stdlib.h rand() % range not from 0 to range

Question

I'm attempting to pick random indexes from a array in C. The problem I'm having is that my random numbers aren't going from the range I expect (from 0 to 2047) as this example says that it will. Just from looking at the indexes I get it seems like I'm getting numbers from 1000-1200 instead of the desired range.

Here's the code which calls rand().

#define MAXPAGES         2048    // maximum number of processes in the system

//until we have a node, pick random indexes.
while(node == NULL){
    table_index = rand() % MAXPAGES;
    node = table[table_index];
    if(node){
       fprintf(stderr, "%d
", table_index);
       if(node->current_pages > 0) break;
       else node = NULL;
    }
}

I call srand only once in my main function.

int main(int argc, char** argv) {
    int i;
    int simulations = 100000;
    // initialize the process hash table
    if(argc > 1){
        removal_algorithm  = atoi(argv[1]);
        if(argc == 3) simulations = atoi(argv[2]);
    }

    srand(time(NULL));
    for (i = 0; i < MAXPAGES; i++) table[i] = NULL;

    printf("Random replacement MMU simulation on %d simulations started
", simulations);
    Simulate(simulations);
    printf("Random MMU simulation completed. only writing page faults:,");
    printf("%d, read / write page faults: %d, total page faults: %d
", read_miss, write_miss, read_miss + write_miss);
}

Example sequence.

AnT stands with Russia · Accepted Answer

Your code obviously prints only those indices, for which table[table_index] is not null. If table[0] is null, it will never print 0. I.e. this code does not necessarily demonstrate the behavior of rand(). Instead it's behavior might be mostly determined by the content of table.

stdlib.h rand() % range not from 0 to range

Answers (1)

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