '*': value = value * partial_value; break; case '/': if(partial_value == 0) throw InterpExc(DIV_BY_ZERO); value = (value) / partial_value; break; case '%': t = (value) / partial_value; value = value - (t * partial_value); break; } } } // Is a unary +, -, ++, or --. void eval_exp4(int &value) { char op; char temp; op = '\0'; if(*token == '+' || *token == '-' || *token == INC || *token == DEC) { temp = *token; op = *token; get_token(); if(temp == INC) assign_var(token, find_var(token)+1); if(temp == DEC) assign_var(token, find_var(token)-1); } eval_exp5(value); if(op == '-') value = -(value); } // Process parenthesized expression. void eval_exp5(int &value) { if((*token == '(')) { get_token(); eval_exp0(value); // get subexpression if(*token != ')') throw InterpExc(PAREN_EXPECTED); get_token(); } else atom(value); } // Find value of number, variable, or function. void atom(int &value) { int i; char temp[MAX_ID_LEN+1]; switch(token_type) { case IDENTIFIER: i = internal_func(token); if(i != -1) { // Call "standard library" function. value = (*intern_func[i].p)(); } else if(find_func(token)) { // Call programmer-created function. call(); value = ret_value; } else { value = find_var(token); // get var's value strcpy(temp, token); // save variable name // Check for ++ or --. get_token(); if(*token == INC || *token == DEC) { if(*token == INC) assign_var(temp, find_var(temp)+1); else assign_var(temp, find_var(temp)-1); } else putback(); } get_token(); return; case NUMBER: // is numeric constant value = atoi(token); get_token(); return; case DELIMITER: // see if character constant if(*token == '\'') { value = *prog; prog++; if(*prog!='\'') throw InterpExc(QUOTE_EXPECTED); prog++; get_token(); return ; } if(*token==')') return; // process empty expression else throw InterpExc(SYNTAX); // otherwise, syntax error default: throw InterpExc(SYNTAX); // syntax error } } // Display an error message. void sntx_err(error_msg error) { char *p, *temp; int linecount = 0; static char *e[]= { "Syntax error", "No expression present", "Not a variable", "Duplicate variable name", "Duplicate function name", "Semicolon expected", "Unbalanced braces", "Function undefined", "Type specifier expected", "Return without call", "Parentheses expected", "While expected", "Closing quote expected", "Division by zero", "{ expected (control statements must use blocks)", "Colon expected" }; // Display error and line number. cout << "\n" << e[error]; p = p_buf; while(p != prog) // find line number of error { p++; if(*p == '\r') { linecount++; } } cout << " in line " << linecount << endl; temp = p; while(p > p_buf && *p != '\n') p--; // Display offending line. while(p <= temp) cout << *p++; cout << endl; } // Get a token. tok_types get_token() { char *temp; token_type = UNDEFTT; tok = UNDEFTOK; temp = token; *temp = '\0'; // Skip over white space. while(isspace(*prog) && *prog) ++prog; // Skip over newline. while(*prog == '\r') { ++prog; ++prog; // Again, skip over white space. while(isspace(*prog) && *prog) ++prog; } // Check for end of program. if(*prog == '\0') { *token = '\0'; tok = END; return (token_type = DELIMITER); } // Check for block delimiters. if(strchr("{}", *prog)) { *temp = *prog; temp++; *temp = '\0'; prog++; return (token_type = BLOCK); } // Look for comments. if(*prog == '/') if(*(prog+1) == '*') // is a /* comment { prog += 2; do // find end of comment { while(*prog != '*') prog++; prog++; } while (*prog != '/'); prog++; return (token_type = DELIMITER); } else if(*(prog+1) == '/') // is a // comment { prog += 2; // Find end of comment. while(*prog != '\r' && *prog != '\0') prog++; if(*prog == '\r') prog +=2; return (token_type = DELIMITER); } // Check for double-ops. if(strchr("!