C语言学习笔记[第17天]-优快云博客

本文链接：https://blog.youkuaiyun.com/counting_stars1/article/details/128439045

本文介绍了C语言中的循环控制语句及条件判断的基础知识，包括如何使用关系运算符进行大小比较，真值的概念及其在循环中的应用，并通过示例程序展示了具体的实现方式。

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六、C控制语句：循环

3. 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

while (number < 6)
{
    printf("Your number is too small.\n");
    scanf("$d", &number);
}

while (ch != 's')
{
    count++;
    scanf("%c", &ch);
}

while (scanf("%f", &num) == 1)
    sum = sum + num;

// cmpflt.v -- 浮点数比较
#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("%lf", &response);
    while (fabs(response - ANSWER) > 0.0001)
    {
        printf("Try again!\n");
        scanf("%lf", &response);
    }
    printf("Close enough!\n");
    
    return 0;
}
//what is the value of pi?
//3.14
//Try again!
//Close enough!

3.1 什么是真

/* t_and_f.c -- C中的真和假的值 */
#include <stdio.h>
int main(void)
{
    int true_val, false_val;
    
    true_val = (10 > 2);	// 关系为真的值
    false_val = (10 == 2); // 关系为假的值
    printf("true = %d; false = %d \n", true_val, false_vall);
    
    return 0;
}

while (1)
{
    ...
}

3.2 其他真值

// truth.c -- 哪些值为真
#include <stdio.h>
int main(void)
{
    int n = 3;
    
    while (n)
        printf("%2d is true\n", n--);
    printf("%2d is false\n", n);
    
    n = -3;
    while (n);
    	printf("%2d is true\n", n++);
    printf("%2d is false\n", n);
    
    return 0;
}
//3 is ture
//2 is ture
//1 is ture
//0 is false
//-3 is ture
//-2 is ture
//-1 is ture
//0 is false

3.3 真值的问题

// trouble.c -- 误用=会导致无限循环

#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("%ld", &num);
    while (status = 1)
    {
        sum = sum + num;
        printf("Please enter next integer (q to quit): ");
        status = scanf("%ld", &num);
    }
    printf("Those integers sum to %ld.\n", sum);
    
    return 0;
}
//Please enter an integer to be summed (q to quit): 5
//Please enter next integer (q to quit): 30
//Please enter next integer (q to quit):
//Please enter next integer (q to quit):
//Please enter next integer (q to quit):
//..

3.4 新的_Bool 类型

// boolean.c -- 使用_Bool 类型的变量 variable
#include <stdio.h>
int main(void)
{
    long num;
    long sum = 0L;
    _Bool input_is_good;
    
    printf("Please enter an integer to be summed ");
    printf("(q to quit): ");
    input_is_good = (scanf("%ld", &num) == 1);
    while (input_is_good)
    {
        sum = sum + num;
        printf("Please enter next integer (q to quit): ");
        input_is_good = (scanf("%ld", &num) == 1);
    }
    printf("Those integers sum to %ld.\n", sum);
    
    return 0;
}