Why is it showing wrong answer?

Question

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Given a number N. The task is to check if the number N is prime and all of its digits are prime are not. If both the number and all of its digits are prime then print "YES" otherwise print "NO" (without quotes).

Input: The first line of input contains an integer T denoting the number of test cases. The only line of each test case contains a single integer N.

Output: For each test case print "YES" if the number N and all of its digits are prime otherwise print "NO".

Constraints:
1 <= T <= 100
1 <= N <= 103

What I have tried:

C++

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

int prime(int z)
{
    int i;
    if(z==1)
    return 0;
    else
    {
    for(i=2;i<=sqrt(z);i++)
    if(z%i==0)
    return 0;
    }
    return 1;
}


int main() {
	int t;
	cin>>t;
	while(t--)
	{
	    int n,p=1,r;
	    cin>>n;
	   
	   
	        if(prime(n))
	        {
	            while(n)
	            {
	               r=n%10;
	               if(prime(r))
	               {
	                   
	                   n/=10;
	               }
	               else
	               {
	                   p=0;
	                   break;
	               }
	            }
	        }
	        else
	        p=0;
	    
	    if(p==0)
	    cout<<"NO\n";
	    else if(p==1)
	    cout<<"YES\n";
	}
	return 0;
}

Posted 22-Jan-19 21:02pm

Prateek Krishna

Updated 22-Jan-19 22:10pm

Richard MacCutchan

v2

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Comments

CPallini 23-Jan-19 4:01am

Could you please provide an example of failing input (the program looks correct to me)?

Prateek Krishna 23-Jan-19 4:15am

there is no such example.
it is showing only wrong answer.

Richard MacCutchan 23-Jan-19 5:36am

What does "wrong answer" mean? Please do not expect us to guess what results your program comes up with.

CPallini 23-Jan-19 6:09am

What is showing 'wrong answer' (please provide a link to)?
I am pretty confident the constraints you have written are not correct (did you mean 10^3?).

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OriginalGriff · Answer 1 · 2019-01-22T21:45:00

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!