<>=+-", *prog)) { switch(*prog) { case '=': if(*(prog+1) == '=') { prog++; prog++; *temp = EQ; temp++; *temp = EQ; temp++; *temp = '\0'; } break; case '!': if(*(prog+1) == '=') { prog++; prog++; *temp = NE; temp++; *temp = NE; temp++; *temp = '\0'; } break; case '<': if(*(prog+1) == '=') { prog++; prog++; *temp = LE; temp++; *temp = LE; } else if(*(prog+1) == '<') { prog++; prog++; *temp = LS; temp++; *temp = LS; } else { prog++; *temp = LT; } temp++; *temp = '\0'; break; case '>': if(*(prog+1) == '=') { prog++; prog++; *temp = GE; temp++; *temp = GE; } else if(*(prog+1) == '>') { prog++; prog++; *temp = RS; temp++; *temp = RS; } else { prog++; *temp = GT; } temp++; *temp = '\0'; break; case '+': if(*(prog+1) == '+') { prog++; prog++; *temp = INC; temp++; *temp = INC; temp++; *temp = '\0'; } break; case '-': if(*(prog+1) == '-') { prog++; prog++; *temp = DEC; temp++; *temp = DEC; temp++; *temp = '\0'; } break; } if(*token) return(token_type = DELIMITER); } // Check for other delimiters. if(strchr("+-*^/%=;:(),'", *prog)) { *temp = *prog; prog++; temp++; *temp = '\0'; return (token_type = DELIMITER); } // Read a quoted string. if(*prog == '"') { prog++; while(*prog != '"' && *prog != '\r' && *prog) { // Check for \n escape sequence. if(*prog == '\\') { if(*(prog+1) == 'n') { prog++; *temp++ = '\n'; } } else if((temp - token) < MAX_T_LEN) *temp++ = *prog; prog++; } if(*prog == '\r' || *prog == 0) throw InterpExc(SYNTAX); prog++; *temp = '\0'; return (token_type = STRING); } // Read an integer number. if(isdigit(*prog)) { while(!isdelim(*prog)) { if((temp - token) < MAX_ID_LEN) *temp++ = *prog; prog++; } *temp = '\0'; return (token_type = NUMBER); } // Read identifier or keyword. if(isalpha(*prog)) { while(!isdelim(*prog)) { if((temp - token) < MAX_ID_LEN) *temp++ = *prog; prog++; } token_type = TEMP; } *temp = '\0'; // Determine if token is a keyword or identifier. if(token_type == TEMP) { tok = look_up(token); // convert to internal form if(tok) token_type = KEYWORD; // is a keyword else token_type = IDENTIFIER; } // Check for unidentified character in file. if(token_type == UNDEFTT) throw InterpExc(SYNTAX); return token_type; } // Return a token to input stream. void putback() { char *t; t = token; for(; *t; t++) prog--; } // Look up a token's internal representation in the // token table. token_ireps look_up(char *s) { int i; char *p; // Convert to lowercase. p = s; while(*p) { *p = tolower(*p); p++; } // See if token is in table. for(i=0; *com_table[i].command; i++) { if(!strcmp(com_table[i].command, s)) return com_table[i].tok; } return UNDEFTOK; // unknown command } // Return index of internal library function or -1 if // not found. int internal_func(char *s) { int i; for(i=0; intern_func[i].f_name[0]; i++) { if(!strcmp(intern_func[i].f_name, s)) return i; } return -1; } // Return true if c is a delimiter. bool isdelim(char c) { if(strchr(" !:;,+-<>'/*%^=()", c) || c == 9 || c == '\r' || c == 0) return true; return false; } listing 2 // A Mini C++ interpreter. #include <iostream> #include <fstream> #include <new> #include <stack> #include <vector> #include <cstring> #include <cstdlib> #include <cctype> #include "mccommon.h" using namespace std; char *prog; // current execution point in source code char *p_buf; // points to start of program buffer // This structure encapsulates the info // associated with variables. struct var_type { char var_name[MAX_ID_LEN+1]; // name token_ireps v_type; // data type int value; // value }; // This vector holds info for global variables. vector<var_type> global_vars; // This vector holds info for local variables // and parameters. vector<var_type> local_var_stack; // This structure encapsulates function info. struct func_type { char func_name[MAX_ID_LEN+1]; // name token_ireps ret_type; // return type char *loc; // location of entry point in program }; // This vector holds info about functions. vector<func_type> func_table; // Stack for managing function scope. stack<int> func_call_stack; // Stack for managing nested scopes. stack<int> nest_scope_stack; char token[MAX_T_LEN+1]; // current token