My program keeps crushing.

This is C++.

1. I am trying to make a program that only has alphabetical value after converting lowercases.

Why am I getting segmentation fault?

string sen; string s = "race a car"; for (int i = 0; i < s.length(); i++) { putchar(tolower(s[i])); }

int counter = 0; for (int i = 0; i < s.length(); i++) { if (s[i] >= 'a' && s[i] <= 'z') sen[counter++] = s[i]; } for (int z = 0; z < sen.length(); z++) { cout << sen[z] << " "; }