CODEWITHSUNDEEP
c++algorithm
John Lui
John Lui

Reputation: 1464

Why do I get an out of bounds exception in this function?

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

int findpali (string s,int dp[][100],int st, int e);
int main (void)
{
    string s;
    cin>>s;
    int dp[100][100];
    for (int i = 0; i < 100; i++ )
        for ( int j = 0; j < 100; j++ )
            dp[i][j] = -1;
    int out = findpali(s,dp,0,s.length()-1);
    cout<<out<<"\n";
    return 0;
}



 int findpali (string s,int dp[][100],int st, int e) // st ->starting position, e -> ending position
    {
        if (s.length() == 1)
        {
            dp[st][e] = 1;
            return 1;
        }
        else if (s.length()==2 && s[st] == s[e])
        {
            dp[st][e] = 2;
            return 2;
        }
        else if (dp[st][e] != -1)
            return dp[st][e];
        else if (s[st] == s[e])
        {
            dp[st][e] = findpali(s.substr(st+1,s.length()-2),dp,st+1,e-1)+2;
            return dp[st][e];
        }
        else
        {
            dp[st][e] = max(findpali(s.substr(st,s.length()-1),dp,st,e-1),findpali(s.substr(st+1,s.length()-1),dp,st+1,e));
            return dp[st][e];
        }
    }

The above function finds the length of the longest palindrome in a given string. I have initialised the given array dp to -1 and then I am storing the different values in the array. However, when I enter a string like ABCD, it says out of bound exception and it aborts. What could be the reason for this? Thanks!

Upvotes: 0

Views: 63

Answers (1)

xashru
xashru

Reputation: 3580

you need to add test for empty string at the start of your function. Also you are storing dp for original string s, so when you update it with a substring of s the result is no longer valid. You can just pass different indices st and e every time you call the function recursively.

Your modified function may look something like this:

int findpali(string &s, int dp[][100], int st, int e)
{
    if(s.length()==0 || st > e) {  //extra test for boundary case
        return 0;
    }
    if(st==e)   // length 1 substring
    {
        dp[st][e]=1;
        return 1;
    } else if(e-st==1 &&  s[st] == s[e])   // length 2 substring
    {
        dp[st][e]=2;
        return 2;
    } else if(dp[st][e] != -1)
        return dp[st][e];
    else if(s[st] == s[e])
    {
        dp[st][e]=findpali(s, dp, st+1, e-1)+2;
        return dp[st][e];
    } else
    {
        dp[st][e]=max(findpali(s, dp, st, e-1), findpali(s, dp, st+1, e));
        return dp[st][e];
    }
}

Upvotes: 1

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