CODEWITHSUNDEEP
c++palindrome
Zain
Zain

Reputation: 25

Checking for palindrome between two indices

I am trying to check whether a the string contains palindrome between the given indices. However I am unable to find the bug in my code.

bool isPal(char a[],int i,int j){

    for(int k=i;k<(j-i)/2;k++)
        if(a[k]!=a[j--])
            return 0;

    return 1;
}

Upvotes: 1

Views: 259

Answers (4)

Jarod42
Jarod42

Reputation: 218088

You modify j inside the loop, changing also the end condition (which were incorrect anyway), you might use instead:

bool isPal(const char a[],int i,int j){
    for (int k = 0; k < (j - i) / 2 ; k++) {
        if (a[i + k] != a[j - k]) {
            return false;
        }
    }
    return true;
}

Minimal change from your code would be:

bool isPal(char a[], int i, int j)
{
    const int mid_index = (j + i) / 2;
    for (int k = i; k < mid_index; k++)
        if (a[k] != a[j--])
            return 0;

    return 1;
}

Upvotes: 2

luizchagasjardim
luizchagasjardim

Reputation: 125

for(int k=i;k<(j-i)/2;k++)

You're starting at i and then going up to j-i-1. This is not what you want. Replace i and j with some values and you'll see.

Upvotes: 0

Walter
Walter

Reputation: 45454

You error was to increment j as but also assume (in the loop condition) that it remains at its input value. You could use the input variables also as loop variables

bool isPalindrome(const char*a, int i, int j) {
    while(i<j)
        if(a[i++] != a[j--])
            return false;
    return true;
}

when setting the correct stop conditions for the loop is straightforward.

Upvotes: 2

Rangeesh
Rangeesh

Reputation: 373

You are changing the value of j using j--, which will affect the condition of your for loop k<(j-1)

Upvotes: 4

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