C语言基础知识-程序流程结构

　　　　　　　　　　　　C语言基础知识-程序流程结构

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　作者：尹正杰

版权声明：原创作品，谢绝转载！否则将追究法律责任。

一.概述

C语言支持最基本的三种程序运行结构：顺序结构，选择结构，循环结构。
　　顺序结构：程序按顺序执行，不发生跳转。
　　选择结构：依据是否满足条件，有选择的执行相应功能。
　　循环结构：依据条件是否满足，循环多次执行某段代码。

二.选择结构

1>.if语句

[[e-mail] /yinzhengjie/code/day002]# cat if_demo.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>

int main(void)
{
    int a = ;
    int b = ;
    if (a > b)
    {
        printf("a > b\n");
    }

    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o if_demo if_demo.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./if_demo
a > b
[[e-mail] /yinzhengjie/code/day002]# 

2>.if ... else语句

[[e-mail] /yinzhengjie/code/day002]# cat if_demo2.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>

int main(void)
{
    int a = ;
    int b = ;
    if (a > b)
    {
        printf("a > b\n");
    }
    else
    {
        printf("a < b\n");
    }

    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o if_demo2 if_demo2.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./if_demo2
a < b
[[e-mail] /yinzhengjie/code/day002]# 

3>.if ... else if ...else语句

[[e-mail] /yinzhengjie/code/day002]# cat if_demo3.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>

int main(void)
{
    int a = ;
    int b = ;
    if (a > b)
    {
        printf("a > b\n");
    }
    else if(a == b)
    {
        printf("a == b\n");
    }
    else
    {
        printf("a < b\n");
    }

    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o if_demo3 if_demo3.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./if_demo3
a == b
[[e-mail] /yinzhengjie/code/day002]#

4>.三目运算符【其实其内部判断条件和if相似，语法结构简单明了】

[[e-mail] /yinzhengjie/code/day002]# cat if_demo4.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>

int main(void)
{
    int a = ;
    int b = ;
    int max;

    if (a > b)
    {
        max = a;
    }
    else
    {
        max = b;
    }
    printf("s1 = %d\n",max);

    a = ;
    b = ;
    max = (a > b ? a:b);　　　　　　//上面一大堆代码，我们仅仅用三目运算符一行简写。三目运算符格式为"表达式？选项1[表达式]:选项2"，即如果表达式为真，选择选项1的结果，如果为假则选择新选项2。
    printf("s2 = %d\n",max);

    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o if_demo4 if_demo4.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./if_demo4
s1 =
s2 =
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# 

5>.switch语句【注意：if 条件语句执行效率差，switch条件语句执行效率相对较高，但是if可以判断一个区间，而switch则只能用来判断一个值】

[[e-mail] /yinzhengjie/code/day002]# cat switch_demo.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>

int main(void)
{
    char c;
    c = getchar();    //注意该方法只会接收第一个字符哟~比如你输入的是100，它只会接收第一个字符“1”

    switch(c)    //参数只能是整型变量
    {
    case '':
        printf("OK\n");
        break;    //switch遇到break就中断了
    case '':
        printf("not OK\n");
        break;
    default:    //如果上面的条件都不满足，那么执行default
        printf("are u OK?\n");
    }

    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o switch_demo switch_demo.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./switch_demo 

OK
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./switch_demo 

not OK
[[e-mail] /yinzhengjie/code/day002]# ./switch_demo 

are u OK?
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./switch_demo 

are u OK?
[[e-mail] /yinzhengjie/code/day002]# 

三.循环结构

1>.while语句

[[e-mail] /yinzhengjie/code/day002]# cat while_demo.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int a = ;
    while(a < )
    {
        printf("a = %d\n",a);
        a++;
        system("sleep 0.5");
    }
    printf("程序执行完毕~\n");

    return EXIT_SUCCESS;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o while_demo while_demo.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./while_demo
a =
a =
a =
a =
a =
a =
a =
a =
a =
程序执行完毕~
[[e-mail] /yinzhengjie/code/day002]# 

