Creating array with non constant sizes

Question

I'm currently working on an assignment where I must find a way to output the longest common subsequence of two strings. In all of the other places I have found implementations of this code, they all share one similarity: multiple arrays are initialized with non-constant variables which I had always learned was not allowed. When I tried to compile the program, I got an error because of this. How is code like this even supposed to compile in the first place?

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

//Prints the Longest common subsequence
void printLCS(char *s1, char *s2, int m, int n);
/* Driver program to test above function */
int main()
{
char s1[] = "ABCBDAB";
char s2[] = "BDCABA";
printLCS(s1, s2, strlen(s1), strlen(s2));
return 0;
}

void printLCS(char *s1, char *s2, const int m, const int n)
{
int L[m + 1][n + 1];

//Building L[m][n] as in algorithm
for (int i = 0; i <= m; i++)
{
    for (int j = 0; j <= n; j++)
    {
        if (i == 0 || j == 0)
            L[i][j] = 0;
        else if (s1[i - 1] == s2[j - 1])
            L[i][j] = L[i - 1][j - 1] + 1;
        else
            L[i][j] = max(L[i - 1][j], L[i][j - 1]);
    }
}

//To print LCS
int index = L[m][n];
//charcater array to store LCS
char LCS[index + 1];
LCS[index] = '\0'; // Set the terminating character

                   //Stroing characters in LCS
                   //Start from the right bottom corner character
int i = m, j = n;
while (i > 0 && j > 0)
{
    //if current character in s1 and s2 are same, then include this character in LCS[]
    if (s1[i - 1] == s2[j - 1])
    {
        LCS[index - 1] = s1[i - 1]; // Put current character in result
        i--; j--; index--;     // reduce values of i, j and index

    }
    // compare values of L[i-1][j] and L[i][j-1] and go in direction of greater value.
    else if (L[i - 1][j] > L[i][j - 1])
        i--;
    else
        j--;
}

// Print the LCS
cout << "LCS of " << s1 << " and " << s2 << " is " << endl << LCS << endl;
}

Specifically the declarations for array L and array LCS.

Sorry if this code is a mess I don't really post here. Any help would be greatly appreciated.

Answer 1

There is a non-standard extension in the GCC compiler which most people use that allows variable length arrays. You really shouldn't use it though, because VLAs have a lot of downsides, which is why they aren't in the C++ standard in the first place. Also, you will probably end up with a stack overflow when your program receives a large input due to attempting to create a big array on the stack.

Make your arrays constant size or use std::vector.