Calculate the maximum pair product

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Hello.
I am learning C#. For this problem, I want to calculate the largest product of 2 elements of an array. To pass the tests of this program both times, how much CPU is used, is important. So I did not use nested loops because it will need more than one second to compile and show output . And here is my new code :

C#

using System;

namespace codeproject
{
    class Program
    {

        public static long FindMaxPairProduct(long[] myarray)
        {
            long result = 0;
            for(int i=0; i<myarray.Length ; i++)
            {
                for(int j=i+1; j<myarray.Length; j++)
                {
                    if(result < myarray[i] * myarray[j])
                        result = myarray[i] * myarray[j];
                }
            }
            return result;
        }

        public static long FindMaxPairProductFaster(long[] myarray)
        {
            int max_index=0;
            for(int i=0; i<myarray.Length; i++)
                if( myarray[i] > myarray[max_index])
                    max_index = i;
            int max_indexx=0 ;
            for(int j=0; j<myarray.Length; j++)
                if(myarray[j] > myarray[max_indexx])
                    if(j != max_index)
                        max_indexx = j;
            checked
            {
                return myarray[max_index] * myarray[max_indexx];
            }
        }
        static void Main(string[] args)
        {
            checked
            {
                long[] newarray = {100000,90000};
                System.Console.WriteLine(FindMaxPairProductFaster(newarray));
            }
        }
    }
}

What I have tried:

But for this input, it shows

10000000000

not 9000000000 and I know that why it is like that because both

C#

max_index

and

C#

max_indexx

are equal to 0. But I do not know how can I fix this problem to get the correct answer.

Posted 1-Aug-21 2:21am

Aylin Naebzadeh

Updated 18-Aug-22 20:25pm

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George Swan 2-Aug-21 3:44am

Try doing something like this:
https://stackoverflow.com/questions/14810444/find-the-second-maximum-number-in-an-array-with-the-smallest-complexity

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Patrice T · Answer 1 · 2021-08-01T15:14:00

Quote:
But for this input, it shows
10000000000 not 9000000000 and I know that why it is like that because both max_index and max_indexx are equal to 0.

Indeed that is the problem, you need to add a check in case first element of list is also themaximum :

C#

int max_index=0;
for(int i=0; i<myarray.Length; i++)
    if( myarray[i] > myarray[max_index])
        max_index = i;
int max_indexx ;
if (max_index == 0)
    max_indexx=1 ;
else
    max_indexx=0 ;
for(int j=0; j<myarray.Length; j++)
    if(myarray[j] > myarray[max_indexx])
        if(j != max_index)
            max_indexx = j;

OriginalGriff · Answer 2 · 2021-08-01T03:17:00

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#

private 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!

But ... think about it: What is the product of two numbers X and Y?
Easy: X * Y
So if you have a collection of numbers, which pair of numbers will produce the largest product?
Simple: the largest one and the second largest.
So a simple single loop through the input to find the largest two numbers will give you the product - you don't even have to multiply anything until the loop is complete ...

Calculate the maximum pair product

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