C++ :: Hangman Game - Generating Random Number

I am recreating a hangman game. I'm trying to generate a random number to choose which word from my words list to use and I've done it a billion times before just like this. So, here's my code:

#include <iostream>
#include <cstdlib> //standard library.
#include <ctime> //for random.
#include <fstream> //for reading and writing to a file.
#include <string>
using namespace std;
int main() {
int num_words;
string word[num_words];

[Code] .....

I have the text file "words.txt" info below.

5
dog
cat
horse
pig
bird

If I run the program as it is, I get a segmentation fault core dumped error, however, if I take all the stuff that reads in the words from the words.txt file, like so:

#include <iostream> //every program has this.
#include <cstdlib> //standard library.
#include <ctime> //for random.
#include <fstream> //for reading and writing to a file.
#include <string>

[Code] ....

it works correctly and generates a random number.

I'm using Linux Mint.