字符串不会打印

Question

I've been doing programming challenges on coderbyte and while doing one, ran into an issue. I want to isolate a word from a string, do some checks on it and then move to another word. The code I'm going to post is supposed to take only the first word and print it out on the screen. When I run it, it doesn't print anything. I thought that maybe I did something wrong in the while loop so I did a simple test. Let's say my input is "This is a test sentence" and instead of word (in cout), I type word[0]. Then it prints "T" just fine. Can you find what the problem is?

#include <iostream>
#include <string>
using namespace std;

int Letters(string str) {
    int i=0;
    int len=str.length();
    string word;
    while(i<len){
        if(isspace(str[i])){word[i]='\0'; break;}
        word[i]=str[i];
        i++;
    }
    cout<<word;
    return 0;
}

int main() {
    int test;
    string str;
    getline(cin, str);
    test=Letters(str);
    return 0;
}

Answer 1

string word;

is default constructed, which is empty initially. Inside while loop, you tried to do:

word[i] = str[i];

It means you tried to access memory that has not been allocated,resulting in undefined behavior .

Try:

word.append(str[i]); 

Answer 2

You can use simpler way to get words from input in C++. It will help you to avoid errors in the future.

#include <iostream>
using namespace std;

int main()
{
  string word;
  while(cin >> word)
    {
      // "word" contains one word of input each time loop loops
      cout << word << endl;
    }
  return 0;
}