分段错误（核心转储）递归函数

Question

I am getting a segmentation fault (core dump) for this recursive function to count vowels within a string which is inputted by the user. My objective is to copy the user inputted string variable into a character array then lowercase and verify whether or not the character is a vowel. After verification, the function should execute the recursive addition of the vowel count. I am passing an integer of 0 as the parameter int L in this function. Any info on what I can do to fix and improve this portion of code would be awesome.

#include <iostream>
#include <cstring>
#include <string>
#include <cctype>

using namespace std;

int vowels(string s, int L)
{
 int sum = 0;
 char str[s.length()-1];

 strcpy(str,s.c_str());

    if(str[L])
    {
        tolower(str[L]);

        if(str[L]!='a'||str[L]!='e'||str[L]!='i'||str[L]!='o'||str[L]!='u')
            sum = 0;
        else
            sum = 1;
        return sum += vowels(s,L++);
    }
    else
        return 0;
}

Answer 1

There are several problems with your code:

1. You pass string by value, which is expensive and you do that very much.
2. You copy your string to an array? Needless. If you want access by index simply use [] operator on string.
3. Why not use iterators instead of index based access and passing strings?
4. Why recursion? Iteration would be great for this job.
5. As Colonel Thirty Two mentioned, tolower doesn't modify the argument.

Why segmentation fault happens?

strcpy copies a string until null-char is reached. So you need a char array of s.length()+1 elements not s.length()-1 , if not segmentation fault happens.

Noting those points, here is a modified code:

int vowels(string::const_iterator beg, string::const_iterator end) {
  int sum = 0;
  if (beg != end) {
    switch (*beg) {
      case 'a':
      case 'A':
      case 'e':
      case 'E':
      case 'i':
      case 'I':
      case 'o':
      case 'O':
      case 'u':
      case 'U':
        return 1 + vowels(++beg, end);
    }
    return vowels(++beg, end);
  }
  return 0;
}

Working code:

#include <string>
#include <iostream>
using namespace std;
int vowels(string::const_iterator beg, string::const_iterator end) {
  int sum = 0;
  if (beg != end) {
    switch (*beg) {
      case 'a':
      case 'A':
      case 'e':
      case 'E':
      case 'i':
      case 'I':
      case 'o':
      case 'O':
      case 'u':
      case 'U':
        return 1 + vowels(++beg, end);
    }
    return vowels(++beg, end);
  }
  return 0;
}
int main() {
  string x = "hello canberk";
  cout << vowels(x.begin(), x.end()) << endl;
  return 0;
}