2>.do ... while语句

[[e-mail] /yinzhengjie/code/day002]# cat narcissus.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int index = ;
    do
    {
        int one = , ten=, hundred=;  //将一个三位数分解个位，十位，百位

        hundred = index / ;        //百位

        ten = index /  % ;        //十位

        one = index % ;             //个位

        if (hundred * hundred * hundred + ten * ten * ten + one * one * one == index)    //各个位数的立方和等于该数本身，那么它就是一个水仙花
        {
            printf("%d是水仙花数\n",index);
        }

        index ++;
    }while(index < );

    return EXIT_SUCCESS;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o narcissus narcissus.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./narcissus
153是水仙花数
370是水仙花数
371是水仙花数
407是水仙花数
[[e-mail] /yinzhengjie/code/day002]# 

3>.for循环

[[e-mail] /yinzhengjie/code/day002]# cat for_demo.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int index = ;
    int sum = ;

    for(index = ;index<=;index++)    //计算0-100之间所有数字的是总和
    {
        sum += index;
    }

    printf("sum = %d\n",sum);

    return EXIT_SUCCESS;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o for_demo for_demo.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./for_demo
sum =
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# 

[[e-mail] /yinzhengjie/code/day002]# cat for_demo2.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    for(int i = ;i<;i++)
    {
        int one = , ten=, hundred=;  //将一个三位数分解个位，十位，百位

        hundred = i / ;        //百位

        ten = i /  % ;        //十位

        one = i % ;             //个位

        if (hundred * hundred * hundred + ten * ten * ten + one * one * one == i)    //各个位数的立方和等于该数本身，那么它就是一个水仙花
        {
            printf("%d是水仙花数\n",i);
        }
    }
    return EXIT_SUCCESS;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o for_demo2  for_demo2.c -std=c99         #注意，在Linux系统我们编译for循环代码时需要指定"-std=c99"，否则会报错"error: ‘for’ loop initial declarations are only allowed in C99 mode"
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./for_demo2
153是水仙花数
370是水仙花数
371是水仙花数
407是水仙花数
[[e-mail] /yinzhengjie/code/day002]# 

使用for循环打印三位数存在的水仙花数字

4>.嵌套循环（循环之间可以相互嵌套）

[[e-mail] /yinzhengjie/code/day002]# cat for_99.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int i,j;　　　 #我们提前声明了变量i和j，如果我们在这里不声明直接在for循环里面声明也是可以的，只不过在Linux操作系统编译时，我们需要指定std的库为c99,默认使用的是c90库。否则会报错"error: ‘for’ loop initial declarations are only allowed in C99 mode"
    for(i = ; i <= ; i++)
    {
        for(j=; j<=i; j++)
        {
            printf("%d x %d = %d\t",i,j,i * j);
        }
        printf("\n");
    }
    return EXIT_SUCCESS;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o for_99 for_99.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./for_99
x  =
x  =      x  =
x  =      x  =      x  =
x  =      x  =      x  =      x  =
x  =      x  =      x  =      x  =      x  =
x  =      x  =      x  =      x  =      x  =      x  =
x  =      x  =      x  =      x  =      x  =      x  =      x  =
x  =      x  =      x  =      x  =      x  =      x  =      x  =      x  =
x  =      x  =      x  =      x  =      x  =      x  =      x  =      x  =      x  =
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]#

 5>.循环语句练习一(猜年龄)

