Help with c++ projecteuler problem

[ETAkoG1T]

Hey I am working on http://projecteuler.net/problem=4 and this is the program I have I don't know why its not working! Please help, looks like it is in an infinite loop for some reason...

int main()
{
	
	long a, b;
	long biggest = 0;
	a = 999;
	b = 999;
	bool trueistrue = 1;
	for(;a != 0 && b != 0;)
	{
		long num, rem,numCopy;
	    long sum = 0;
		numCopy = a * b;
		num = a * b;
		while(num!=0)
		{
			rem=num%10;
			num=num/10;
			sum=sum*10+rem;
		}
		if(numCopy == num)
		{
			if(biggest < num)
				biggest = num;
		}
		if(trueistrue)
			b = b - 1;
		else 
			a = a - 1;
		if(b == 0)
		{
			trueistrue = 0;
			b = a - 1;
			a = 999;
		}
		if(a == 0)
		{
			trueistrue = 1;
			a = b - 1;
			b = 999;
		}
	}
	cout << biggest << " is  the answer a = " << a << " b = " << b;
	cin.get();
	cin.get();
	return 0;
}

[Imadatobanisa]

May you tell us more about your problem?...

[ETAkoG1T]

This is the code from the time of writing:

int main()
{
	
	long a, b;
	long biggest = 1;
	a = 999;
	b = 999;
	bool trueistrue = 1;
	for(;a != 0 || b != 0;)
	{
		if(b == 1 && a == 2)
			break;
		long num, rem,numCopy;
	    long sum = 0;
		numCopy = a * b;
		num = a * b;
		while(num!=0)
		{
			rem=num%10;
			num=num/10;
			sum=sum*10+rem;
		}
		if(a * b == num)
		{
			if(biggest < a * b)
				biggest = a * b;
		}
		if(trueistrue)
			b = b - 1;
		else 
			a = a - 1;
		
		if(b == 0)
		{
			trueistrue = 0;
			b = a - 1;
			a = 999;
		}
		if(a == 0)
		{
			trueistrue = 1;
			a = b - 1;
			b = 999;
		}
	}
	cout << biggest << " is  the answer a = " << a << " b = " << b;
	cin.get();
	cin.get();
	return 0;
}

The problem now is that biggest doesn't change from its original value. I don't know why, that is what I am wondering about, I don't need anyone to write my program just tell me why biggest doesn't change. As for what the thing I am making is you can click that link in my original post...

[ETAkoG1T]

Can someone help me understand

while(num!=0)
		{
			rem=num%10;
			num=num/10;
			sum=sum*10+rem;
		}

I found it online and made the rest of the program. It may have a flaw that makes my program un usable.

[ETAkoG1T]

I solved it, took longer than it should! But I still don't understand all of this:

while(num!=0)
		{
			rem=num%10;
			num=num/10;
			sum=sum*10+rem;
		}

[JLBorges]

> But I still don't understand all of this

To check if the number d₀d₁d₂d₃ is a palindrome:

check if d₀d₁d₂d₃ == d₃d₂d₁d₀

d₃d₂d₁d₀ == d₃ * 1000 + d₂ * 100 + d₁ * 10 + d₀ == ( ( ( ( d₃ * 10 ) + d₂ ) *10 ) + d₁ ) * 10 ) + d₀



> took longer than it should!

a. For positive a and b, a*b is greater than a*(b-1), a*(b-2) etc., we can omit those checks.

b. 789 * 687 == 687 * 789, we need not check both

c. The check for a number being a palindrome is far more expensive than the check if a number is greater than another number. We can place the inexpensive check first.

#include <iostream>

bool is_palindrome( int number )
{
    int n = number, reverse_number = 0 ;
    while( n )
    {
        reverse_number = reverse_number * 10 + n % 10 ;
        n /= 10 ;
    }
    return number == reverse_number ;
}

int main()
{
    enum { SMALLEST = 100, LARGEST = 999 } ;
    int largest_palindrome = 0 ;

    int num1 = 0, num2 = 0 ;
    for( int i = LARGEST ; i >= SMALLEST ; --i )
        for( int j = i-1 ; j >= SMALLEST ; --j )
        {
            int product = i * j ;
            if( ( product > largest_palindrome ) && is_palindrome(product) )
            {
                num1 = i ;
                num2 = j ;
                largest_palindrome = product ;
                break ; // no need to check for smaller values of j
            }
        }

    std::cout << num1 << " * " << num2 << " == " << largest_palindrome << '\n' ;
    // 993 * 913 == 906609
}

[ETAkoG1T]

thank you very much that helped me very much! :)

[TheIdeasMan]

Just to further your education a little:


bool trueistrue = 1;

Stick to the the idea that bool variables are either true or false - then re-evaluate this piece of code to come to an inescapable conclusion.

With use of this variable in this code:

		if(a == 0)
		{
			trueistrue = 1;

This translates to: if a is false, then trueistrue is true. A lesson in nightmare confusion. lol

if(a == 0)
//can be written

if(!a)   {}  // if not a, in other words if a equals false

//can also do this
if (a) // is variable a true?

bool Quit = false;
//user wants to quit
Quit = true;

With this:

for(;a != 0 || b != 0;) {}

We have for(initialise none; end-condition; increment none) {}

A while loop looks like this while(end-condition){} Do you see my point here?

I hope this has cleared up some concepts for you, good luck with your coding, and have fun !!

[ETAkoG1T]

Thanks for the lesson, I didn't understand very much of that but atleast I got the program to work and I understand the algorythm decently. The reason I called it trueistrue is just because it is so fun to make trueistrue false :P But I made the program overly complicated and stupid in the first few posts...

Edit: Actually TheIdeasMan I understand it :)