The function shown in your listing generates "random" integers that are greater than or equal to min and less than or equal to max.
Here's the idea:
The standard C library function rand() generates a sequence of integers between 0 and some large value. (The largest value that rand() can generate for a given compiler is given by a constant RAND_MAX, which is defined in <stdlib.h>.) Every time you call rand() you get another number between 0 and RAND_MAX.
Now for non-negative integers, the % operator gives the remainder that is obtained by dividing the first number by the second. So, if you take any int, say Z, and let x = Z % 11, the result in x will always be greater than or equal to zero and less than or equal to 10. (That's how division works: the remainder is greater than or equal to zero and less than the divisor.) This program assumes that the remainders will be "random" also (since they were obtained by dividing a "random" number by a constant).
So if we want to obtain a random number between -5 and 5, we see that there are 11 possible values of integers (count them: -5, -4, ..., 4, 5). The number if integer values between min and max is given by (max - min) + 1.
In other words the value x in the following expression is greater than equal to 0 and less than or equal to 10 (where min = -5, max = 5):
Then, since we want to find a "random" integer number between min and max, one way is to use the formula
x = rand() % (max - min + 1);
Note that, although your example calls getrand(5.,5.) the arguments are converted to ints since that's the way the function and its prototype are defined.
x = rand() % (max - min + 1) + min;
If you want to get a floating point number between two given numbers, say fmin and fmax, here's a way
First find a floating point number between 0 and 1. Then perform a scaling function that maps the interval [0,1] to [fmin,fmax].
Here's the function (remember RAND_MAX is the largest possible value that can be returned by rand()).
Of course you can do it in one step:
x = (double)rand()/RAND_MAX; /* this makes x between 0 and 1 */
x = x * (fmax - fmin) + fmin;
Here's a function built like the one you had, but the inputs and output are doubles:
x = ((double)rand()/RAND_MAX ) * (fmax - fmin) + fmin;
double getfrand(double fmin, double fmax)
static int Init=0;
rc= ((double)rand()/RAND_MAX) * (fmax - fmin) + fmin;