2d vector c++ use in Longest Common Substring - c++

Question

I am working on codechef practice problem in which I have to find longest common substring. ( Its practice problem so don't down vote )
Following wiki and some resources online I got the algorithm
https://en.wikipedia.org/wiki/Longest_common_substring_problem
After understanding I wrote the algorithm in c++ , but its compiling but not running successfully . Throwing error while assigning value to vector matrix .
An invalid parameter was passed to a function that considers invalid parameters fatal.
1) Whats wrong with my LCSubstring function
#include<iostream>
#include<string>
#include<vector>
using namespace std;
int max(int a, int b) {
return a > b ? a : b;
}
int LCSubString(string str1 , string str2) {
// create 2d matrix
vector<vector<int>> matrix;
int maxlength = 0;
for (int i = 0; i <= str1.length(); i++) {
for (int j = 0; j <= str2.length(); j++) {
if (i == 0 && j == 0) {
matrix[i][j] = 0;
continue;
}
if (str1[i - 1] == str2[j - 1]) {
matrix[i][j] = matrix[i - 1][j - 1] + 1;
maxlength = max(maxlength, matrix[i][j]);
}
else {
matrix[i][j] = 0;
}
}
}
return maxlength;
}
int main()
{
int t;
int count = 0;
string str;
int len;
cin >> t;
while (t--) {
cin>> str;
len = LCSubString(str, "chef");
if (len >= 2) {
count++;
}
}
cout << count << endl;
return 0;
}

Answer 1

You did not properly initialize std::vector<std::vector<int>> matrix. They do not automatically resize when you access them with [], you need to set the proper size beforehand, like this:
vector<vector<int>> matrix(str.length()+1);
for (int i = 0; i <= str1.length(); ++i)
matrix[i] = vector<int>(str2.length()+1, 0);
Additionally, you are not properly checking for i == 0 or j == 0. If only one of them is zero then the first if in your loop won't hit and you subsequently try to read string[-1]. I don't know whether it would be correct for your algorithm, but try using logical or (||) instead of and (&&).