第3章 Python 的控制语句

3.1 结构化程序设计；
3.2 条件判断语句；
     3.2.1 if 条件语句；
if (表达式):
    语句1
else:
    语句2
x = input("x:")
x = int(x)
x = x + 1
print (x)
输入：x:9
     10

执行 if 内的程序
a = input("a:")
a = int(a)
b = input("b:")
b = int(b)
if (a > b):
    print (a, ">", b)
输出：a:8
     b:3
     8 > 3
     a:2
     b:7

if else 语句
a = input("a:")
a = int(a)
b = input("b:")
b = int(b)
if (a > b):
    print (a, ">", b)
else:
    print (a, "<", b)

        3.2.2 if...elif...else 判断语句
score = float(input("score:"))
if 90 <= score <= 100:
    print ("A")
elif 80 <= score <= 90:
    print ("B")
elif 60 <= score <= 80:
    print ("C")
else:
    print ("D")

x = -1
y = 99
if (x >= 0):
    if (x > 0):
        y = 1
    else:
        y = 0
else:
    y = -1
print ("y =", y)

        3.2.4 switch 语句的替代方案
使用字典实现switch语句
from __future__ import division
x = 1
y = 2
operator = "*"
result = {"+": x + y, "-": x - y, "*": x * y, "/": x / y}
print (result.get(operator))
输出：2  x*y

class switch(object):
    def __init__(self, value):
        self.value = value
        self.fall = False

    def __iter__(self):
        yield self.match
        raise StopIteration

    def match(self, *args):
        if self.fall or not args:
            return True
        elif self.value in args:
            self.fall = True
            return True
        else:
            return False

operator = "+"
x = 1
y = 2
for case in switch(operator):
    if case("+"):
        print (x + y)
        break
    if case("-"):
        print (x - y)
        break
    if case("*"):
        print (x * y)
        break
    if case("/"):
        print (x / y)
        break
    if case():
        print ""

3.3 循环语句 3.3.1 while 循环 numders = input("输入几个数字，用逗号分割：").split(",") print (numders) x = 0 while x < len(numders): print (numders[x]) x += 1 输出：输入几个数字，用逗号分割：123 ['123'] 123 x = float(input("输入x的值：")) i = 0 while(x != 0): if (x > 0): x -= 1 else: x += 1 i = i + 1 print ("第%d次循环：" % (i, x)) else: print ("x等于0：",x) i = 1 while i > 0: i = i + 1 print (i) break 3.3.2 for 循环 for x in range(-1, 2): if x > 0: print ("正数：", x) elif x == 0: print ("零：", x) else: print ("负数：", x) else: print ("循环结束") 输出：负数： -1 零： 0 正数： 1 循环结束 x = 0 while x < 5: print (x) x = x + 2 for x in range(0, 5, 2): print (x) 输出：0 2 4 3.3.3 break 和 continue 语句 operator = "+" x = 1 y = 2 for case in switch(operator): if case("+"): print (x + y) break if case("-"): print (x - y) break if case("*"): print (x * y) break if case("/"): print (x / y) break if case(): print "" x = int(input("输入x的值：")) y = 0 for y in range(0, 100): if x == y: print ("找到数字：", x) break else: print ("没找到") x = 0 for i in [1,2,3,4,5]: if x == i: continue x += i print ("x的值为", x) 3.4 结构化程序实例 def bubbleSort(numbers): for j in range(len(numbers) - 1, -1, -1): for i in range(j): if numbers[i] > numbers[i + 1]: numbers[i], numbers[i + 1] = numbers[i + 1], numbers[i] print (numbers) def main(): numbers = [23, 12, 9, 15, 6] bubbleSort((numbers)) if __name__ == "__main__": main() 输出：[12, 23, 9, 15, 6] [12, 9, 23, 15, 6] [12, 9, 15, 23, 6] [12, 9, 15, 6, 23] [9, 12, 15, 6, 23] [9, 12, 15, 6, 23] [9, 12, 6, 15, 23] [9, 12, 6, 15, 23] [9, 6, 12, 15, 23] [6, 9, 12, 15, 23] 3.6 习题