c语言之控制语句：循环

#include<stdio.h>

int main(void)
{
    long num;
    long sum = 0L;
    int status;
    printf("Please enter an integer to be summed:");
    printf("(q to quit):");
    status = scanf_s("%ld", &num);
    while (status)
    {
        sum = sum + num;
        printf("Please enter next integer (q to quit):");
        status = scanf_s("%ld", &num);
    }
    printf("Those integers sum to %ld.\n", sum);
    system("pause");
    return 0;
}

status = scanf_s("%ld",&num);
while(status=1)
{
    /*循环行为*/
    status = scanf_s("%ld",&num)
}
//替换代码
while(scanf_s("%ld",&num)=1)
{
    /*循环行为*/
    status = scanf_s("%ld",&num)
}

while语句

while(expression)
statement

statement部分可以是以分号结尾的简单语句，也可以是用花括号括起来的复合语句。其中，如果expression为真，执行。

在构建while循环时，必须让测试表达式的值有所变化。

while(expression)
printf("sth");//如果没有花括号，只有紧跟着while后面的第一条语句才算循环
other statement

/*不加分号*/
#include<stdio.h>

int main(void)
{
    int n = 0;
    while (n++ < 3) 
    printf("n is %d\n", n);
    printf("That's all this program does.\n");
    system("pause");
    return 0;
}
/*
result:
n is 1
n is 2
n is 3
That's all this program does.
*/

/*加分号*/
#include<stdio.h>

int main(void)
{
    int n = 0;
    while (n++ < 3); //加分号则会让编译器认为这句号是单独的一句
    printf("n is %d\n", n);
    printf("That's all this program does.\n");
    system("pause");
    return 0;
}
/*
result:
n is 4
That's all this program does.
*/

用关系运算符和表达式比较大小

运算符	含义
<	小于
<=	小于或等于
==	等于
>=	大于或等于
>	大于
!=	不等于

注：当使用浮点数时，尽量只是用“<”或“>”，因为浮点数的舍入误差会导致在逻辑上应该相等的两数却不相等。

value_1 == value_! //判断
value_1 = value_2  //赋值

#include<math.h>
#include<stdio.h>

int main(void)
{
    const double ANSWER = 3.14159;
    double response;
    printf("What is the value of pi?\n");
    scanf_s("%f", &response);
    while (fabs(response - ANSWER) > 0.0001) //fabs()函数取绝对值
    {
        printf("Try again!\n");
        scanf_s("%lf", &response);
    }
    printf("Close enough!\n");
    system("pause");
    return 0;
}

新的_Bool类型

#include<stdio.h>

int main(void)
{
    long num;
    long sum = 0L;
    _Bool input_is_good;  //定义Bool类型
    printf("Please enter an integer to be summed");
    printf("(q to quit):");
    input_is_good = (scanf_s("%ld", &num) == 1);
    while (input_is_good)
    {
        sum = sum + num;
        printf("Please enter next integer(q to quit):");
        input_is_good = (scanf_s("%ld", &num) == 1);
    }
    printf("Those integers sum to %ld.\n", sum);
    return 0;
}

优先级和关系运算符

高优先级组：<、<=、>、>=

低优先级组：==、！=

不确定循环和计数循环

#include<stdio.h>

int main(void)
{
    const int NUMBER = 22;
    int count = 1;
    while (count <= NUMBER) {
        printf("Be my Valentine!\n");
        count++;
    }
    system("pause");
    return 0;
}

for 循环

#include<stdio.h>

int main(void)
{
    const int NUMBER = 22;
    int count = 1;
    for (count = 1; count <= NUMBER; count++)//第一个是表达式初始化，第二个是测试条件，第三个是表达式执行更新。
        printf("Be my Valentine!\n");
    system("pause");
    return 0;
}

#include<stdio.h>

int main(void)
{
    char ch;
    for (ch = 'a'; ch <= 'z'; ch++)
        printf("The ASCII value for %c is %d.\n", ch, ch);
    system("pause");
    return 0;
}

