why the stl function does not work properly

Question

So this is a code chef question in this we are given a string of binary count we just have to count the zeroes from the last till the first 1 come(start from the last)

I tried to use the function __builtin_ctz and also tried s.size()-s.find_last_of('1')-1 and by a loop just simply by counting the number of zeroes i would like to know why my simple loop code is giving me correct answer and why my all fancy methods are getting me wrong answer in codechef

here are the codes

#include <iostream>
using namespace std;

int main() {
    long int N;
    cin >> N;
    string s;
    cin >> s;
    int count = 0;

    for (long int i = s.size()-1; i>=0; i--)
    {
        if (s[i] =='0')
            count++;
        if (s[i] == '1')
            break;
    }
    cout << count<<endl;
    return 0;
}

Above code is giving correct answer

#include <iostream>
#include<math.h>
using namespace std;
#include<bits/stdc++.h>

int main() {
    long int N;
    cin >> N;
    char s[N+1];
    cin >> s;
    string p =s;
    long int num = strtol(s,NULL,2); 
    cout<<__builtin_ctz(num)<<endl;
    
    return 0;
}

Not giving correct answer

#include <iostream>
#include<math.h>
using namespace std;
#include<bits/stdc++.h>

int main() {
    long int N;
    cin >> N;
    char s[N+1];
    cin >> s;
    string p =s;
    long int num = strtol(s,NULL,2);

    if (num%2 == 1) { cout<<"0\n"; }
    else
    {
        cout<<p.size()-p.find_last_of('1')-1<<endl;
    }
    return 0;
}

when i am trying my test cases that i have figured out all are giving same answer. Can any anone help me find out what is going wrong? here is the link of question https://www.codechef.com/problems/MAX2

The problem says `N` can be as large as 10^5 (a hundred thousand). `long int` contains 64 bits, at best. `strtol` can't squeeze an ocean into a thimble. — Igor Tandetnik, May 02 '23 at 03:57
C++ **must** be learnt using a [good c++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) instead of by solving random online puzzles. — Jason, May 02 '23 at 10:36

score 0 · Answer 1 · answered May 02 '23 at 10:33

0

Always remember that C/C++ does not allow for dynamic memory allocation that way.

cin >> N ;
char a[N] ; // this does not work 
// Memory is allocated at compile time and not at runtime.

you might want to allocate the memory at runtime using either a malloc/calloc or a new operator. (and don't forget to free/delete it afterward)

answered May 02 '23 at 10:33

FRANCESCO AVANZI

21
2

Thankyou for your answer i will surely remember that for future references – Naman Gupta May 03 '23 at 04:45
you are more than welcome. the whole point of this site is to answer when you can and pick up when you can't do things. ;) – FRANCESCO AVANZI May 07 '23 at 08:34

why the stl function does not work properly

1 Answers1