tok_types token_type; // token type token_ireps tok; // internal representation int ret_value; // function return value bool breakfound = false; // true if break encountered int main(int argc, char *argv[]) { if(argc != 2) { cout << "Usage: minicpp <filename>\n"; return 1; } // Allocate memory for the program. try { p_buf = new char[PROG_SIZE]; } catch (bad_alloc exc) { cout << "Could Not Allocate Program Buffer\n"; return 1; } // Load the program to execute. if(!load_program(p_buf, argv[1])) return 1; // Set program pointer to start of program buffer. prog = p_buf; try { // Find the location of all functions // and global variables in the program. prescan(); // Next, set up the call to main(). // Find program starting point. prog = find_func("main"); // Check for incorrect or missing main() function. if(!prog) { cout << "main() Not Found\n"; return 1; } // Back up to opening (. prog--; // Set the first token to main strcpy(token, "main"); // Call main() to start interpreting. call(); } catch(InterpExc exc) { sntx_err(exc.get_err()); return 1; } catch(bad_alloc exc) { cout << "Out Of Memory\n"; return 1; } return ret_value; } // Load a program. bool load_program(char *p, char *fname) { int i=0; ifstream in(fname, ios::in | ios::binary); if(!in) { cout << "Cannot Open file.\n"; return false; } do { *p = in.get(); p++; i++; } while(!in.eof() && i < PROG_SIZE); if(i == PROG_SIZE) { cout << "Program Too Big\n"; return false; } // Null terminate the program. Skip any EOF // mark if present in the file. if(*(p-2) == 0x1a) *(p-2) = '\0'; else *(p-1) = '\0'; in.close(); return true; } // Find the location of all functions in the program // and store global variables. void prescan() { char *p, *tp; char temp[MAX_ID_LEN+1]; token_ireps datatype; func_type ft; // When brace is 0, the current source position // is outside of any function. int brace = 0; p = prog; do { // Bypass code inside functions while(brace) { get_token(); if(tok == END) throw InterpExc(UNBAL_BRACES); if(*token == '{') brace++; if(*token == '}') brace--; } tp = prog; // save current position get_token(); // See if global var type or function return type. if(tok==CHAR || tok==INT) { datatype = tok; // save data type get_token(); if(token_type == IDENTIFIER) { strcpy(temp, token); get_token(); if(*token != '(') // must be global var { prog = tp; // return to start of declaration decl_global(); } else if(*token == '(') // must be a function { // See if function already defined. for(unsigned i=0; i < func_table.size(); i++) if(!strcmp(func_table[i].func_name, temp)) throw InterpExc(DUP_FUNC); ft.loc = prog; ft.ret_type = datatype; strcpy(ft.func_name, temp); func_table.push_back(ft); do { get_token(); } while(*token != ')'); // Next token will now be opening curly // brace of function. } else putback(); } } else { if(*token == '{') brace++; if(*token == '}') brace--; } } while(tok != END); if(brace) throw InterpExc(UNBAL_BRACES); prog = p; } // Interpret a single statement or block of code. When // interp() returns from its initial call, the final // brace (or a return) in main() has been encountered. void interp() { int value; int block = 0; do { // Don't interpret until break is handled. if(breakfound) return; token_type = get_token(); // See what kind of token is up. if(token_type == IDENTIFIER || *token == INC || *token == DEC) { // Not a keyword, so process expression. putback(); // restore token to input stream for // further processing by eval_exp() eval_exp(value); // process the expression if(*token != ';') throw InterpExc(SEMI_EXPECTED); } else if(token_type==BLOCK) // block delimiter? { if(*token == '{') // is a block { block = 1; // interpreting block, not statement // Record nested scope. nest_scope_stack.push(local_var_stack.size()); } else // is a }, so reset scope and return { // Reset nested scope. local_var_stack.resize(nest_scope_stack.top()); nest_scope_stack.pop(); return; } } else // is keyword switch(tok) { case CHAR: case INT: // declare local variables putback(); decl_local(); break; case RETURN: // return from function call func_ret(); return; case IF: // process an if statement exec_if(); break; case ELSE: // process an else statement find_eob(); // find end of else block // and continue execution break; case WHILE: // process a while loop exec_while(); break; case DO: // process a do-while loop exec_do(); break; case FOR: // process a for loop exec_for(); break; case BREAK: // handle break breakfound = true; // Reset nested scope. local_var_stack.resize(nest_scope_stack.top()); nest_scope_stack.pop(); return; case SWITCH: // handle a switch statement exec_switch(); break; case COUT: // handle console output exec_cout(); break; case CIN: // handle console input exec_cin(); break; case END: exit(0); } } while (tok != END && block); return; } // Return the entry point of the specified function. // Return NULL if not found. char *find_func(char *name) { unsigned i; for(i=0; i < func_table.size(); i++) if(!strcmp(name, func_table[i].func_name)) return func_table[i].loc; return NULL; } // Declare a global variable. void decl_global() { token_ireps vartype; var_type vt; get_token(); // get type vartype = tok; // save var type // Process comma-separated list. do { vt.v_type = vartype; vt.value = 0; // init to 0 get_token(); // get name // See if variable is a duplicate. for(unsigned i=0; i < global_vars.size(); i++) if(!strcmp(global_vars[i].var_name, token)) throw InterpExc(DUP_VAR); strcpy(vt.var_name, token); global_vars.push_back(vt); get_token(); } while(*token == ','); if(*token != ';') throw InterpExc(SEMI_EXPECTED); } // Declare a local variable. void decl_local() { var_type vt; get_token(); // get var type vt.v_type = tok; // store type vt.value = 0; // init var to 0 // Process comma-separated list. do { get_token(); // get var name // See if variable is already the name // of a local variable in this scope. if(!local_var_stack.empty()) for(int i=local_var_stack.size()-1; i >= nest_scope_stack.top(); i--) { if(!strcmp(local_var_stack[i].var_name, token)) throw InterpExc(DUP_VAR); } strcpy(vt.var_name, token); local_var_stack.push_back(vt); get_token(); } while(*token == ','); if(*token != ';') throw InterpExc(SEMI_EXPECTED); } // Call a function. void call() { char *loc, *temp; int lvartemp; // First, find entry point of function. loc = find_func(token); if(loc == NULL) throw InterpExc(FUNC_UNDEF); // function not defined else { // Save local var stack index. lvartemp = local_var_stack.size(); get_args(); // get function arguments temp = prog; // save return location func_call_stack.push(lvartemp); // push local var index prog = loc; // reset prog to start of function get_params(); // load the function's parameters with // the values of the arguments interp(); // interpret the function prog = temp; // reset the program pointer if(func_call_stack.empty()) throw InterpExc(RET_NOCALL); // Reset local_var_stack to its previous state. local_var_stack.resize(func_call_stack.top()); func_call_stack.pop(); } } // Push the arguments to a function onto the local // variable stack. void get_args() { int value, count, temp[NUM_PARAMS]; var_type vt; count = 0; get_token(); if(*token != '(') throw InterpExc(PAREN_EXPECTED); // Process a comma-separated list of values. do { eval_exp(value); temp[count] = value; // save temporarily get_token(); count++; } while(*token == ','); count--; // Now, push on local_var_stack in reverse order. for(; count>=0; count--) { vt.value = temp[count]; vt.v_type = ARG; local_var_stack.push_back(vt); } } // Get function parameters. void get_params() { var_type *p; int i; i = local_var_stack.size()-1; // Process comma-separated list of parameters. do { get_token(); p = &local_var_stack[i]; if(*token != ')' ) { if(tok != INT && tok != CHAR) throw InterpExc(TYPE_EXPECTED); p->v_type = tok; get_token(); // Link parameter name with argument already on // local var stack. strcpy(p->var_name, token); get_token(); i--; } else break; } while(*token == ','); if(*token != ')') throw InterpExc(PAREN_EXPECTED); } // Return from a function. void func_ret() { int value; value = 0; // Get return value, if any. eval_exp(value); ret_value = value; } // Assign a value to a variable. void assign_var(char *vname, int value) { // First, see if it's a local variable. if(!local_var_stack.empty()) for(int i=local_var_stack.size()-1; i >= func_call_stack.top(); i--) { if(!strcmp(local_var_stack[i].var_name, vname)) { if(local_var_stack[i].v_type == CHAR) local_var_stack[i].value = (char) value; else if(local_var_stack[i].v_type == INT) local_var_stack[i].value = value; return; } } // Otherwise, try global vars. for(unsigned i=0; i < global_vars.size(); i++) if(!strcmp(global_vars[i].var_name, vname)) { if(global_vars[i].v_type == CHAR) global_vars[i].value = (char) value; else if(global_vars[i].v_type == INT) global_vars[i].value = value; return; } throw InterpExc(NOT_VAR); // variable not found } // Find the value of a variable. int find_var(char *vname) { // First, see if it's a local variable. if(!local_var_stack.empty()) for(int i=local_var_stack.size()-1; i >= func_call_stack.top(); i--) { if(!strcmp(local_var_stack[i].var_name, vname)) return local_var_stack[i].value; } // Otherwise, try global vars. for(unsigned i=0; i < global_vars.size(); i++) if(!strcmp(global_vars[i].var_name, vname)) return global_vars[i].value; throw InterpExc(NOT_VAR); // variable not found } // Execute an if statement. void exec_if() { int cond; eval_exp(cond); // get if expression. if(cond) // if true, process target of IF { // Confirm start of block. if(*token != '{') throw InterpExc(BRACE_EXPECTED); interp(); } else { // Otherwise skip around IF block and // process the ELSE, if present. find_eob(); // find start of next line get_token(); if(tok != ELSE) { // Restore token if no ELSE is present. putback(); return; } // Confirm start of block. get_token(); if(*token != '{') throw InterpExc(BRACE_EXPECTED); putback(); interp(); } } // Execute a switch statement. void exec_switch() { int sval, cval; int brace; eval_exp(sval); // Get switch expression. // Check for start of block. if(*token != '{') throw InterpExc(BRACE_EXPECTED); // Record new scope. nest_scope_stack.push(local_var_stack.size()); // Now, check case statements. for(;;) { brace = 1; // Find a case statement. do { get_token(); if(*token == '{') brace++; else if(*token == '}') brace--; } while(tok != CASE && tok != END && brace); // If no matching case found, then skip. if(!brace) break; if(tok == END) throw InterpExc(SYNTAX); // Get value of the case statement. eval_exp(cval); // Read and discard the : get_token(); if(*token != ':') throw InterpExc(COLON_EXPECTED); // If values match, then interpret. if(cval == sval) { brace = 1; do { interp(); if(*token == '{') brace++; else if(*token == '}') brace--; } while(!breakfound && tok != END && brace); // Find end of switch statement. while(brace) { get_token(); if(*token == '{') brace++; else if(*token == '}') brace--; } breakfound = false; break; } } } // Execute a while loop. void exec_while() { int cond; char *temp; putback(); // put back the while temp = prog; // save location of top of while loop get_token(); eval_exp(cond); // check the conditional expression // Confirm start of block. if(*token != '{') throw InterpExc(BRACE_EXPECTED); if(cond) interp(); // if true, interpret else // otherwise, skip to end of loop { find_eob(); return; } prog = temp; // loop back to top // Check for break in loop. if(breakfound) { // Find start of loop block. do { get_token(); } while(*token != '{' && tok != END); putback(); breakfound = false; find_eob(); // now, find end of loop return; } } // Execute a do loop. void exec_do() { int cond; char *temp; // Save location