Question

I was writing a program that tell the user to input a random string, and then print out all the duplicates and the number of time each of them repeats. I was running it through gdb and this is the output:

Here is the program:

#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main()
{
  //Read a string word by word and put into a vector
  //loop through the vector:
  //  If there are two words duplicate:
  //    loop through another vector (used to store duplicate word)
  //    compare if these two words are the same as the duplicate word stored
  //      If the same: ++count[i]
  //      If not: push_back(count) ; ++count[i]

  string word;
  vector<string> sentence;
  vector<string> duplicate;
  vector<int> times;
  int count = 1;

  while (cin >> word) {
    if (word == "ctrlz") {
      break;
    }
  sentence.push_back(word);    
  }

  vector<string>::size_type i = 0;
  vector<string>::size_type j = 0;

  while (i != sentence.size()) {
    if (sentence[i] == sentence[i+1]) {
      while (j != sentence.size()) {
        if (duplicate.size() == 0) {
          duplicate.push_back(sentence[i]);
          times.push_back(count);
          ++times[0];
        }
        else { 
          if (sentence[i] != duplicate[j]) {
            duplicate.push_back(sentence[i]);
            times.push_back(count);
            ++times[j+1];          
          }
          else {
            ++times[j];
          }
        }
      ++j;
      }
    }
  ++i;  
  }

  while (i != duplicate.size()) {
    cout << duplicate[i] << ' ';
    ++i;
  }

  return 0;
}

I got this after running gdb:

(gdb) run 
Starting program: /home/phongcao/C++/6.12
phong phong phong phong phong phong 
ctrlz

Program received signal SIGSEGV, Segmentation fault.
0x001c58d9 in std::string::size() const () from /usr/lib/libstdc++.so.6 
(gdb) 

What does this output mean? How can I fix this segmentation fault?

Answer 1

Some bugs:

if (sentence[i] == sentence[i+1]) {

Your loop permits i to be size()-1 – So you’re reading one past the end of the vector sentence.

while (i != sentence.size()) {
    while (j != sentence.size()) {
     ...
     ++j;
   }
   ++i;
}

j is never reset – next iteration of the outer loop it’ll start at sentence.size() – You likely don’t want that.

You should solve this with a std::map<std::string, int>:

std::map<std::string, int> words;
while (cin >> word) {
    if (word == "ctrlz") {
         break;
    }
    words[word] += 1;
}
for (std::map<std::string>::const_iterator it = words.begin(); it != words.end(); ++it) { 
     if (it->second > 1) {
         cout << it->second << " copies of " << it->first << endl;
     }
}