[[e-mail] /yinzhengjie/code/day002]# cat guess_age.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>
#include <time.h>

int main(void)
{
    srand((unsigned int)time(NULL));        //加入随机数种子
    int num = rand()% + ;
    int value;
    for(;;)
    {
        printf("猜猜看我年龄多大(请输入一个整数)>>>: ");
        scanf("%d",&value);
        if(value > num)
        {
            printf("我有那么老吗?\n");
        }
        else if(value < num)
        {
            printf("我看起来这么小吗?\n");
        }
        else
        {
            printf("太棒了，你猜对啦~\n");
            break;
        }
    }
    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o guess_age guess_age.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./guess_age
猜猜看我年龄多大(请输入一个整数)>>>:
我看起来这么小吗?
猜猜看我年龄多大(请输入一个整数)>>>:
我有那么老吗?
猜猜看我年龄多大(请输入一个整数)>>>:
我有那么老吗?
猜猜看我年龄多大(请输入一个整数)>>>:
我看起来这么小吗?
猜猜看我年龄多大(请输入一个整数)>>>:
我有那么老吗?
猜猜看我年龄多大(请输入一个整数)>>>:
我看起来这么小吗?
猜猜看我年龄多大(请输入一个整数)>>>:
太棒了，你猜对啦~
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# 

 6>.循环语句练习二(打印等腰三角形)

[[e-mail] /yinzhengjie/code/day002]# cat isosceles_triangle.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>

int main(void)
{
    int row;
    printf("请输入要打印等腰三角形的行数:>>> ");
    scanf("%d",&row);
    for (int i = ;i <= row;i++)
    {
        for (int j =;j <= row - i;j++)
        {
            printf(" ");
        }

        for (int k = ;k <= i *  -;k++)
        {
            printf("*");
        }
        printf("\n");
    }
    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o isosceles_triangle isosceles_triangle.c -std=c99
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./isosceles_triangle
请输入要打印等腰三角形的行数:>>>
  *
 ***
*****
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./isosceles_triangle
请输入要打印等腰三角形的行数:>>>
         *
        ***
       *****
      *******
     *********
    ***********
   *************
  ***************
 *****************
*******************
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# 

四.跳转语句break和contiune语句

1>.break语句

　　在switch条件语句和循环语句中都可以使用break语句：
　　　　当它出现在switch条件语句时，作用是终止某个case并跳出switch结构。
　　　　当它出现在循环语句中，作用是跳出当前内循环语句，执行后面的代码。
　　　　当它出现嵌套循环语句中，跳出最近的内层循环语句，执行后面的代码。

[[e-mail] /yinzhengjie/code/day002]# cat do_while.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int a = ;
    do
    {
        a++;
        if (a == )
        {
            break;
        }
    }while(a);    //需要注意的是，尽管没有上面的if条件判断语句，该循环并非死循环，只是执行的次数较多而已，因为a是一个有符号int类型的数字，而int类型是有上限的

    printf("%d\n",a);

    return EXIT_SUCCESS;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o do_while do_while.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./do_while 

[[e-mail] /yinzhengjie/code/day002]# 

[[e-mail] /yinzhengjie/code/day002]# cat do_while.c

2>.continue语句

　　在循环语句中，如果希望立即终止本次循环，并执行下一次循环，此时就需要使用continue语句。

[[e-mail] /yinzhengjie/code/day002]# cat continue_demo.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int index = ;
    while (index < )
    {
        index++;
        if(index %  ==  || index %  ==  || index /  == )    //过滤掉带7和7的倍数的数字
        {
            continue;
        }
        printf("数字:%d\n",index);
    }

    return EXIT_SUCCESS;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o continue_demo continue_demo.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./continue_demo
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
数字:
[[e-mail] /yinzhengjie/code/day002]# 

[[e-mail] /yinzhengjie/code/day002]# cat continue_demo.c #过滤掉带7和7的倍数的数字

 3>.goto语句（无条件跳转，尽量少用）

[[e-mail] /yinzhengjie/code/day002]# cat goto_demo.c
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:[e-mail]
*/

#include <stdio.h>

int main(void)
{
    goto END;        //无条件跳转到END的标识
    printf("第一行字母:a\n");

END:            //我们定义的END标识,需要注意的是，这里是冒号(":")，而非分号(“;”)哟~
    printf("第二行字母:A\n");

    return ;
}
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# gcc -o goto_demo goto_demo.c
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]# ./goto_demo
第二行字母:A
[[e-mail] /yinzhengjie/code/day002]#
[[e-mail] /yinzhengjie/code/day002]#