C++中函数备忘录

1.

int rand(void);

function: creat random value ,从srand(seed)中指定的seed开始，返回一个[seed,RAND_MAX(0X7fff)]间的随机整数；

void srand(unsigned seed);

参数seed是rand()的种子，用来初始化rand()的起始值。

可以认为rand()在每次被调用的时候，它会查看：

1） 如果用户在此之前调用过srand(seed)，给seed指定了一个值，那么它会自动调用srand(seed)一次来初始化它的起始值。

2） 如果用户在此之前没有调用过srand(seed)，它会自动调用srand(1)一次。

可以查看：https://blog.csdn.net/u013745804/article/details/82379266；

 

 

2. function:floor （向下取整）

c11: double floor (double x);

        floor floor (float x);

long double floor (long double x);

        doulbe floor (T x); //整型的附加重载；

与之对应的函数 ceil : 向上取整。

3. STL中resize(); 和 reserve();

reszie();

This function will %resize the %vector to the specified

* number of elements. If the number is smaller than the

* %vector's current size the %vector is truncated, otherwise

* the %vector is extended and new elements are populated with

* given data.

reserve();

This function attempts to reserve enough memory for the

* %vector to hold the specified number of elements. If the

* number requested is more than max_size(), length_error is

* thrown.

* The advantage of this function is that if optimal code is a

* necessity and the user can determine the number of elements

* that will be required, the user can reserve the memory in

* %advance, and thus prevent a possible reallocation of memory

* and copying of %vector data.