C控制语句

C控制语句

  1. default、goto
  2. 更复杂的测试表达式中用逻辑运算符组合关系表达式
  3. C条件表达式
  4. switch语句
  5. continue、goto语句
  6. I/O函数getchar()和putchar()
  7. ctype.h宏文件字符分析函数

if分支语句

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/29/2023, 10:38:10 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/29/2023, 10:38:10 PM
 * Description: if branching statement
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    const int FREEZING = 0;
    float temperature;
    int cold_days = 0;
    int all_days = 0;

    printf("Enter the list of daily low temperatures. \n");
    printf("Use Celsius, and enter q to quit. \n");
    while (scanf("%f", &temperature) == 1)
    {
        all_days++;
        if (temperature < FREEZING) // if 复合语句
        {
            cold_days++;
        }
        if (all_days != 0)
        {
            printf("%d days total: %.lf%% were below freezing.\n",
            all_days, 100.0 * (float) cold_days / all_days);
        }
        if (all_days == 0)
            printf("No data entered!\n"); // if简单语句
        
        getchar();

        return 0;
    }
}

if...else

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/29/2023, 10:51:26 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/29/2023, 10:51:26 PM
 * Description: if...else branching statements
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    const int FREEZING = 10;
    float temperature;
    int cold_days = 0;
    int all_days = 0;

    printf("Enter the list of daily low temperatures. \n");
    printf("Use Celsius, and enter q to quit. \n");
    while (scanf("%f", &temperature) == 1)
    {
        all_days++;
        if (temperature < FREEZING)
        {
            cold_days++;
        } else {
            printf("!!!");
        }
        
        getchar();

        return 0;
    }
    
}

getchar()和Putchar()字符处理函数

getchar()

主要作用:

从输入队列中返回下一个字符.--->ch = getchar();等效于scanf("%c", &ch);

putchar()

主要作用:

打印参数--->putchar(ch);等效于printf("%c", ch);

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/29/2023, 10:59:44 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/29/2023, 10:59:44 PM
 * Description: getchar()函数和putchar()函数的结合使用
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>
# define SPACE ' '

int main(void)
{
    char ch;

    ch = getchar();
    while (ch != '\n')
    {
        if (ch == SPACE)
            putchar(ch);
        else
            putchar(ch + 1);
            /**
             * ch被转为int类型
             * int类型的计算结果被传递给接受一个int类型参数的putchar()
             * 根据最后一个字节确定显示哪个字符
            */
        ch = getchar();
    }

    putchar(ch);
    
    return 0;
}

ctype.h系列字符函数

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/29/2023, 11:07:43 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/29/2023, 11:07:43 PM
 * Description: ctype.h宏文件函数用法
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>
# include<ctype.h>

int main(void)
{
    char ch;

    while ((ch = getchar()) != '\n')
    {
        if (isalpha(ch))
            putchar(ch + 1);
        else
            putchar(ch);
    }
    
    putchar(ch);

    getchar();

    return 0;
}

ctype.h宏文件当中的测试函数:

  • isxdigit() ---> 十六进制字数字符
  • tolower() ---> 如果参数是大写字符,返回小写字符,否则,返回原始参数
  • toupper() ---> 如果参数是小写字符,返回大写字符,否则,返回原始参数

else...if

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/29/2023, 11:30:09 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/29/2023, 11:30:09 PM
 * Description: 电费经典题目
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>
# define RATE1 0.13230
# define RATE2 0.15040
# define RATE3 0.30025
# define RATE4 0.34025
# define BREAK1 360.0
# define BREAK2 468.0
# define BREAK3 720.0
# define BASE1 (RATE1 * BREAK1)
# define BASE2 (BASE1 + (RATE2 * (BREAK2 - BREAK1)))
# define BASE3 (BASE2 + (RATE3 * (BREAK3 - BREAK2)))

int main(void)
{
    double kwh;
    double bill;

    printf("请输入使用的电量(kwh):\n");
    scanf("%lf", &kwh);
    if (kwh <= BREAK1)
        bill = RATE1 * kwh;
    else if (kwh < BREAK2)
        bill = BASE1 + (RATE2 * (kwh - BREAK1)); // 360 - 480 kwh
    else if (kwh < BREAK3)
        bill = BASE2 + (RATE3 * (kwh - BREAK2)); // 468 - 720 kwh
    else
        bill = BREAK3 + (RATE4 * (kwh - BREAK3)); // 超过720 kwh
    printf("用电: %.lf kwh, 电费为: $%1.2f. \n", kwh, bill);

    getchar();

    getchar();
    
    return 0;
}

需要注意,简单的if语句当中else与哪个if匹配:

示例代码:

# include<stdio.h>

int main(void)
{
    int number;
    scanf("%d", &number);

    if (number > 6)
        if (number < 12)
            printf("关闭!");
    else
    /**
     * 在简单的if语句当中,else与最近的if匹配.
     * 除非是用{}括起来的语句才会有层级关系,类似
     * if()
     * {
     *     if()
     *     {
     *         
     *     }
     * }
     * else 
     * {
     *     
     * }
     * 这个当中的else与最外层的if匹配
    */
        printf("继续!\n");

    getchar();

    getchar();

    return 0;
}

变式:

