循环
让一段代码重复执行，直到满足设定条件为止。
1 递增和递减运算符
自增或自减，改变的是变量本身的值。如int number=6; number++;则执行完后number自变为7。
2 for循环
打印26个英文字母。1
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int main()
{
char ch='a';
int i;
for(i=1; i<=26; i++)
printf("%c ", ch++); //这时char的值可以和int互换
return 0;
}
上面for(…)中，第一个表达式是初始条件；第二个表达式必须是一个逻辑表达式，逻辑终点就是在这里确定，最开始i=1，符合i<=26的条件，循环继续；第三个表达式是循环变量，否则会成为死循环，跳不出来。每个表达式之间用“；”隔开。
也可以在for循环里设置多个变量，如：1
2for(i=1, j=2; i<=5; i++, j+=2) //先i=1，再j=2，再比较i是否大于5，再i++，最后j+=2
printf("%5d", i*j);
3 再谈递增和递减运算符
下面三条语句效果相同：1
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3count = count +1;
count += 1;
++count;
而第三条语句，++放在count后和count前有很大区别，如在下例中：1
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3label a:
int count = 5;
total = ++count + 6; //total = 12 & count = 6
执行完第二条语句时，count自增为6并参与运算然后再赋值到左边，total = 6 + 6。但，1
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3label b:
int count = 5;
total = count++ + 6; //total = 11 & count = 6
它是在这条赋值语句执行完后才自增为6，也就是（total = 5 + 6）后再变成6。可以认为label b中第二条语句等价于：1
2total = count + 6;
++count;
4 再谈for
4.1 前n个自然数的和：
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PS:上面的第9、10行可简化为一行：for(; i<=n; sum += i++);
for()语句，括号内是从左到右执行的。1
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int main()
{
int m=1,n=3;
for(;m<=8;++m)
{
if(m==n)
break;
}
printf("%d",m); //3
return 0;
}
这个自然数和只作为一个例子，求其和可直接：printf(“%ld”, n*(n+1)/2);
4.2 循环内的break语句
上一章中谈到switch时有引入关键字break，它的意思是跳出语句块执行下一条语句。所以在类似for(;;)这样无限死循环中，可用它来跳出，如：1
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8char answer = 0;
for(;;)
{
printf("继续输入吗？(y/n)");
scanf("%c", &answer);
if(tolower(answer) == 'n')
break;
}
这里，如果不输入“n”或“N”，则会一直读取输入，执行循环。
5 嵌套循环
循环内还有循环：1
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int main()
{
int i=1;
for(; i<=6; i++)
{
int j=1,sum=0;
for(; j<=i; sum+=j++);
printf("%d\t%d\n",i,sum);
}
return 0;
}
printf语句位于外循环里面，循环里又有一个for循环，每经历一次外循环，初始化j和sum，然后从1开始一直加到当时的i。
写成下面这种方式更容易理解：1
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int main()
{
int i=1;
for(; i<=6; i++)
{
int j=1,sum=1; //每次外循环都初始化这两个量
printf("1 ");
while(j<i) //另一种循环语句
{
sum+=++j; //sum原值为1，因此除第一次外循环不参与加之外，它从2开始累加，直到加到第i次循环的i
printf("+ %d ",j);
}
printf(" = %d\n",sum);
}
return 0;
}
6 do-while循环
for和while循环都是很检查条件，再执行循环体，也就是如果条件不满足，则一次都不会执行。而do-while语句则至少会执行一次。1
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int main()
{
int num,renum=0;
printf("输入一个正数：");
scanf("%d", &num);
int temp=num;
do{
renum = 10*renum + temp%10; //往左移一位，加入尾数
temp/=10; //往右移一位，这样可以丢掉已经加的尾数
}while(temp);
printf("%d", renum);
return 0;
}
上例将一个正整数反转。
7 continue
它和break一样，可以跳出循环。只continue是跳过本次循环，而break是跳到最近一个外循环。以查寝为例，当查到某个寝室有妹子时，continue跳过这个寝室，查下一个寝室；而break是直接跳过本层，查下一层。
8 小游戏
屏幕显示一串数字，一秒后删除，如果输入正确3次，增加数字长度。下面是原书代码。原作者提示：当代码可以编译时，应该编译它，但不执行，确保每次写的代码都可以编译。1
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106/* Program 4.12 Simple Simon */
int main(void)
{
/* Records if another game is to be played */
char another_game = 'Y';
/* true if correct sequence entered, false otherwise */
int correct = false;
/* Number of sequences entered successfully */
int counter = 0;
int sequence_length = 0; /* Number of digits in a sequence */
time_t seed = 0; /* Seed value for random number sequence */
int number = 0; /* Stores an input digit */
time_t now = 0; /* Stores current time - seed for random values */
int time_taken = 0; /* Time taken for game in seconds */
/* Describe how the game is played */
printf("\nTo play Simple Simon, ");
printf("watch the screen for a sequence of digits.");
printf("\nWatch carefully, as the digits are only displayed"
" for a second! ");
printf("\nThe computer will remove them, and then prompt you ");
printf("to enter the same sequence.");
printf("\nWhen you do, you must put spaces between the digits. \n");
printf("\nGood Luck!\nPress Enter to play\n");
scanf("%c", &another_game);
/* One outer loop iteration is one game */
do
{
correct = true; /* By default indicates correct sequence entered */
counter = 0; /* Initialize count of number of successful tries*/
sequence_length = 2; /* Initial length of a digit sequence */
time_taken = clock(); /* Record current time at start of game */
/* Inner loop continues as long as sequences are entered correctly */
while(correct)
{
/* On every third successful try, increase the sequence length */
sequence_length += counter++%3 == 0;
/* Set seed to be the number of seconds since Jan 1,1970 */
seed = time(NULL);
now = clock(); /* record start time for sequence */
/* Generate a sequence of numbers and display the number */
srand((unsigned int)seed);
int i = 1; /* Initialize the random sequence */
for(; i <= sequence_length; i++)
printf("%d ", rand() % 10); /* Output a random digit */
/* Wait one second */
for( ;clock() - now < CLOCKS_PER_SEC; );
/* Now overwrite the digit sequence */
printf("\r");
int r = 1; /* go to beginning of the line */
for(; r <= sequence_length; r++)
printf(" "); /* Output two spaces */
if(counter == 1) /* Only output message for the first try */
printf("\nNow you enter the sequence - don't forget"
" the spaces\n");
else
printf("\r"); /* Back to the beginning of the line */
/* Check the input sequence of digits against the original */
srand((unsigned int)seed);
int q = 1; /* Restart the random sequence */
for(; q <= sequence_length; q++)
{
scanf("%d", &number); /* Read an input number */
if(number != rand() % 10) /* Compare against random digit */
{
correct = false; /* Incorrect entry */
break; /* No need to check further... */
}
}
printf("%s\n", correct? "Correct!" : "Wrong!");
}
/* Calculate total time to play the game in seconds)*/
time_taken = (clock() - time_taken) / CLOCKS_PER_SEC;
/* Output the game score */
printf("\n\n Your score is %d", --counter * 100 / time_taken);
fflush(stdin);
/* Check if new game required*/
printf("\nDo you want to play again (y/n)? ");
scanf("%c", &another_game);
}while(toupper(another_game) == 'Y');
return 0;
}
9 练习
9.1 乘法表
1 | #include <stdio.h> |
9.2 ASCII码表
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9.3 扩展ASCII码表
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9.4 猜迷游戏
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