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Negative and Float random numbers

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Started by mello October 07, 2006 12:18 PM

3 comments, last by Ebola0001 17 years, 6 months ago

mello

122

Author

October 07, 2006 12:18 PM

Hello community, Oh... today I've a really huge problem! I've a function called draw_forest() with the following prototype:

void draw_forest(GLubyte color[], int color_variation, float max_height, float min_height);

The function calls another function to draw several trees in the screen with a color variation and with the height between min_height and max_height. If color[]={100, 100, 100} (colors in the 0-255 range) and color_variation=30 then the tree should be drawn with color components in the range 70-130. Well, for the height I've this code:


float height_range = height_max - height_min;
float height_temp = height_min + (rand()%height_range);

This solution doesn't work because rand() doesn't accept floats. rand() also doesn't return negative numbers so I'm having trouble with the colors, too. Does anyone have some tips on how to solve this? Thanks a lot!

zix99

205

October 07, 2006 12:23 PM

Hi,

I've created a set of functions just for this purpose. (I'm assuming you're using C++)

float random(){	return (float)rand() / (float)RAND_MAX;}float random(float max){	return (float)rand()*max / (float)RAND_MAX;}float random(float min, float max){	return random() * (max-min) + min;}int randomi(int max){	return rand() % max;}int randomi(int min, int max){	return rand() % (max-min) + min;}

As you can see, randomi (overloaded), accepts a min and max value which can be negative. The random() function also has min and max capabilities for floating point numbers.

Hope those are helpful
~zix

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Scet

960

October 07, 2006 12:31 PM

Quote:Original post by mello
his solution doesn't work because rand() doesn't accept floats.

No, it's the % operator that doesn't accept floats. Convert height_range to an int ( (rand()%(int)height_range) ) or use the function fmod from the cmath header.

Quote:Original post by mello
rand() also doesn't return negative numbers so I'm having trouble with the colors, too.

If colors have to be between 0 and 255 why do you need negative numbers? Anyway just rand a higher number a subtract a certain amount. Examples:

-127 to 128 = ( rand() % 256 ) - 127

-3 to 9 = ( rand % 13 ) - 3

or

(where A is negative and B is positive )
A to B = ( rand() % ( ( B - A ) + 1 ) ) + A

Demus79

362

October 07, 2006 12:59 PM

int randomi(int min, int max){	return rand() % (max-min) + min;}

There is one fundamental problem with the code above. A singular number produced by the code is random and it is in between the min and max ranges.
The problem however is that eventually the code produces more random numbers near the minimum value (unless, RAND_MAX is divisible by max-min).

To prove this, you can create like 10000 random numbers and check which numbers occure more often.

Cheers !

Ebola0001

138

October 07, 2006 03:02 PM

Yay... i know this one... here is my solution to random numbers

i use two overloaded functions "rand(int,int)" and "rand(float,float)"

// INT rand(x,y)int rand(int min, int max)                                                      // Generates random INT between X and Y{                                                                               //float r = rand();                                                               // Generate random numberfloat i;                                                                        // number to hold scaled random numberi = (r/RAND_MAX)*(max-min)+min;                                                 // Scale the random number to 0-1 then multiply it by the new range, and add the offsetreturn i;                                                                       // Return the scaled random number}                                                                               //// FLOAT rand(x,y)float rand(float min, float max)                                                // Generates random INT between X and Y{                                                                               //float r = rand();                                                               // Generate random numberfloat i;                                                                        // number to hold scaled random numberi = (r/RAND_MAX)*(max-min)+min;                                                 // Scale the random number to 0-1 then multiply it by the new range, and add the offsetreturn i;                                                                       // Return the scaled random number}

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