int x = rand() % 3;
x will choose a 'random' number between 0 and 2
if x <= 1 then white ( 66%) if x == 2 then blue ( 33%)
Pregunta
I have one bag with three equal balls. I made code to simulate the number of times each ball comes off (it works flawlessly so far).
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/time.h>
#include <string.h>
#define N 50 /* Number of simulations*/
#define P 3 /* Number of of balls*/
unsigned long long millitime2(){
struct timeval tv;
gettimeofday(&tv, NULL);
return (unsigned long long)(tv.tv_usec);
}
int main() {
int i;
int num = 0;
int *v;
if((v = malloc(N*sizeof(int))) == NULL){
printf("\n\tMEMORY ERROR");
exit(1);
}
memset(v,0,N);
printf("\nexpected freq: %f\n\n", ((float)1/(float)P)*100);
for (i=0; i<N; i++){
srand(millitime2());
num = (rand()%P);
v[num]++;
}
for(i=0;i<P;i++){
printf("ball:%d picked:%d/%d freq:%f\n",i+1,v[i],N,((float)v[i]/(float)N)*100);
}
printf("\n");
return 0;
}
But now the study I'm doing requires that from three balls; one ball is blue and two balls are white.
What do I have to change in the line with rand()
, so it spits a blue ball one out of three times (~33%) and a white ball two out of three times(~66%)?
Solución
int x = rand() % 3;
x will choose a 'random' number between 0 and 2
if x <= 1 then white ( 66%) if x == 2 then blue ( 33%)
Otros consejos
make your own function
#define BLUE 1
#define WHITE 2
int whichBall()
{
int val = rand() % P;
if (val == 0)
return BLUE;
return WHITE;
}
this will return BLUE 33% of the time and WHITE 66% of the time
for 10 balls 3 blue 3 white and 4 black do:
#define BLUE = 3
#define WHITE = 3
#define BLACK = 4
int whichBall()
{
int val = rand() % P;
if (val < BLUE)
return BLUE;
if (val < BLUE + WHITE)
return WHITE;
if (val < BLUE + WHITE + BLACK)
return BLACK
return BLACK;//This line should never be reached but is included so it compiles
}
One workaround for this is:
Imagine having a bag with 10 balls, 3 red, 3 blue, 3 green, and 1 white.
Get an array[10].
Fill [0][1][2] with 'red', [3][4][5] with 'blue', [6][7][8] with 'green' and [9] 'white'.
Call rand()%10 = index of array.
You will get approximately 30% red, 30% blue, 30% green and 10% white.