CODEWITHSUNDEEP
c++infinite-loop
Kaushal Waghela
Kaushal Waghela

Reputation: 5

infinite for loop with correct limits

can you please explain why this code is going in infinite loop? I am unable to find the error. It is working fine with small values of n and m.

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

int main()
{
    long long n=1000000, m=1000000;
    long long k = 1;

    for (long long i = 0; i < n; i++)
    {
        for (long long j = 0; j < m; j++)
        {
            k++;
        }

    }

    cout << k;

    return 0;
}

Upvotes: 0

Views: 237

Answers (3)

user9427593
user9427593

Reputation:

when I Run this code I got this as my output

1000000000001

reference : I had run this code in the code chef IDE(https://www.codechef.com/ide) you can try in this IDE once, I guess there is some problem with your IDE or might be some other issue

it took me less than 20 sec to run this(on clock😁)

But when I put the same code in CODE::BLOCKS its taking long time(like you said infinite loops running) and the reason is quite simple it should do 1000000000000 runs

however this brings me a questions what's difference between code chef IDE and code::blocks compiler (Got a New question from your question 😁🤔(Difference Between Code-Chef IDE and Code::Blocks))

Finally answer is try on with code chef IDE that's it , this code runs fast there🤣

Hope this Helps you 😃

Upvotes: 0

Zongru Zhan
Zongru Zhan

Reputation: 536

It is a typical target for optimization.

  1. Build with -Ofast.
g++ t_duration.cpp -Ofast -std=c++11 -o a_fast

#time ./a_fast
1000000000001
real    0m0.002s
user    0m0.000s
sys     0m0.002s

it takes almost no time to return the output.

  1. Build with -O1.
 g++ t_duration.cpp -O1 -std=c++11 -o a_1


 #./a_1
419774 ms

About 420 seconds to complete the calculation.

Upvotes: 1

user13961569
user13961569

Reputation:

It's not infinite, but that k++ operation has to run for 1,000,000 * 1,000,000 = 1,000,000,000,000 times. It's not infinite, but it takes too long. That's exactly why it works well with small n and m values.

Upvotes: 1

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