Chancer redistributes the uniform distribution of randFloat(0,1) to any distribution you'd like.

Chancer.h

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

#include <vector>
using namespace std ;

float randFloat()
{
  return (float)rand() / RAND_MAX ;
}

float randFloat( float min, float max )
{
  return min+(max-min)*rand() / RAND_MAX ;
}

int chance1( vector<float>& chances )
{
  float r = randFloat() ;
  for( int i = 0 ; i < chances.size() ; i++ )
    if( r <= chances[i] )
      return i ;
  return (int)chances.size()-1; // if the last entry is 1.0, and randFloat() is [0,1] 
  // then we won't get to this last line, but this is a failsafe
}

int chance2( vector<float>& chances )
{
  float r = randFloat() ; // by far the most expensive thing here.
  float total=0.f;
  for( int i = 0 ; i < chances.size() ; i++ )
  {
    total += chances[i] ;
    if( r <= total )
      return i ;
  }
  
  // You get OOB the chances array if none of your choices were selected
  return (int)chances.size() ;
}

void testChances1()
{
  // 20% 0, 20% 1, 50% 2, 10% 3
  vector<float> chances = { .2, .4, .9, 1. } ;
  vector<int> res = {0,0,0,0};
  
  int runs=10000;
  for( int i = 0 ; i < runs ; i++ )
    res[ chance1( chances ) ] ++ ;
  for( int i = 0 ; i < res.size() ; i++ )
    printf( "%d: %.2f%%\n", i, 100.*res[i]/runs ) ;
}

void testChances2()
{
  // 20% 0, 20% 1, 50% 2, 10% 3 (classified as "other")
  vector<float> chances = { .2, .2, .5 } ;
  vector<int> res = {0,0,0,0};
  
  int runs=10000;
  for( int i = 0 ; i < runs ; i++ )
    res[ chance2( chances ) ] ++ ;
  for( int i = 0 ; i < res.size() ; i++ )
    printf( "%d: %.2f%%\n", i, 100.*res[i]/runs ) ;
}

int main(int argc, const char * argv[])
{
  //testChances1() ;
  testChances2() ;
    
  return 0;
}