of top of do loop. putback(); // put back do temp = prog; get_token(); // get start of loop block // Confirm start of block. get_token(); if(*token != '{') throw InterpExc(BRACE_EXPECTED); putback(); interp(); // interpret loop // Check for break in loop. if(breakfound) { prog = temp; // Find start of loop block. do { get_token(); } while(*token != '{' && tok != END); // Find end of while block. putback(); find_eob(); // Now, find end of while expression. do { get_token(); } while(*token != ';' && tok != END); if(tok == END) throw InterpExc(SYNTAX); breakfound = false; return; } get_token(); if(tok != WHILE) throw InterpExc(WHILE_EXPECTED); eval_exp(cond); // check the loop condition // If true loop; otherwise, continue on. if(cond) prog = temp; } // Execute a for loop. void exec_for() { int cond; char *temp, *temp2; int paren ; get_token(); // skip opening ( eval_exp(cond); // initialization expression if(*token != ';') throw InterpExc(SEMI_EXPECTED); prog++; // get past the ; temp = prog; for(;;) { // Get the value of the conditional expression. eval_exp(cond); if(*token != ';') throw InterpExc(SEMI_EXPECTED); prog++; // get past the ; temp2 = prog; // Find start of for block. paren = 1; while(paren) { get_token(); if(*token == '(') paren++; if(*token == ')') paren--; } // Confirm start of block. get_token(); if(*token != '{') throw InterpExc(BRACE_EXPECTED); putback(); // If condition is true, interpret if(cond) interp(); else // otherwise, skip to end of loop { find_eob(); return; } prog = temp2; // go to increment expression // Check for break in loop. if(breakfound) { // Find start of loop block. do { get_token(); } while(*token != '{' && tok != END); putback(); breakfound = false; find_eob(); // now, find end of loop return; } // Evaluate the increment expression. eval_exp(cond); prog = temp; // loop back to top } } // Execute a cout statement. void exec_cout() { int val; get_token(); if(*token != LS) throw InterpExc(SYNTAX); do { get_token(); if(token_type==STRING) { // Output a string. cout << token; } else if(token_type == NUMBER || token_type == IDENTIFIER) { // Output a number. putback(); eval_exp(val); cout << val; } else if(*token == '\'') { // Output a character constant. putback(); eval_exp(val); cout << (char) val; } get_token(); } while(*token == LS); if(*token != ';') throw InterpExc(SEMI_EXPECTED); } // Execute a cin statement. void exec_cin() { int val; char chval; token_ireps vtype; get_token(); if(*token != RS) throw InterpExc(SYNTAX); do { get_token(); if(token_type != IDENTIFIER) throw InterpExc(NOT_VAR); vtype = find_var_type(token); if(vtype == CHAR) { cin >> chval; assign_var(token, chval); } else if(vtype == INT) { cin >> val; assign_var(token, val); } get_token(); } while(*token == RS); if(*token != ';') throw InterpExc(SEMI_EXPECTED); } // Find the end of a block. void find_eob() { int brace; get_token(); if(*token != '{') throw InterpExc(BRACE_EXPECTED); brace = 1; do { get_token(); if(*token == '{') brace++; else if(*token == '}') brace--; } while(brace && tok != END); if(tok==END) throw InterpExc(UNBAL_BRACES); } // Determine if an identifier is a variable. Return // true if variable is found; false otherwise. bool is_var(char *vname) { // See if vname a local variable. if(!local_var_stack.empty()) for(int i=local_var_stack.size()-1; i >= func_call_stack.top(); i--) { if(!strcmp(local_var_stack[i].var_name, vname)) return true; } // See if vname is a global variable. for(unsigned i=0; i < global_vars.size(); i++) if(!strcmp(global_vars[i].var_name, vname)) return true; return false; } // Return the type of variable. token_ireps find_var_type(char *vname) { // First, see if it's a local variable. if(!local_var_stack.empty()) for(int i=local_var_stack.size()-1; i >= func_call_stack.top(); i--) { if(!strcmp(local_var_stack[i].var_name, vname)) return local_var_stack[i].v_type; } // Otherwise, try global