C:回文:不同的strlen值

Question

我正在编写一个函数来反转字符串并检查该字符串是否为回文。当我用像“abba”这样明显的回文来测试函数时，函数说它不是回文。此外，正向字符串和反向字符串的字符串长度也不同!

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

char forward [] = "abba"; //The string to be reversed

int size = (sizeof(forward)/sizeof(forward[0]) - 1);
int j = 0;
char reverse [10];

void reverser(char forward []) {

    printf("%s", "String forward: ");
    for (int i = 0; i < size; i++) { //Function for printing out the forward string.
        printf("%c", forward[i]);
    }

    printf("\n");

    printf("%s", "String reversed: ");

    for (int i = size, j = 0; i >= 0; --i, ++j) { //Function for reversing the string and printing it.
        reverse[j] = forward[i];
        printf("%c", reverse[j]);       
    }

    printf("\n");

    if (strcmp(forward, reverse) != 0) { //Using strcmp to check if the forward and reversed string are the same. 
        printf("Not a palindrome!\n");
    }
    else{
        printf("A palindrome!\n");
    }

    printf("Forward string length: %d\n",strlen(forward));
    printf("Reversed string length: %d\n",strlen(reverse));
} 

int main () {
    reverser(forward);      
}

输出:
字符串转发:abba
字符串反转:abba
不是回文!
正向字符串长度:9
反转字符串长度:0

Answer 1

在这个循环中

for (int i = size, j = 0; i >= 0; --i, ++j) { //Function for reversing the string and printing it.
    reverse[j] = forward[i];
    printf("%c", reverse[j]);       
    }

forward[i] 是当变量 i 的初始值等于 size

时字符串的终止零

尝试下面的循环实现

for ( int i = size, j = 0; i != 0; ++j) { //Function for reversing the string and printing it.
    reverse[j] = forward[--i];
    printf("%c", reverse[j]);       
    }

还要考虑到，对于将存储由运算符 sizeof 或函数 strlen 返回的值的变量，最好使用类型 size_t.

所以在这些语句中

printf("Forward string length: %d\n",strlen(forward));
printf("Reversed string length: %d\n",strlen(reverse));

使用格式说明符 %zu 而不是 %d

例如

printf("Forward string length: %zu\n",strlen(forward));
printf("Reversed string length: %zu\n",strlen(reverse));

这里的一些回答者说你必须以零结束字符串reverse。然而，它已经是零终止的，因为它是在任何函数之外声明的，因此它具有静态存储持续时间，因此默认情况下它是零初始化的。