1，if语句
1 #include <stdio.h> 2 3 /* 4 *if (expr) 5 * stat 6 *else if (expr) 7 * stat 8 *else 9 * stat 10 * 11 *expr: expression 12 *stat: statement -> expr; | { ... } 13 */ 14 15 #define EPISILON 0.0000001 16 17 int main(void) 18 { 19 double d = 5.0; 20 21 // 对浮点数进行判断, 最好给一个判定范围 22 // 而不是直接使用 == 23 if (5.0 - EPISILON < d && d < 5.0 + EPISILON) 24 { 25 printf("d == 5.0 "); 26 } 27 28 unsigned u = -1; 29 int i = -1; 30 if (u > 0) 31 { 32 printf("u > 0 ! "); 33 } 34 35 if (i > 0) 36 printf("i > 0 @ "); 37 else 38 printf("i < 0 ! "); 39 40 int a; 41 printf("pls input a num: "); 42 scanf("%d", &a); 43 if (a > 5) 44 { 45 printf("the num you inputed is bigger than 5! "); 46 } 47 else if (a < 3) 48 { 49 printf("the num you inputed if less than 3! "); 50 } 51 else 52 { 53 printf("the num is 3 or 4 or 5. "); 54 } 55 56 return 0; 57 }
结果：
d == 5.0
u > 0 !
i < 0 !
pls input a num: 3
the num is 3 or 4 or 5.
2，for语句
1 #include <stdio.h> 2 3 /* 4 *for (init_expr; cond_expr; update_expr) 5 * stat 6 * 7 *--> init_expr 8 * --> cond_expr(if true,...) 9 * --> stat 10 * --> updata_expr 11 * --> cond_expr ... 12 */ 13 14 int main(void) 15 { 16 int i; 17 18 for (i = 0; i < 5; i++) 19 printf("i = %d ", i); 20 21 printf("for loop over. after for loop: "); 22 23 printf("i = %d ", i); 24 25 return 0; 26 }
结果：
i = 0
i = 1
i = 2
i = 3
i = 4
for loop over. after for loop:
i = 5
3，while ....do 语句
1 #include <stdio.h> 2 3 int main(void) 4 { 5 int i; 6 7 i = 0; 8 while (i < 5) 9 { 10 printf("i = %d ", i); 11 i++; 12 } 13 14 printf("after while, i = %d ", i); 15 16 // 无论如何会执行一次. 17 do { 18 printf("i = %d ", i); 19 i++; 20 } while (i < 5); 21 22 printf("after do while, i = %d ", i); 23 24 return 0; 25 }
结果：
i = 0
i = 1
i = 2
i = 3
i = 4
after while, i = 5
i = 5
after do while, i = 6
4，continue和break的区别
1 #include <stdio.h> 2 3 // continue 结束本次循环 4 // break 跳出本层循环 5 int main(void) 6 { 7 int i, j; 8 9 for (i = 0; i < 10; i++) 10 { 11 if (i == 3) 12 continue; 13 14 j = 0; 15 while (1) 16 { 17 printf("%d ", j); 18 if (j >= i) 19 break; 20 j++; 21 } 22 putchar(' '); 23 } 24 25 return 0; 26 }
结果：
0
0 1
0 1 2
0 1 2 3 4
0 1 2 3 4 5
0 1 2 3 4 5 6
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8 9
少了0 1 2 3这一排就是continue的作用
5，goto跳转
1 #include <stdio.h> 2 3 int main(void) 4 { 5 int i = 0; 6 7 labeli: 8 9 printf("i = %d ", i); 10 i++; 11 if (i < 5) 12 goto labeli; 13 14 return 0; 15 }
结果：goto是无条件跳转
i = 0
i = 1
i = 2
i = 3
i = 4
6，swtich语句
1 #include <stdio.h> 2 3 int main(void) 4 { 5 char i = 0; 6 7 printf("pls input a char: "); 8 scanf("%c", &i); 9 10 printf("i = %d ", i); 11 switch (i) 12 { 13 case 'a': 14 case 'A': 15 printf("you inputed a or A. "); 16 break; 17 case 'b': 18 case 'B': 19 printf("you inputed b or B. "); 20 break; 21 case 'c': 22 case 'C': 23 printf("you inputed c or C. "); 24 break; 25 default: 26 break; 27 } 28 29 return 0; 30 }
结果：多分支选择语句
pls input a char: a
i = 97
you inputed a or A.