/**
 * @Author: Lucifer
 * @Date: 4/29/2023, 11:43:48 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/29/2023, 11:43:48 PM
 * Description: else匹配的if变式
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    int number;
    scanf("%d", &number);

    if (number > 6)
    {
        if (number < 12)
            printf("关闭!");
    }
    else // 这个else匹配的if是结构体({})外的if
        printf("继续!");
    
    getchar();

    getchar();

    return 0;
}

求约数:

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 11:24:29 AM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 11:24:29 AM
 * Description: 约数算法
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    int num;
    int div;

    printf("输入一个数:\n");
    scanf("%d", &num);
    for ( div = 2; div < num; div++)
        if (num % div == 0)
            printf("%d is divisible by %d\n", num, div);
    /**
     * 算法缺陷:
     * 如果144%2得0,说明2是144的约数.
     * 144/2得72,说明72也是144约数
     * 所以num%div可以获得两个约数
     * 那么实际上不需要循环到143,只需要循环12次即可停止循环
    */
    
    getchar();

    getchar();

    return 0;
}

约束算法优化2:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 11:39:10 AM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 11:39:10 AM
 * Description: 约数算法优化
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    // 测试数只需要到num的平方即可
    int num;
    int div;

    printf("输入一个数:\n");
    scanf("%d", &num);

    for ( div = 2; (div * div) <= num; div++) // 使用等于是因为要注意完全平方数
    /**
     * 使用平方而不是平方根原因
     * 整数乘法比平方根快
    */
    {
        if (num % div == 0)
            printf("%d 是 %d 和 %d 的积.\n", num, div, num / div); // num / div需要注意的是如果num是完全平方数(一个数等于两个相同整数之积)
    }
    
    getchar();

    getchar();

    return 0;
}

约束算法优化3:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 11:39:27 AM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 11:39:27 AM
 * Description: 约束算法优化2
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    int num;
    int div;

    printf("请输入一个数:\n");
    scanf("%d", &num);

    for ( div = 2; (div * div) <= num; div++)
    {
        if (num % div == 0)
        {
            if (div * div != num)
                printf("%d 是 %d 和 %d 的积.\n", num, div, num / div);
            else
                printf("%d 是 %d 的完全平方数.\n", num, div);
        }
        else
            printf("%d 不是 2 的约数\n", num);
    }
    
    getchar();
    
    getchar();

    return 0;
}

素数算法:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 12:08:21 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 12:08:21 PM
 * Description: 素数约数算法
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>
# include<stdbool.h>

int main(void)
{
    unsigned long num; // 待测的数
    unsigned long div; // 可能的约数
    bool isPrime; // 素数标记

    printf("输入一个整数来分析:\n");
    printf("输入q或者quit退出\n");
    while (scanf("%lu", &num) == 1)
    {
        for (div = 2, isPrime = true; (div * div) <= num; div++)
        {
            if (num % div == 0)
            {
                if ((div * div) != num) // 非完全平方数
                    printf("%lu 是 %lu 和 %lu 的约数.\n", num, div, num / div);
                else
                    printf("%lu 是 %lu 的完全平方数.\n", num, div);
                isPrime = false;
            }
            else
            {
                printf("该数不是2的约数.\n");
                break;
            }
            if (isPrime)
                printf("%lu 是素数.\n", num);
            printf("请输入另一个整数来分析:\n");
            printf("输入q或者quit退出\n");
        }
        printf("退出!");
    }
    getchar();

    return 0;
}

条件表达式: ? :

运行逻辑:

判断表达式的true或false.如果为true输出? :之间的内容,如果为false输出:后的内容

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 2:26:36 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 2:26:36 PM
 * Description: 条件运算符在实际中的运用
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>
# define COVERAGE 350

int main(void)
{
    int sq_feet;
    int cans;

    printf("Enter number of square feet to be painted: \n");
    while (scanf("%d", &sq_feet) == 1)
    {
        cans += sq_feet / COVERAGE;
        cans += (sq_feet % COVERAGE == 0) ? 0 : 1; // 先判断sq_feet % COVERAGE的true或false
        printf("You need %d %s of paint.\n", cans, cans == 1 ? "can" : "cans");
        printf("Enter next value (q or quit to quit):\n");
    }

    getchar();

    return 0;
}

辅助循环: Continue 和 Break

continue语句:

作用,跳过本次迭代剩余部分,进入下一轮迭代:

