C5 - 5.6 Pointers to Pointers

下面是一个经典例子

writing a program that will sort a set of text lines into alphabetic order, a
stripped-down version of the UNIX program sort.

 

自己做的答案

除了main函数，还有函数 pzcmp, pzprint, pzsort 还有 getline

 

我觉得pzcmp和getline非常重要，需要特别掌握。

 

其中，getline是逐行输入，原型为 int getline(char *s, int lim), 把每一行包括末尾的回车一起读到字符数组s中，返回实际读入的字节数。EOF不读。lim为一次最多能够读入的字节数。

int getline(char *s, int lim){
    int nr = 0;
    for(; lim > 0 && (*s = getchar()) != EOF && *s != '\n'; lim--, s++)
        nr++;
    if(*s == '\n')
        nr++;
    return nr;
}

 

pzcmp是比较从指定位置为起点到以回车符为结尾的两个字符串。

int pzcmp(char **lp, int i, int j){
    char *s1 = lp[i], *s2 = lp[j];
    while(*s1 == *s2){
        if(*s1 == '\n')
            return 0;
        s1++;
        s2++;
    }
    if(*s1 == '\n')
        return 0 - *s2;
    if(*s2 == '\n')
        return *s1;
    return *s1 - *s2;
}

 

整个程序的源代码：

#include<stdio.h>
int pzcmp(char **lp, int i, int j){
    char *s1 = lp[i], *s2 = lp[j];
    while(*s1 == *s2){
        if(*s1 == '\n')
            return 0;
        s1++;
        s2++;
    }
    if(*s1 == '\n')
        return 0 - *s2;
    if(*s2 == '\n')
        return *s1;
    return *s1 - *s2;
}
void pzprint(const char *s){
    while(putchar(*s), *s++ != '\n')
        ;
}
void pzsort(char *s){
    char *lp[100];
    int i, n = 1;
    lp[0] = s;
    while(*s){
        if(*s == '\n' && *(s + 1) != '\0')
            lp[n ++] = s + 1;
        s ++;
    }
    int j, k;
    char *t;
    for(i = 0; i < n; i ++){
        k = i;
        for(j = i; j < n; j ++){
            if(pzcmp(lp, k, j) > 0)
                k = j;
        }
        t = lp[k];
        lp[k] = lp[i];
        lp[i] = t;
    }
    
    for(i = 0; i < n; i ++){
        pzprint(lp[i]);
    }
}
//read a line ended by EOF or '\n', or by approching the limit.

//but it doesn't add a '\0' to the end.

int getline(char *s, int lim){
    int nr = 0;
    for(; lim > 0 && (*s = getchar()) != EOF && *s != '\n'; lim--, s++)
        nr++;
    if(*s == '\n')
        nr++;
    return nr;
}
int main(){
    char s[1000], *p = s;
    /*
    int n;
    scanf("%d", &n);
    getchar();
    while(n > 0){
        p += getline(p, s + 1000 - p);
        n --;
    }
    */
    int nr;
    while((nr = getline(p, s + 1000 - p)))
        p += nr;
    *p = '\0';
    pzsort(s);
}