可以省略一个或多个表达式（但是不能省略分号），只要在循环体中包含能结束循环的语句即可。另外，第一个表达式不一定要给变量赋初值，也可以使用printf()。但是，在执行循环体的其他部分之前，只对第一个表达式求值一次或执行一次。

#include<stdio.h>

int main(void)
{
    int num = 0;
    for (printf("Keep entering number!\n"); num != 6;)
        scanf_s("%d", &num);
    printf("That's one I want!\n");
    return 0;
}

其他赋值运算符

表达式	含义
scores += 20	scores = scores + 20
dimes -= 2	dimes = dimes - 20
bunnies *= 2	bunnies = bunnies * 20
time /= 2.73	time = time / 2.73
reduce %= 3	reduce = reduce % 3

逗号运算符

#include<stdio.h>

int main(void)
{
    const int FIRST_OZ = 46, NEXT_OZ = 20;
    int ounces, cost;
    printf("ounces cost\n");
    for (ounces = 1, cost = FIRST_OZ; ounces <= 16; ounces++, cost += NEXT_OZ) //同时有多个值，且从左到右顺序执行
        printf("%5d      $%4.2f\n", ounces, cost / 100.0);
    system("pause");
    return 0;
}

houseprice = 249,500 //分为houseprice=249和500这两条表达式语句
houseprice =(249,500)//等于houseprice=500
char ch,date；//分隔符

出口条件循环：do while

#include<stdio.h>

int main(void)
{
    const int secret_code = 13;
    int code_entered;
    do
    {
        printf("To enter the yuan therapy clud,\n");
        printf("please enter the scret code number:");
        scanf_s("%d", &code_entered);
    } while (code_entered != secret_code);
    printf("Congratulation! You are cured!\n");
    system("pause");
    return 0;
}

/*
do
statement
while (expression);
*/

do while循环在执行完循环体后才执行测试条件，所以至少执行循环体一次，while循环体适用于那些至少要迭代一次的循环。

嵌套循环

/*打印字母串*/
#include<stdio.h>
#define ROWS 6
#define CHARS 10

int main(void)
{
    int row;
    char ch;
    for (row = 0; row < ROWS; row++) {
        for (ch = 'A'; ch < ('A' + CHARS); ch++)
            printf("%c", ch);
        printf("\n");
    }
    system("pause");
    return 0;
}

数组

float debts[n];//声明一个含有20个元素的数组
debts[n]=num;//为数组元素赋值，编译器不会查找元素不在等的错误

用于识别数组元素的数字被称为下标、索引或偏移量。

/*打印字母串*/
#include<stdio.h>
#define SIZE 10
#define PAR 72

int main(void)
{
    int index, score[SIZE];
    int sum = 0;
    float average;
    printf("Enter %d golf scores:\n", SIZE);
    for (index = 0; index < SIZE; index++)
        scanf_s("%d",&score[index]);
    printf("\n");
    for (index = 0; index < SIZE; index++)
        sum += score[index];
    average = (float)sum / SIZE;
    printf("Sum of scores = %d,average = %.2f\n", sum, average);
    printf("That's a handicap of %.0f.\n", average - PAR);
    system("pause");
    return 0;
}

使用函数返回值的循环

#include<stdio.h>
double power(double n, int p); 
/*C编译器运行到这里还不知道power函数是什么，所以要先声明该函数的返回值类型等，而如果该函数写在main函数的前面，则不需要在此处声明该函数*/

int main(void)
{
    double x, xpow;
    int exp;
    printf("Enter a number and the positive integer power");
    printf(" to which\nthe number will be raised. Enter q");
    printf(" to quit.\n");
    while (scanf_s("%lf%d", &x, &exp) == 2)
    {
        xpow = power(x, exp);
        printf("%.3g to the power %d is %.5g.\n", x, exp, xpow);
        printf("Enter next pair of numbers or q to quit.\n");
    }
    printf("Hope you enjoyed this power trip -- bye!\n");
    system("pause");
    return 0;
}

double power(double n, int p)
{
    double pow = 1;
    int i;
    for (i = 1; i <= p; i++)
        pow *= n;
    return pow;
}