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 2:50:08 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 2:50:08 PM
 * Description: while循环当中的continue语句,当执行到这以后执行的是测试条件
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    const float MIN = 0.0f;
    const float MAX = 100.0f;

    float score;
    float total = 0.0f;
    int n = 0;
    float min = MAX;
    float max = MIN;

    printf("Enter the first score (q to quit):");
    while (scanf("%f", &score) == 1)
    {
        if (score < MIN || score > MAX)
        {
            printf("%0.1f is an invalid value. Try again:", score);
            continue; // 回到while循环的测试条件当中
        }
        printf("Accepting %0.1f:\n", score);
        min = (score < min) ? score : min;
        max = (score > max) ? score : max;
        total += score;
        n++;    
        printf("Enter next score (q to quit):");
    }
    if (n > 0)
    {
        printf("Average of %d scores is %0.1f.\n", n, total / n);
        printf("Low = %0.1f, high = %0.1f\n", min, max);
    }
    else
        printf("No valid scores were entered.\n");

    getchar();

    return 0;
}

在for循环当中执行到continue以后下一个执行的是更新表达式求值,然后在是测试条件:

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 3:09:02 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 3:09:02 PM
 * Description: for循环当中的continue使用
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    int count;
    char ch;

    for (count = 0; count < 10; count++)
    {
        ch = getchar();
        if (ch == '\n')
            continue;
        putchar(ch);
    }

    getchar();

    return 0;
}

break语句:

作用:跳出循环,执行循环体后续的内容

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 3:19:24 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 3:19:24 PM
 * Description: break在循环当中的使用,这是一个计算面积的程序
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    float length, width;

    printf("Enter the length of the rectangle:\n");
    while (scanf("%f", &length) == 1)
    {
        printf("长度 = %0.2f: \n", length);
        printf("输入宽度:\n");
        if (scanf("%f", &width) != 1)
            break;
        printf("宽度: %0.2f: \n", width);
        printf("Area = %0.2f: \n", length * width);
        printf("Enter the length of the rectangle:\n");
    }

    printf("Done.\n");

    getchar();

    return 0;
}

上诉代码的控制语句可以改写成:

while(scanf("%f %f", &length, &width) == 2)
{
    
}

嵌套循环的break只会跳出当前层的循环:

示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 3:29:37 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 3:29:37 PM
 * Description: break在嵌套循环当中的使用
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>

int main(void)
{
    int p, q;

    scanf("%d", &p);
    while (p > 0)
    {
        printf("%d\n", p);
        scanf("%d", &q);
        while (q > 0)
        {
            printf("%d\n", p * q);
            if (q > 100)
                break; // 跳出内层循环
            scanf("%d", &q);
        }
        if (q > 100)
            break; // 跳出外层循环
        scanf("%d", &p);
    }
    
    getchar();

    return 0;
}

多重选择:Switch

switch语句扫描的是标签,该标签可以是单个标签也可以是多个标签:

单标签示例代码:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 4:14:23 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 4:14:23 PM
 * Description: switch在循环当中的时候,单标签形式
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>
# include<ctype.h>

int main(void)
{
    char ch;

    printf("Give me a letter of the alphabet, and I will give ");
    printf("an animal name\n beginning with that letter.\n");
    printf("Please type in a letter; type # to end my act. \n");
    while ((ch = getchar()) != '#')
    {
        if ('\n' == ch) // 尤达表达式
            continue;
        if (islower(ch))
            switch (ch)
            {
            case 'a':
                printf("A \n");
                break;
            case 'b':
                printf("B \n");
                break;
            case 'c':
                printf("C \n");
                break;
            case 'd':
                printf("D \n");
                break;
            case 'e':
                printf("E \n");
                break;
            case 'f':
                printf("F \n");
                break;
            default:
                printf("结束!");
            }
            else
                printf("I recognize only lowercase letters.\n");
            while (getchar() != '\n')
                continue; // 跳过输入行剩余部分
            printf("Please type another letter or a #.\n"); // 循环结束
    }
    printf("Bye!\n");

    getchar();

    return 0;
}

多重标签的使用:

/**
 * @Author: Lucifer
 * @Date: 4/30/2023, 4:31:26 PM
 * @LastEditors: Lucifer
 * @LastEditTime: 4/30/2023, 4:31:26 PM
 * Description: switch多重标签 ---> 统计一个句子当中的字符出现的次数
 * Copyright: Copyright (©)}) 2023 Your Name. All rights reserved.
 */
# include<stdio.h>
# include<ctype.h>

int main(void)
{
    char ch;
    int a_ct, e_ct, i_ct, o_ct, u_ct;

    a_ct = e_ct = i_ct = o_ct = u_ct = 0;

    printf("Enter some text; enter # to quuit.\n");
    // ch = isupper(ch); ---> 转成大写判断,就可以变成单标签
    while ((ch = getchar()) != '#')
    {
        switch (ch)
        {
        case 'a':
        case 'A': a_ct++;
                  break;
        case 'e':
        case 'E': e_ct++;
                  break;
        case 'i':
        case 'I': i_ct++;
                  break;
        case 'o': 
        case 'O': o_ct++;
                  break;
        case 'u':
        case 'U': u_ct++;
                  break;
        default: break;
        } // switch结束
    } // while循环结束

    printf("number of vowels: A E I O U\n");
    printf("        %4d %4d %4d %4d %4d\n", a_ct, e_ct, i_ct, o_ct, u_ct);

    getchar();

    getchar();

    return 0;
}
posted @   俊king  阅读(48)  评论(0)  编辑  收藏  举报
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