C语言字符串反转方案失效原因求助（含题目要求及部分代码）

Hey there! Let's figure out why your string reversal code isn't working as expected. First, let's recap the problem statement to make sure we're on the same page:

Write a function reverse (s) that reverses the character strings. Use it to write a program that reverses its input a line at a time

Your code got cut off at the while(len = ...) part, but I can walk through the most common pitfalls people hit with this problem, show you a working implementation, and explain how to fix typical issues.

Common Issues That Break String Reversal #

Before diving into code, let's cover the mistakes that usually cause problems here:

• Missing null terminator: C strings rely on a \0 to mark the end. If your ngetline function doesn't add this, your reversal logic or printf will read garbage memory.
• Off-by-one errors: When swapping characters in the reverse function, it's easy to miscalculate the last index (using len instead of len-1, for example).
• Incorrect line reading: If your ngetline includes the newline character (\n) in the string, reversing it will put the newline at the start of the line, leading to weird output.
• Ignoring EOF: Your loop needs to handle the end-of-file condition properly to avoid infinite loops or unexpected behavior.
• Non-standard main function: Old C allows main() without a return type, but modern compilers will warn about this—always use int main() for portability.

Working Implementation #

Here's a complete, tested version of the program that fixes these issues:

/*Write a function reverse (s) that reverses the character strings. 
Use it to write a program that reverses its input a line at a time*/
#include <stdio.h>
#define MAXLENGTH 1000

// Reads a line into s, returns the length of the line (excluding newline)
int ngetline(char s[], int lim) {
    int c, i;
    // Read characters until we hit newline, EOF, or reach the buffer limit
    for (i = 0; i < lim - 1 && (c = getchar()) != EOF && c != '\n'; ++i) {
        s[i] = c;
    }
    // Uncomment the line below if you want to include the newline in the reversed string
    // if (c == '\n') s[i++] = c;
    // Critical: Add the null terminator to mark the end of the string
    s[i] = '\0';
    return i;
}

// Reverses the string s using its length (avoids recalculating length inside the function)
void nreverse(char s[], int len) {
    int start = 0;
    int end = len - 1;
    char temp;
    // Swap characters from the start and end, moving toward the center
    while (start < end) {
        temp = s[start];
        s[start] = s[end];
        s[end] = temp;
        start++;
        end--;
    }
}

// Standard main function with proper return type
int main() {
    int len;
    char line[MAXLENGTH];
    // Keep reading lines until ngetline returns 0 (EOF or empty line)
    while ((len = ngetline(line, MAXLENGTH)) > 0) {
        nreverse(line, len);
        printf("%s\n", line);
    }
    return 0;
}

How to Adapt This to Your Code #

If you want to stick with your original function signatures, here are key fixes to apply:

1. Fix ngetline: Make sure it adds the \0 terminator and handles EOF correctly. If you want to exclude the newline (most people do for clean output), don't write it to the string.
2. Fix nreverse: Use the line length to avoid off-by-one errors. If you don't want to pass the length, you can calculate it inside the function by looping until you hit \0:

   void nreverse(char s[]) {
       int len = 0;
       while (s[len] != '\0') len++; // Calculate string length
       int start = 0, end = len - 1;
       char temp;
       while (start < end) {
           temp = s[start];
           s[start] = s[end];
           s[end] = temp;
           start++; end--;
       }
   }
   

3. Fix the main loop: Use while ((len = ngetline(line, MAXLENGTH)) > 0) to stop when EOF is reached.

Testing the Program #

To test it, compile and run the code, then type a few lines of text. Press Ctrl+D (on Unix/macOS) or Ctrl+Z (on Windows) to trigger EOF. You should see each line reversed in the output.

内容的提问来源于stack exchange，提问作者BlankQQ