vars. for(unsigned i=0; i < global_vars.size(); i++) if(!strcmp(global_vars[i].var_name, vname)) return local_var_stack[i].v_type; return UNDEFTOK; } listing 3 // ***** Internal Library Functions ***** // Add more of your own, here. #include <iostream> #include <cstdlib> #include <cstdio> #include "mccommon.h" using namespace std; // Read a character from the console. // If your compiler supplies an unbuffered // character intput function, feel free to // substitute it for the call to cin.get(). int call_getchar() { char ch; ch = getchar(); // Advance past () get_token(); if(*token != '(') throw InterpExc(PAREN_EXPECTED); get_token(); if(*token != ')') throw InterpExc(PAREN_EXPECTED); return ch; } // Write a character to the display. int call_putchar() { int value; eval_exp(value); putchar(value); return value; } // Return absolute value. int call_abs() { int val; eval_exp(val); val = abs(val); return val; } // Return a randome integer. int call_rand() { // Advance past () get_token(); if(*token != '(') throw InterpExc(PAREN_EXPECTED); get_token(); if(*token != ')') throw InterpExc(PAREN_EXPECTED); return rand(); } listing 4 // Common declarations used by parser.cpp, minicpp.cpp, // or libcpp.cpp, or by other files that you might add. // const int MAX_T_LEN = 128; // max token length const int MAX_ID_LEN = 31; // max identifier length const int PROG_SIZE = 10000; // max program size const int NUM_PARAMS = 31; // max number of parameters // Enumeration of token types. enum tok_types { UNDEFTT, DELIMITER, IDENTIFIER, NUMBER, KEYWORD, TEMP, STRING, BLOCK }; // Enumeration of internal representation of tokens. enum token_ireps { UNDEFTOK, ARG, CHAR, INT, SWITCH, CASE, IF, ELSE, FOR, DO, WHILE, BREAK, RETURN, COUT, CIN, END }; // Enumeration of two-character operators, such as <=. enum double_ops { LT=1, LE, GT, GE, EQ, NE, LS, RS, INC, DEC }; // These are the constants used when throwing a // syntax error exception. // // NOTE: SYNTAX is a generic error message used when // nothing else seems appropriate. enum error_msg { SYNTAX, NO_EXP, NOT_VAR, DUP_VAR, DUP_FUNC, SEMI_EXPECTED, UNBAL_BRACES, FUNC_UNDEF, TYPE_EXPECTED, RET_NOCALL, PAREN_EXPECTED, WHILE_EXPECTED, QUOTE_EXPECTED, DIV_BY_ZERO, BRACE_EXPECTED, COLON_EXPECTED }; extern char *prog; // current location in source code extern char *p_buf; // points to start of program buffer extern char token[MAX_T_LEN+1]; // string version of token extern tok_types token_type; // contains type of token extern token_ireps tok; // internal representation of token extern int ret_value; // function return value extern bool breakfound; // true if break encountered // Exception class for Mini C++. class InterpExc { error_msg err; public: InterpExc(error_msg e) { err = e; } error_msg get_err() { return err; } }; // Interpreter prototypes. void prescan(); void decl_global(); void call(); void putback(); void decl_local(); void exec_if(); void find_eob(); void exec_for(); void exec_switch(); void get_params(); void get_args(); void exec_while(); void exec_do(); void exec_cout(); void exec_cin(); void assign_var(char *var_name, int value); bool load_program(char *p, char *fname); int find_var(char *s); void interp(); void func_ret(); char *find_func(char *name); bool is_var(char *s); token_ireps find_var_type(char *s); // Parser prototypes. void eval_exp(int &value); void eval_exp0(int &value); void eval_exp1(int &value); void eval_exp2(int &value); void eval_exp3(int &value); void eval_exp4(int &value); void eval_exp5(int &value); void atom(int &value); void sntx_err(error_msg error); void putback(); bool isdelim(char c); token_ireps look_up(char *s); int find_var(char *s); tok_types get_token(); int internal_func(char *s); bool is_var(char *s); // "Standard library" prototypes. int call_getchar(); int call_putchar(); int call_abs(); int call_rand(); listing 5 /* Mini C++ Demonstration Program #1. This program demonstrates all features of C++ that are recognized by Mini C++. */ int i, j; // global vars char ch; int main() { int i, j; // local vars // Call a "standard library' function. cout << "Mini C++ Demo Program.