5. 循环控制 Repetition

Loop（循环）

5.1 循环的基本结构

重复代码段要有4个部分
1 循环语句

while
for
do-while
->for 和 while在C中可以互换，所以选择两个任意效果是一样的。
->其他语言中的习惯，for用来计数器控制循环，while用来条件式控制循环。

2 条件式
3 初始化
4 改变

5.2 while statement

#include <stdio.h>
#define MAXCOUNT 4

int main()
{
    int count;
    float num, total, average;
    
    printf("Only enter %d numbers\n\n", MAXCOUNT);
    
    count = 1;
    
    total = 0.0;
    
    while (count <= MAXCOUNT) {
        printf("Enter a number: ");
        scanf("%f", &num);
        total += num;
        count++;
    }
    
    average = total / MAXCOUNT;
    
    printf("The sum is %f\n", total);
    printf("The average is %f\n", average);
    
    return 0;
    
}

break
强制一个即时中断||立即从switch，while，for，do-while语句中退出
break;

continue
只用于while，do-while，for。
continue;

空语句
;

5.4 for statement

#include <stdio.h>
#define MAXCOUNT 4

int main()
{
    int count;
    float num, total, average;
    
    printf("Only enter %d numbers\n\n", MAXCOUNT);
    
    total = 0.0;
    
    for (count = 0; count < MAXCOUNT; count++) {
        printf("\nEnter a number:");
        scanf("%f", &num);
        total += num;
    }
    
    average = total / MAXCOUNT;
    
    printf("The sum is %f\n", total);
    printf("The average is %f\n", average);
    
    return 0;
    
}

5.5 Loop programming Techs

1 selection within loop

#include <stdio.h>
#define MAXNUMS 5

int main()
{
    int i;
    float number;
    float postotal = 0.0f;
    float negtotal = 0.0f;
    
    for (i = 1; i <= MAXNUMS; i++) {
        printf("Enter a number (positive or negative): ");
        scanf("%f", &number);
        //在循环内选择
        if (number > 0) {
            postotal += number;
        }
        else
            negtotal += number;
        
    }
    
    printf("\nThe positive total is %f", postotal);
    printf("\nThe negative total is %f\n", negtotal);
    
    return 0;
}

2 Input data Validation

#include <stdio.h>

int main()
{
    int month;
    
    printf("\nEnter a month between 1 and 12: ");
    scanf("%d", &month);
    
    while (month <1 || month >12) {
        printf("Input Error");
        printf("\nEnter a month between 1 and 12: ");
        scanf("%d", &month);
    }
    
    printf("The month accepted is %d\n", month);
    
    return 0;
    
}

5.6 嵌套循环

至少两层循环inner loop和outer loop

#include <stdio.h>

int main()
{
    int i,j;
    
    for(i = 1; i <= 5; i++){
        printf("\ni is now %d\n", i);
        for (j = 1; j<=4; j++) {
            printf("j=%d", j);
        }
    }
    return 0;
}

语法

1 while
while(表达式)
语句;

2 for
for(statement) {
compound statement
}

3 跟别人说的
do
statement;
while (expression)