Random Number Generator - not generating a random number every time

Question

I am trying to create a random number / letter using rand() however it just returns the same number / letter every time it is called.

void Location::pick() {
    srand(time(NULL));
    x = rand() % fieldSize + 1;
    int locy = rand() % fieldSize + 1; // rand y
    switch (locy) {
    case 1: y = 'a'; break;
    case 2: y = 'b'; break;
    case 3: y = 'c'; break;
    case 4: y = 'd'; break;
    case 5: y = 'e'; break;
    }
}

fieldSize is set to 5. I use srand() at the top. Below is where I call the function.

void Fleet::deployFleet() {
    bool newLoc = true;
    Location tmp;

    for (int i = 0; i < fleetSize; i++) {
        tmp.pick();
        ships[i].setLocation(tmp);
    }
}

fleetSize is set to 5. ships[] is an array of 5 ships. setLocation() just sets the location of the ship to the given parameter.

Don't use `rand()`, especially not `rand() % n`. Read more [here](http://www.azillionmonkeys.com/qed/random.html). Use `srand` with time as the parameter instead. — Eli Sadoff, Nov 07 '16 at 16:39
reopened as the code is a little different. Highly related: http://stackoverflow.com/questions/37701567/calling-a-random-number-generating-member-function-doesnt-produce-entirely-rand — NathanOliver, Nov 07 '16 at 16:40
Everything about this code is broken. Multiple seedings, using `time(NULL)` as a seed, using `rand()`, using `%` to limit the range of the numbers... I don't even know where to begin. — nwp, Nov 07 '16 at 16:41

score 2 · Accepted Answer · answered Nov 07 '16 at 16:42

Pseudo-random generators will generate the same sequence of "random" numbers for the same seed.

You are repeatedly seeding the generator with the time in seconds. You're doing it 5 times very quickly. So the time is always the same for each iteration. Hence you get the first number of said sequence and it'll always be the same.

Random Number Generator - not generating a random number every time

1 Answers1