反转C中没有strtok的字符串 [英] Reverse a string without strtok in C

问题描述

正在开发使用RPN(反向波兰表示法)的程序.

Working on a program that uses RPN (Reverse Polish Notation).

我有一个函数，可以在不使用 strtok 或触发 printf 的情况下反转字符串的所有单词(与在线和此处找到的所有解决方案不同).

I have a function that reverses all the words of string without using strtok or triggering printf (unlike all the solutions found online and here).

该函数实际上会部分起作用，因为它会打印给定字符串中除最后一个字符串以外的所有单词，我需要帮助找出正在发生的事情.

The function actually works partially as it prints all the words of a given string except the last one and I need help figuring out what's going on.

char *extract(char s[]) {
    if (s[0] == '\0') 
        return NULL;
    int i = 0;
    char *p = NULL;
    while (s[i] != '\0') {
        if (s[i] == ' ')
            p = s + i;
        i++;
    }
    if (p != NULL) {
        *p = '\0';
        return p + 1;
    }
}

然后在main中这样调用它:

And then it's called in main like this:

char s[MAX] = "5 60 +";
while(s != NULL){
    printf("%s\n", extract(s));
}

输出为 + 60 ，光标最终等待某事但预期输出应为 + 60 5

The output is + 60 with the cursor endessly waiting for something but the expected output should be + 60 5

推荐答案

一种标准方法是反转字符串中的每个单词，然后反转整个字符串.

A standard approach is to reverse each word within a string and then to reverse the whole string.

您在这里.

#include <stdio.h>
#include <string.h>

static char * reverse( char *s, size_t n )
{
    for ( size_t i = 0; i < n / 2; i++ )
    {
        char c = s[ i ];
        s[ i ] = s[ n - i - 1 ];
        s[ n - i - 1 ] = c;
    }

    return s;
}

char * reverse_by_words( char *s )
{
    const char *delim = " \t";

    char *p = s;

    while ( *p )
    {
        p += strspn( p, delim );

        if ( *p )
        {
            char *q = p;

            p += strcspn( p, delim );

            reverse( q, p - q );
        }
    }

    return reverse( s, p - s );
}

int main(void) 
{
    char s[] = "5 60 +";

    puts( s );

    puts( reverse_by_words( s ) );

    return 0;
}

程序输出为

5 60 +
+ 60 5

如果您想保持前导空格和尾随空格在原始字符串中的位置，则函数可以采用以下方式

If you want to keep leading and trailing spaces as they were in the original string then the functions can look the following way

#include <stdio.h>
#include <string.h>

static char *reverse( char *s, size_t n )
{
    for ( size_t i = 0; i < n / 2; i++ )
    {
        char c = s[i];
        s[i] = s[n - i -1 ];
        s[n - i - 1] = c;
    }

    return s;
}

char * reverse_by_words( char *s )
{
    const char *delim = " \t";

    char *first = s, *last = s;

    for ( char *p = s;  *p; )
    {
        p += strspn( p, delim );

        if ( last == s ) first = last = p;


        if ( *p )
        {
            char *q = p;

            p += strcspn( p, delim );

            last = p;

            reverse( q, p - q );
        }
    }

    reverse( first, last - first );

    return s;
}

int main(void) 
{
    char s[] = "\t\t\t5 60 +";

    printf( "\"%s\"\n", s );

    printf( "\"%s\"\n", reverse_by_words( s ) );

    return 0;
}

程序输出为

"           5 60 +"
"           + 60 5"

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