How to simplify this for loop with many if else inside ?

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Hi all, first I want to say thanks to this community, because it's my first post in here =)

I'm very fresh in C++/CLI and I've only a basic question:
How to simplify this loop ?

I want to set x/y coordinates for a 30*30 square in the following order:
y = 0 -> x from 0 to 29;
y = 1 -> x from 0 to 29;
...
y = 29 -> x from 0 to 29;
Since in every case (except of j==0) y = j/30, I think there is a chance to get this smaller. But I have no idea to get a better expression for x.

Thank you for any suggestions and/or hints :)

for (int j=0; j<900; j++)
{
if ( j == 0)
{
	y = 0;
	x = 0;
} else if ( j >= 1 && j <= 29 )
{
	y = j/30;
	x = j;
} else if ( j >= 30 && j <= 59 )
{
	y = j/30;
	x = j-30;
} else if ( j >= 60 && j <= 89 )
{
	y = j/30;
	x = j-60;
} else if ( j >= 90 && j <= 119 )
{
	y = j/30;
	x = j-90;
} else if ( j >= 120 && j <= 149 )
{
	y = j/30;
	x = j-120;
} else if ( j >= 150 && j <= 179 )
{
	y = j/30;
	x = j-150;
} else if ( j >= 180 && j <= 209 )
{
	y = j/30;
	x = j-180;
} else if ( j >= 210 && j <= 239 )
{
	y = j/30;
	x = j-210;
} else if ( j >= 240 && j <= 269 )
{
	y = j/30;
	x = j-240;
} else if ( j >= 270 && j <= 299 )
{
	y = j/30;
	x = j-270;
} else if ( j >= 300 && j <= 329 )
{
	y = j/30;
	x = j-300;
} else if ( j >= 330 && j <= 359 )
{
	y = j/30;
	x = j-330;
} else if ( j >= 360 && j <= 389 )
{
	y = j/30;
	x = j-360;
} else if ( j >= 390 && j <= 419 )
{
	y = j/30;
	x = j-390;
} else if ( j >= 420 && j <= 449 )
{
	y = j/30;
	x = j-420;
} else if ( j >= 450 && j <= 479 )
{
	y = j/30;
	x = j-450;
} else if ( j >= 480 && j <= 509 )
{
	y = j/30;
	x = j-480;
} else if ( j >= 510 && j <= 539 )
{
	y = j/30;
	x = j-510;
} else if ( j >= 540 && j <= 569 )
{
	y = j/30;
	x = j-540;
} else if ( j >= 570 && j <= 599 )
{
	y = j/30;
	x = j-570;
} else if ( j >= 600 && j <= 629 )
{
	y = j/30;
	x = j-600;
}else if ( j >= 630 && j <= 659 )
{
	y = j/30;
	x = j-630;
} else if ( j >= 660 && j <= 689 )
{
	y = j/30;
	x = j-660;
} else if ( j >= 690 && j <= 719 )
{
	y = j/30;
	x = j-690;
} else if ( j >= 720 && j <= 749 )
{
	y = j/30;
	x = j-720;
} else if ( j >= 750 && j <= 779 )
{
	y = j/30;
	x = j-750;
} else if ( j >= 780 && j <= 809 )
{
	y = j/30;
	x = j-780;
} else if ( j >= 810 && j <= 839 )
{
	y = j/30;
	x = j-810;
} else if ( j >= 840 && j <= 869 )
{
	y = j/30;
	x = j-840;
} else if ( j >= 870 && j <= 899 )
{
	y = j/30;
	x = j-870;
}
}

Posted 9-Aug-10 17:20pm

Charakterlos

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4 solutions

Solution 1

Sounds like the classic case for nested loops:

for (y = 0; y < 30; y++)
  {
  for (x = 0; x < 30; x++)
    {
    // whatever you want to do for each (x,y)
    }
  }

Posted 9-Aug-10 17:28pm

Peter_in_2780

Solution 2

Use Math.Floor to get the whole number values you need to do this in a single loop.

int n = (int) Math.Floor(j/30);
x = n * 30;

Something like that.

Posted 9-Aug-10 17:39pm

Christian Graus

Comments

Niklas L 10-Aug-10 4:16am

The floor function is a bit superfluous. Since j is already an int, int n = j/30; should be sufficient.

Solution 4

Aescleal, you are right. It seems logical, but I didn't catch that connection between y and * 30.
Thank you, now the loop is damn short!

Posted 9-Aug-10 21:58pm

Charakterlos

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Aescleal · Accepted Answer · 2010-08-09T19:59:00

Try hoisting everything you can out of the if statements - first off the assignment to y is the same whatever values you're using:

C++

for (int j=0; j<900; j++)
{
    y = j/30;

    if ( j == 0)
    { 
        x = 0;
    }
    else if ( j >= 1 && j <= 29 )
    {
        x = j;
    } 
    else if ( j >= 30 && j <= 59 )
    {
        x = j-30;
    }

    // ad nauseum...

}

Then either notice that you're subtracting 30 * y off j to get x:

for( int j = 0; j < 900; j++ )
{
    y = j / 30;
    x = j - y * 30;
}

or use nested loops the way Christian suggested.

Admittedly most compilers would probably notice that the loop is a very bad way of saying:

y = x = 29;

but I'll assume that x and y are just placeholders for something for now.

Cheers,

Ash