7，输出缓存区效果
1 #include <stdio.h> 2 3 /* 4 * 0 1 2 5 * stdin, stdout, stderr 6 * */ 7 8 /* 9 *输出缓冲区的刷新条件: 10 *1. 缓冲区满 11 *2. 遇到' ' 12 *3. 强制刷新(fflush(stdout)) 13 */ 14 15 int main(void) 16 { 17 printf("hello"); 18 19 fflush(stdout); 20 21 while (1) 22 { 23 sleep(1); 24 } 25 26 return 0; 27 }
结果：如果没有fflush(stdout);语句，则hello不会显示出来，如果变成printf("hello ");hello就可显示了，也就是上面的printf有问题的话，下面要刷新
8，scanf输入以及输入缓存区效果
1 #include <stdio.h> 2 3 /* 4 *scanf 根据格式去标准输入缓冲区取字符/字符串 5 *如果该字符/字符串不符合scanf 参数指定的格式 6 *则scanf 直接返回(不取东西) 7 *scanf的返回值代表取的参数个数. 8 *如果scanf的返回值为0, 代表scanf没有取到东西. 9 */ 10 int main(void) 11 { 12 char ch; 13 int a; 14 15 printf("pls input a int: "); 16 scanf("%d", &a); 17 printf("a = %d ", a); 18 19 printf("------------ "); 20 21 /* 清空标准输入缓冲区 */ 22 while (getchar() != ' ') 23 ; 24 25 printf("pls input a char: "); 26 scanf("%c", &ch); 27 printf("ch = %c ", ch); 28 29 return 0; 30 }
结果：如果没有清空，就无法输入了。对于下面只接受一个是因为下面scanf只允许接受一个字符
有清空
pls input a int: 54
a = 54
------------
pls input a char: 56
ch = 5
无清空
pls input a int: 54
a = 54
------------
pls input a char: ch =
9，输入输出简单应用，选择
1 #include <stdio.h> 2 3 int main(void) 4 { 5 int quit = 0; 6 int op; 7 8 int ret; 9 10 printf("欢迎来到xxx系统. "); 11 while (!quit) 12 { 13 retry: 14 printf("==> 1. a 操作. "); 15 printf("==> 2. b 操作. "); 16 printf("==> 3. c 操作. "); 17 printf("==> 4. d 操作. "); 18 printf("==> 5. 退出. "); 19 printf("请选择操作[1 - 5]: "); 20 21 ret = scanf("%d", &op); 22 /* 清空标准输入缓冲区 */ 23 while (getchar() != ' ') 24 ; 25 if (ret == 0) 26 { 27 printf("姿势不对, 再试一次. "); 28 goto retry; 29 } 30 31 switch (op) 32 { 33 case 1: 34 printf("----------a------------ "); 35 break; 36 case 2: 37 printf("----------b------------ "); 38 break; 39 case 3: 40 printf("----------c------------ "); 41 break; 42 case 4: 43 printf("----------d------------ "); 44 break; 45 case 5: 46 quit = 1; 47 break; 48 default: 49 break; 50 } 51 } 52 53 return 0; 54 }
结果：
欢迎来到xxx系统.
==> 1. a 操作.
==> 2. b 操作.
==> 3. c 操作.
==> 4. d 操作.
==> 5. 退出.
请选择操作[1 - 5]: 1
----------a------------
==> 1. a 操作.
==> 2. b 操作.
==> 3. c 操作.
==> 4. d 操作.
==> 5. 退出.
请选择操作[1 - 5]: 2
----------b------------
10，随机数的产生
1 #include <stdio.h> 2 3 int main(void) 4 { 5 int i; 6 7 int sec; 8 9 sec = time(NULL); 10 printf("1970 --> now has %d sec. ", sec); 11 12 // 给 rand 做一个随机数种子 13 srand(sec); 14 15 i = rand() % 100; 16 printf("i = %d ", i); 17 18 i = rand() % 100; 19 printf("i = %d ", i); 20 21 return 0; 22 }
结果：time函数是记载时间1970年1月1日00:00:00（称为UNIX系统的Epoch时间）到当前时刻的秒数
再用时间做随机数种子，因为每运行一次，时间改变一次，每次随机就不一样了
1970 --> now has 1377135061 sec.
i = 35
i = 49