Why it is showing wrong answer.

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You are given a number N. You need to convert it to 1 in minimum number of operations.
The operations allowed are as follows:

If N is even then divide the number by 2.
If N is odd then you can either add 1 to it or subtract 1 from it.

The first line of input contains T denoting the number of testcases. T testcases follow. Each testcase contains 1 line of input containing N.

What I have tried:

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

int p(int x)
{
    int z=0;
    while(x)
    {
        if(x&1)
        z++;
        x>>=1;
    }
    if(z==1)
    return z;
    return 0;
}

int main()
 {
	int t;
	cin>>t;
	while(t--)
	{
	    int n,ctr=0;
	    cin>>n;
	    if(n==1)
	    ctr=0;
	    else
	    {
	    while(n>1)
	    {
	        if(n%2==0)
	        {
	            ctr++;
	        n/=2;
	        }
	        else 
	        {
	            if(p(n-1))
	            n-=1;
	            else if(p(n+1))
	            n+=1;
	            else 
	            n-=1;
	            ctr++;
	        }
	    }
	    }
	    cout<<ctr<<endl;
	}
	return 0;
}

Posted 25-Mar-19 5:58am

Prateek Krishna

Updated 25-Mar-19 10:59am

KarstenK

v2

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4 solutions

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OriginalGriff · Answer 1 · 2019-03-25T06:05:00

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!

KarstenK · Answer 2 · 2019-03-25T08:20:00

You need to debug your code. Some important tips:

1. use braces for EVERY SINGLE IF/ELSE STATEMENT and align the code
(I hope it is clear now)
2. make some output in the return cases.
3. write some test code*
4. last but not least: USE THE DEBUGGER

*you need to make function from the while code in your main.

Patrice T · Answer 3 · 2019-03-25T08:32:00

Quote:
Why it is showing wrong answer.

When asking this kind of question, it is a good idea to show a couple sample data with actual answer and expected one. Since it looks like a challenge from a site, it is also a good idea to give link.

Learn to indent properly your code, it show its structure and it helps reading and understanding. It also helps spotting structures mistakes.

C++

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

int p(int x)
{
    int z=0;
    while(x)
    {
        if(x&1)
            z++;
        x>>=1;
    }
    if(z==1)
        return z;
    return 0;
}

int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        int n,ctr=0;
        cin>>n;
        if(n==1)
            ctr=0;
        else
        {
            while(n>1)
            {
                if(n%2==0)
                {
                    ctr++;
                    n/=2;
                }
                else
                {
                    if(p(n-1))
                        n-=1;
                    else if(p(n+1))
                        n+=1;
                    else
                        n-=1;
                    ctr++;
                }
            }
        }
        cout<<ctr<<endl;
    }
    return 0;
}

Indentation style - Wikipedia[^]

Professional programmer's editors have this feature and others ones such as parenthesis matching and syntax highlighting.
Notepad++ Home[^]
ultraedit[^]

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.
[Update]
Advice: Learn to check your code (it is part of the job).
change your program to list the set of operations done to solve each values.
For this problem, make a sample set with values 1, 2, 3, 4, 5, 6, 7, 8, 9 ...
write the values in base 2 and write each step of reduction and with base 2 too.
solve by hand, and solve with your program, compare results.
See when you have differences, try to spot a pattern, use the debugger to see what your program is doing, sometime it don't do like you by hand.

CPallini · Answer 4 · 2019-03-25T10:59:00

Solution 4

Consider the input number (perform the tests in the given order):

Is it zero? Add 1 and you've done.
Is it 1? Do nothing you already have the result.
Is it even? Divide by 2 until you obtain 1.
IS it odd? Subtract one and goto step 3.

Is it hard to implement?

Posted 25-Mar-19 10:59am

CPallini

Why it is showing wrong answer.

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