\n\n"; // Call a programmer-defined function. print_alpha(); cout << "\n"; // Demonstrate do and for loops. cout << "Use loops.\n"; do { cout << "Enter a number (0 to quit): "; cin >> i; // Demonstrate the if if(i < 0 ) { cout << "Numbers must be positive, try again.\n"; } else { for(j = 0; j <= i; ++j) { cout << j << " summed is "; cout << sum(j) << "\n"; } } } while(i != 0); cout << "\n"; // Demonstrate the break in a loop. cout << "Break from a loop.\n"; for(i=0; i < 100; i++) { cout << i << "\n"; if(i == 5) { cout << "Breaking out of loop.\n"; break; } } cout << "\n"; // Demonstrate the switch cout << "Use a switch.\n"; for(i=0; i < 6; i++) { switch(i) { case 1: // can stack cases case 0: cout << "1 or 0\n"; break; case 2: cout << "two\n"; break; case 3: cout << "three\n"; break; case 4: cout << "four\n"; cout << "4 * 4 is "<< 4*4 << "\n"; // break; // this break is optional // no case for 5 } } cout << "\n"; cout << "Use a library function to generate " << "10 random integers.\n"; for(i=0; i < 10; i++) { cout << rand() << " "; } cout << "\n"; cout << "Done!\n"; return 0; } // Sum the values between 0 and num. // This function takes a parameter. int sum(int num) { int running_sum; running_sum = 0; while(num) { running_sum = running_sum + num; num--; } return running_sum; } // Print the alphabet. int print_alpha() { cout << "This is the alphabet:\n"; for(ch = 'A'; ch<='Z'; ch++) { putchar(ch); } cout << "\n"; return 0; } listing 6 // Nested loop example. int main() { int i, j, k; for(i = 0; i < 5; i = i + 1) { for(j = 0; j < 3; j = j + 1) { for(k = 3; k ; k = k - 1) { cout << i <<", "; cout << j << ", "; cout << k << "\n"; } } } cout << "done"; return 0; } listing 7 // Assigments as operations. int main() { int a, b; a = b = 5; cout << a << " " << b << "\n"; while(a=a-1) { cout << a << " "; do { cout << b << " "; } while((b=b-1) > -5); cout << "\n"; } return 0; } listing 8 // This program demonstrates a recursive function. // A recursive function that returns the // factorial of i. int factr(int i) { if(i<2) { return 1; } else { return i * factr(i-1); } } int main() { cout << "Factorial of 4 is: "; cout << factr(4) << "\n"; cout << "Factorial of 6 is: "; cout << factr(6) << "\n"; return 0; } listing 9 // A more rigorous example of function arguments. int f1(int a, int b) { int count; cout << "Args for f1 are "; cout << a << " " << b << "\n"; count = a; do { cout << count << " "; } while(count=count-1); cout << a << " " << b << " " << a*b << "\n"; return a*b; } int f2(int a, int x, int y) { cout << "Args for f2 are "; cout << a << " " << x << " " << y << "\n"; cout << x / a << " "; cout << y*x << "\n"; return 0; } int main() { f2(10, f1(10, 20), 99); return 0; } listing 10 // Exercise the loop statements. int main() { int a; char ch; // The while. cout << "Enter a number: "; cin >> a; while(a) { cout << a*a << " "; --a; } cout << "\n"; // The do-while. cout << "\nEnter characters, 'q' to quit.\n"; do { // Use two "standard library" functions. ch = getchar(); putchar(ch); } while(ch != 'q'); cout << "\n\n"; // the for. for(a=0; a<10; ++a) { cout << a << " "; } cout << "\n\nDone!\n"; return 0; } listing 11 // Demonstrate nested scopes. int x; // global x int main() { int i; i = 4; x = 99; // global x is 99 if(i == 4) { int x; // local x int num; // local to if statement x = i * 2; cout << "Outer local x before loop: " << x << "\n"; while(x--) { int x; // another local x x = 18; cout << "Inner local x: " << x << "\n"; } cout << "Outer local x after loop: " << x << "\n"; } // Can't refer to num here because it is local // to the preceding if block. // num = 10; cout << "Global x: " << x << "\n"; }
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