如何调试导致堆栈溢出错误的递归 function？

Question

The problem I'm trying to solve is KR2 exercise 1-22; 'write a program to "fold" long input lines into two or more shorter lines after the last non-blank character that occurs before the n-th column of input. Make sure your program does something intelligent with very long lines, and if there are no blanks or tabs before the specified column'.

I've attempted to do with a recursive function which just gets stuck.

Example of erroneous behavior:
hello world << input
output >>
hell
o wo
o wo
o wo
o wo
....
...
Expected behavior:
hello world << input
output >>
hell
o wo
rld

Second example of erroneous behavior:
he llo world << input
output >>
he
llo
llo llo
llo llo
llo
....
...
Secondary expected behavior:
he llo world << input
output >>
he
llo
wor
ld

See code below:

#include <stdio.h>
#define PAGEWIDTH 5
#define MAXLINE 1000

int getline(char line[], int maxline);
void fold(char line[], int start);

/* "fold" long input lines into two or more shorter lines after
  the last non-blnk character that occurs before the n-th
  column of input */
int main()
{
    int len;                /* current line length*/
    char line[MAXLINE];     /* current input lne*/

    while ((len = getline(line, MAXLINE)) > 0)
        if (len > PAGEWIDTH)
            fold(line, 0);
    return 0;
}

/* getline : read a line into s, return length */
int getline(char s[], int lim)
{
    int c, i;

    c = 0;
    for (i = 0; i < lim - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
        s[i] = c;
    if (c == '\n') {
        s[i] = c;
        ++i;
    }
    s[i] = '\0';

    return i;
}

void fold(char line[], int start)
{
    int i, lstnb;
    i = lstnb = 0;

    for (i = 0; i < PAGEWIDTH-1 && line[start + i] != '\0'; ++i) {
        if (line[start + i] != ' ')
            lstnb = i;
    }
    for (i = 0; i <= lstnb; ++i)
        putchar(line[start + i]);

    putchar('\n');
    if(line[lstnb + 1] != '\0')
        fold(line, lstnb + 1);
}

Answer 1

good question. First, I think that a recursive solution isn't ideal here. I got it working for the inputs you gave, but considering how small PAGEWIDTH is, you'll likely run into issues with larger inputs. Using a loop would make your life easier and result in a more reliable function.

That said, your issue lies in the recursive call you make in the fold() function, you are providing the wrong start index. This means you don't move forward in the line with each call. Also, you don't clearly define a base case (ie when to end the recursion). You can do this by modifying your for loop conditions or passing the string length (n) to the function and checking whether start > n.

Checking base case and the recursive call is always a good place to start when debugging recursive functions. This is normally where my issues come from and it's really easy to assume you did them correctly and overlook them while debugging.

I'm not sure what you are looking for in terms of an answer, but if those suggestions don't get you any closer, I pasted a fix in a pastebin here . I didn't post it here in case you wanted to solve it on your own.

Hope that works, let me know if there's something I missed.