《The C Programming Language, 2nd Edition》K&R Chapter 3 - Control Flow 笔记

控制流

if-else语句中，else总与最近的前一个没有else的if匹配。
if语句嵌套时，建议使用{}，否则容易产生歧义，例如：

if (n > 0)
    for (i = 0; i < n; i++)
        if (s[i] > 0) {
            printf("...");
            return i;
        }
else /* WRONG */
    printf("error -- n is negative\n");

else-if语句末尾else部分是可选的。

switch语句的default分支也是可选的。

for语句的表达式都是可选的。如for ( ; ; );无限循环。

goto语句用法如下：

    for ( ... )
        for ( ... ) {
            ...
            if (disaster)
                goto error;
        }
    ...
error:
/* clean up the mess */

goto语句标号(label)的作用域是所处的函数。

运算符

逗号运算符从左向右求值。分隔实参的逗号，分隔声明中变量的逗号都不是逗号运算符，以下为逗号运算符：

for (i = 0, j = strlen(s) - 1; i < j; i++, j--)
    c = s[i], s[i] = s[j], s[j] = c;

例程

二分查找(折半查找)

/* binsearch: find x in v[0] <= v[1] <= ... <= v[n-1] */
int binsearch(int x, int v[], int n)
{
    int low, high, mid;

    low = 0;
    high = n - 1;
    while (low <= high) {
        mid = (low+high)/2;
        if (x < v[mid])
            high = mid - 1;
        else if (x > v[mid])
            low = mid + 1;
        else /* found match */
            return mid;
    }
    return -1; /* no match */
}

统计数字、空白符及其它字符

#include <stdio.h>

main() /* count digits, white space, others */
{
    int c, i, nwhite, nother, ndigit[10];

    nwhite = nother = 0;
    for (i = 0; i < 10; i++)
        ndigit[i] = 0;
    while ((c = getchar()) != EOF) {
        switch (c) {
        case '0': case '1': case '2': case '3': case '4':
        case '5': case '6': case '7': case '8': case '9':
            ndigit[c-'0']++;
            break;
        case ' ':
        case '\n':
        case '\t':
            nwhite++;
            break;
        default:
            nother++;
            break;
        }
    }
    printf("digits =");
    for (i = 0; i < 10; i++)
        printf(" %d", ndigit[i]);
    printf(", white space = %d, other = %d\n",
        nwhite, nother);
    return 0;
}

atoi函数(将字符串转换为整数)

#include <ctype.h>

/* atoi: convert s to integer; version 2 */
int atoi(char s[])
{
    int i, n, sign;

    for (i = 0; isspace(s[i]); i++) /* skip white space */
        ;
    sign = (s[i] == '-') ? -1 : 1;
    if (s[i] == '+' || s[i] == '-') /* skip sign */
        i++;
    for (n = 0; isdigit(s[i]); i++)
        n = 10 * n + (s[i] - '0');
    return sign * n;
}

希尔(shell)排序(先比较较远的元素，最后比较相邻的元素)

/* shellsort: sort v[0]...v[n-1] into increasing order */
void shellsort(int v[], int n)
{
    int gap, i, j, temp;
    
    for (gap = n/2; gap > 0; gap /= 2)
        for (i = gap; i < n; i++)
            for (j=i-gap; j>=0 && v[j]>v[j+gap]; j-=gap) {
                temp = v[j];
                v[j] = v[j+gap];
                v[j+gap] = temp;
            }
}

反转字符串中的字符

#include <string.h>

/* reverse: reverse string s in place */
void reverse(char s[])
{
    int c, i, j;

    for (i = 0, j = strlen(s)-1; i < j; i++, j--) {
        c = s[i];
        s[i] = s[j];
        s[j] = c;
    }
}

itoa函数(将整数转换为字符串)

/* itoa: convert n to characters in s */
void itoa(int n, char s[])
{
    int i, sign;

    if ((sign = n) < 0) /* record sign */
        n = -n; /* make n positive */
    i = 0;
    do { /* generate digits in reverse order */
        s[i++] = n % 10 + '0'; /* get next digit */
    } while ((n /= 10) > 0); /* delete it */
    if (sign < 0)
        s[i++] = '-';
    s[i] = '\0';
    reverse(s);
}

trim函数(删除字符串末尾的空白字符)

/* trim: remove trailing blanks, tabs, newlines */
int trim(char s[])
{
    int n;

    for (n = strlen(s)-1; n >= 0; n--)
        if (s[n] != ' ' && s[n] != '\t' && s[n] != '\n')
            break;
    s[n+1] = '\0';
    return n;
}

Fly_Fire1000的博客