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
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #include <stdio.h> int main(void) { int a = 100; int b = 20; if (a > b) { printf("a > b\n"); } return 0; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo if_demo.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo a > b [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#
2>.if ... else语句
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo2.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #include <stdio.h> int main(void) { int a = 100; int b = 200; if (a > b) { printf("a > b\n"); } else { printf("a < b\n"); } return 0; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo2 if_demo2.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo2 a < b [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#
3>.if ... else if ...else语句
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo3.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #include <stdio.h> int main(void) { int a = 300; int b = 300; if (a > b) { printf("a > b\n"); } else if(a == b) { printf("a == b\n"); } else { printf("a < b\n"); } return 0; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo3 if_demo3.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo3 a == b [root@node101.yinzhengjie.org.cn /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】
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo4.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #include <stdio.h> int main(void) { int a = 100; int b = 300; int max; if (a > b) { max = a; } else { max = b; } printf("s1 = %d\n",max); a = 10; b = 20; 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 0; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo4 if_demo4.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo4 s1 = 300 s2 = 20 [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /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】
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat switch_demo.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #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 '1': printf("OK\n"); break; //switch遇到break就中断了 case '2': 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 0; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o switch_demo switch_demo.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 1 OK [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 2 not OK [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 3 are u OK? [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 5 are u OK? [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#
三.循环结构
1>.while语句
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat while_demo.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #define _CRT_SECURE_NO_WARNINGS #include <stdio.h> #include <stdlib.h> #include <string.h> int main(void) { int a = 1; while(a < 10) { printf("a = %d\n",a); a++; system("sleep 0.5"); } printf("程序执行完毕~\n"); return EXIT_SUCCESS; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o while_demo while_demo.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./while_demo a = 1 a = 2 a = 3 a = 4 a = 5 a = 6 a = 7 a = 8 a = 9 程序执行完毕~ [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#
2>.do ... while语句
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat narcissus.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #define _CRT_SECURE_NO_WARNINGS #include <stdio.h> #include <stdlib.h> #include <string.h> int main(void) { int index = 100; do { int one = 0, ten=0, hundred=0; //将一个三位数分解个位，十位，百位 hundred = index / 100; //百位 ten = index / 10 % 10; //十位 one = index % 10; //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 < 1000); return EXIT_SUCCESS; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o narcissus narcissus.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /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 [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#
3>.for循环
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat for_demo.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #define _CRT_SECURE_NO_WARNINGS #include <stdio.h> #include <stdlib.h> #include <string.h> int main(void) { int index = 100; int sum = 0; for(index = 0;index<=100;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; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o for_demo for_demo.c [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./for_demo sum = 5050 [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat for_demo2.c /* @author :yinzhengjie blog:http://www.cnblogs.com/yinzhengjie EMAIL:y1053419035@qq.com */ #define _CRT_SECURE_NO_WARNINGS #include <stdio.h> #include <stdlib.h> #include <string.h> int main(void) { for(int i = 100;i<1000;i++) { int one = 0, ten=0, hundred=0; //将一个三位数分解个位，十位，百位 hundred = i / 100; //百位 ten = i / 10 % 10; //十位 one = i % 10; //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; } [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /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" [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# [root@node101.yinzhengjie.org.cn /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 [root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#