random number between multiple ranges in c

Since you already have a way to generate a random number within one range, you can certainly generate it between two ranges:

First, determine how many possible values there are.

int num_values = 1 + range1high - range1low;
num_values += (1 + range2high - range2low);
// continue for additional ranges

Next generate a random number between 0 and num_values.

Finally, adjust that number based on the base of range 1's low. If it exceeds range 1's high, subtract the size of range 1 and re-base it on range 2. Continue to re-base for additional ranges.

The following program has a hard coded sample of generating a number in the ranges [2,5] or [100,200], and an exercise that generates enough data to normally hit every value within the valid ranges. This function could be improved upon significantly, but it works fine for a hard-coded set of ranges:

#include <stdio.h>
#include <stdlib.h>


int randomInRanges()
{
  int range1low = 2,   range1high = 5;
  int range2low = 100, range2high = 200;

  int range1size = 1 + range1high - range1low;
  int range2size = 1 + range2high - range2low;

  int numValues = range1size + range2size;

  int rawRandom = rand() % (numValues);

  /* is the value within range 1? */
  if (rawRandom < range1size) return range1low + rawRandom; /* return a range 1 value */
  rawRandom -= range1size;

  /* is it in range 2? */
  if (rawRandom < range2size) return range2low + rawRandom; /* return a range 2 value */
  rawRandom -= range1size; /* not necessary since we've seen the last range, but part of the pattern */

  printf("Impossible!\n");
  exit(0);
  return -1;
}

int main(int argc, char *argv[])
{
  int values[300] = {0, };
  int i;
  /* seed the random number generator */
  srand(time(NULL));

  for (i = 0; i < 10000; ++i)
  {
    values[randomInRanges()]++;
  }
  for (i = 0; i < 300; ++i)
  {
    if (values[i]) printf("[%d]: %d\n", i, values[i]);
  }

  return 0;
}

What I suggest you do is select a range randomly, then, generate the number within that range.

srand(time(NULL));
int r = rand() % 3; //supposing we have 3 ranges to choose from
int min;
int max;

switch(r) {
case(0): // [1, 4]
    min = 1;
    max = 4;
case(1): // [6, 7]
    min = 6;
    max = 7;
case(2): // [10, 15]
    min = 10;
    max = 15;
}

int range = max - min;
r = (rand() % range) + min;