Control
控制语句：循环，分支，跳转
Loop
while
while (expression)
statementwhile + scanf，循环输入
// power.c -- raises numbers to integer powers
#include <stdio.h>
double power(double n, int p); // ANSI prototype
int main(void)
{
double x, xpow;
int exp;
printf("Enter a number and the positive integer power");
printf(" to which\nthe number will be raised. Enter q");
printf(" to quit.\n");
while (scanf("%lf%d", &x, &exp) == 2)
{
xpow = power(x,exp); // function call
printf("%.3g to the power %d is %.5g\n", x, exp, xpow);
printf("Enter next pair of numbers or q to quit.\n");
}
printf("Hope you enjoyed this power trip -- bye!\n");
return 0;
}
double power(double n, int p) // function definition
{
double pow = 1;
int i;
for (i = 1; i <= p; i++)
pow *= n;
return pow; // return the value of pow
}
(base) kimshan@MacBook-Pro output % ./"power"
Enter a number and the positive integer power to which
the number will be raised. Enter q to quit.
1 2
1 to the power 2 is 1
Enter next pair of numbers or q to quit.
1.1 2
1.1 to the power 2 is 1.21
Enter next pair of numbers or q to quit.
100 2
100 to the power 2 is 10000
Enter next pair of numbers or q to quit.
100.1 2
100 to the power 2 is 10020
Enter next pair of numbers or q to quit.
100 11.1
100 to the power 11 is 1e+22
Enter next pair of numbers or q to quit.
q
Hope you enjoyed this power trip -- bye!while + scanf，输入一个字符串
// cypher2.c -- alters input, preserving non-letters
#include <stdio.h>
#include <ctype.h> // for isalpha()
int main(void)
{
char ch;
//while ((ch = getchar()) != '\n')
while ((ch = getchar()) != EOF)
{
if (isalpha(ch)) // if a letter,
putchar(ch + 1); // display next letter
else // otherwise,
putchar(ch); // display as is
}
putchar(ch); // display the newline
return 0;
}
(base) kimshan@MacBook-Pro output % ./"cypher2"
ABCDabcd
BCDEbcdedo while
do
statement
while(expression);for
for(init; condition; update)
statement等价于
init
for(;condition;)
{
statement
update
}可以省略init，condition，update，statement任意位置
另外可以用逗号插入多个内容在一个语句里：for(int i=1,j=2; i+j<100; i++,j++)
Branch
if, else
Demo
// electric.c -- calculates electric bill
#include <stdio.h>
#define RATE1 0.13230 // rate for first 360 kwh
#define RATE2 0.15040 // rate for next 108 kwh
#define RATE3 0.30025 // rate for next 252 kwh
#define RATE4 0.34025 // rate for over 720 kwh
#define BREAK1 360.0 // first breakpoint for rates
#define BREAK2 468.0 // second breakpoint for rates
#define BREAK3 720.0 // third breakpoint for rates
#define BASE1 (RATE1 * BREAK1)
// cost for 360 kwh
#define BASE2 (BASE1 + (RATE2 * (BREAK2 - BREAK1)))
// cost for 468 kwh
#define BASE3 (BASE1 + BASE2 + (RATE3 *(BREAK3 - BREAK2)))
//cost for 720 kwh
int main(void)
{
double kwh; // kilowatt-hours used
double bill; // charges
printf("Please enter the kwh used.\n");
scanf("%lf", &kwh); // %lf for type double
if (kwh <= BREAK1)
bill = RATE1 * kwh;
else if (kwh <= BREAK2) // kwh between 360 and 468
bill = BASE1 + (RATE2 * (kwh - BREAK1));
else if (kwh <= BREAK3) // kwh betweent 468 and 720
bill = BASE2 + (RATE3 * (kwh - BREAK2));
else // kwh above 680
bill = BASE3 + (RATE4 * (kwh - BREAK3));
printf("The charge for %.1f kwh is $%1.2f.\n", kwh, bill);
return 0;
}
?:（条件运算符）
if(condition)
statement1
else
statement2等价于
condition ? statement1 : statement2注意， Rust 里边的 if else 是表达式，但是C 里边 if else 并不是表达式
#include <stdio.h>
int main()
{
int a = 100;
int b = a == 100 ? a + 1 : a + 2;
int c = (if (a == 100) a + 1 else a + 2); // wrong!
return 0;
}break, continue
break和 continue 都是针对本轮循环
switch
本质就是 if else
Demo
/* animals.c -- uses a switch statement */
#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\nbeginning with that letter.\n");
printf("Please type in a letter; type # to end my act.\n");
while ((ch = getchar()) != '#')
{
if('\n' == ch)
continue;
if (islower(ch)) /* lowercase only */
switch (ch)
{
case 'a' :
printf("argali, a wild sheep of Asia\n");
break;
case 'b' :
printf("babirusa, a wild pig of Malay\n");
break;
case 'c' :
printf("coati, racoonlike mammal\n");
break;
case 'd' :
printf("desman, aquatic, molelike critter\n");
break;
case 'e' :
printf("echidna, the spiny anteater\n");
break;
case 'f' :
printf("fisher, brownish marten\n");
break;
default :
printf("That's a stumper!\n");
} /* end of switch */
else
printf("I recognize only lowercase letters.\n");
while (getchar() != '\n')
continue; /* skip rest of input line */
printf("Please type another letter or a #.\n");
} /* while loop end */
printf("Bye!\n");
return 0;
}
如果没有 break 就会进入下一个代码块，可以利用这个特性，制作多重标签
Demo
// vowels.c -- uses multiple labels
#include <stdio.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 quit.\n");
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;
} // end of switch
} // while loop end
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);
return 0;
}
// (base) kimshan@MacBook-Pro output % ./"vowels"
// Enter some text; enter # to quit.
// A
// a
// #
// number of vowels: A E I O U
// 2 0 0Jump
goto
唯一的 goto 使用场景：跳出多重循环
#include <stdio.h>
int main()
{
for (int i = 0; i < 3; ++i)
{
for (int j = 0; j < 3; ++j)
{
for (int k = 0; k < 3; ++k)
{
printf("i: %d, j: %d, k: %d\n", i, j, k);
// 假设当 i == 1, j == 2, k == 2 时，我们需要跳出所有循环
if (i == 1 && j == 2 && k == 2)
{
goto end_of_loops;
}
}
}
}
end_of_loops:
printf("跳出所有循环。\n");
test:
printf("不调用也会 print\n");
return 0;
}
// (base) kimshan@MacBook-Pro output % ./"a"
// i: 0, j: 0, k: 0
// i: 0, j: 0, k: 1
// i: 0, j: 0, k: 2
// i: 0, j: 1, k: 0
// i: 0, j: 1, k: 1
// i: 0, j: 1, k: 2
// i: 0, j: 2, k: 0
// i: 0, j: 2, k: 1
// i: 0, j: 2, k: 2
// i: 1, j: 0, k: 0
// i: 1, j: 0, k: 1
// i: 1, j: 0, k: 2
// i: 1, j: 1, k: 0
// i: 1, j: 1, k: 1
// i: 1, j: 1, k: 2
// i: 1, j: 2, k: 0
// i: 1, j: 2, k: 1
// i: 1, j: 2, k: 2
// 跳出所有循环。
// 不调用也会 print最后更新于
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