Why is it showing segmentation fault (SIGSEGV)...the task is to print largest prime factor.

Question

1.00/5 (2 votes)

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Given a number N, the task is to find the largest prime factor of that number.

Input:
The first line of input contains an integer T, denoting the number of test cases. Then T test cases follow. Each test case contains an integer N.

Output:
For each test case, in a new line, print the largest prime factor of N.

Constraints:
1 <= T <= 100
2 <= N <= 10^10

Example:
Input:
2
6
15
Output:
3
5

What I have tried:

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

void check(long long int n)
{
    long long int p=2,i;
    bool prime[n+1];
    memset(prime,true,sizeof(prime));
    for(p=2;p*p<=n;p++)
    {
        if(prime[p])
        {
          for(i=2*p;i<=n;i+=p)
          prime[i]=false;
        }
    }
    if(prime[n])
    {
        cout<<n<<endl;
    }
    else
    {
    for(i=n/2;i>=2;i--)
   {
       if(prime[i])
       {
         if(n%i==0)
         {
         cout<<i;
         break;
         }
       }
   }
   cout<<"\n";
    }
}


int main() {
	int t;
	cin>>t;
	while(t--)
	{
	    long long int n;
	    cin>>n;
	    if(n==1)
	    cout<<n<<endl;
	    else if(n==2)
	    cout<<n<<endl;
	    else if(n==3)
	    cout<<n<<endl;
	    else
	    check(n);
	}
	return 0;
}

Posted 16-Jan-19 2:13am

Prateek Krishna

Updated 17-Jan-19 1:52am

v2

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Comments

CPallini 16-Jan-19 8:46am

Are you sure the contraints are correct (your code doesn't crash within such constraints).

Prateek Krishna 16-Jan-19 8:50am

Yes,

Constraints:
1 <= T <= 100
2 <= N <= 10^10

CPallini 16-Jan-19 9:01am

Sorry I misread that.
You can (that is you must) implement it without allocating the prime array.

Prateek Krishna 16-Jan-19 9:29am

how?

5 solutions

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OriginalGriff · Answer 1 · 2019-01-16T03:15:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Richard MacCutchan · Answer 2 · 2019-01-16T03:36:00

Solution 2

Most likely because of the following:

C++

bool prime[n+1];
memset(prime,true,sizeof(prime));

The sizeof operator is a compile time operation. But since the compiler does not know how big the array is it cannot correctly calculate the value. You should not use dynamic arrays in this way as not all compilers support the feature. Use the C++ new operator so you ensure you get the correct result.

Posted 16-Jan-19 3:36am

Richard MacCutchan

Comments

[no name] 16-Jan-19 9:49am

A 5
"Use the C++ new operator": Or as an alternative something like (n+1)*sizeof(bool)
Anyway memset(prime,true,...); looks suspect to me

Richard MacCutchan 16-Jan-19 10:01am

You do not need to use sizeof when allocating on the stack, or using new. The type specification makes clear what number of bytes to allocate per element.

For example if you entered


int foo[2 * sizeof(int)];

it would allocate 8 items rather than 2.

Patrice T · Answer 3 · 2019-01-16T09:59:00

Quote:
Why is it showing segmentation fault (SIGSEGV)

This mean that somewhere in your code, you try to access memory that you don't own.
By using the debugger, you will see what your code is really doing, and you will see where the problem happen.

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

KarstenK · Answer 4 · 2019-01-16T23:24:00

Solution 4

use the debugger to find the problems in your code.

tip: make some output to find it faster where your problems still are. I would guess again that some division with zero is in your backlog.

Posted 16-Jan-19 23:24pm

KarstenK

Comments

Maciej Los 18-Jan-19 3:57am

5ed!

CPallini · Answer 5 · 2019-01-17T01:52:00

Solution 5

It doesn't make sense to (trying to) allocate an array of, 10^10 items: the runtime cannot provide you with it and you actually don't need it.
You could store just the actual prime numbers in a vector. However there are far simpler solutions to such a problem (you are allowed to find them yourself, it is
matter of Googling a bit).

Posted 17-Jan-19 1:52am

CPallini

Comments

Maciej Los 18-Jan-19 3:57am

5ed!

CPallini 18-Jan-19 5:33am

Thank you!

Why is it showing segmentation fault (SIGSEGV)...the task is to print largest prime factor.

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