Thread: Random Numbers

Problem: Acquiring a random integer number between 1 and 100. (rand() % 100) + 1 has failed. Suggestions?

Story: I'm trying to write a simple guessing game; I think of a number, an integer, betwixt 1 and 100, while you try to guess it. I use (rand() % 100) + 1 to get the (supposedly) random number. Over the dozen or so times I've done it, I have consistently gotten 84 as the secret number. By my calculations, that's roughly a chance of 1 in 10^24; one in a million billion billion. Clearly, there's a problem somewhere. My full code follows.

Code:

#include <stdio.h>
main () {
	int guess, high, low;
	high = 100;
	low = 0;
	char line[80];
         printf("I'll think of a number and you try to guess it.\n");
	printf("If you get it right, you get a prize. Come on, friend. ");
	while (1) {
		int secret = rand() % 100 + 1;
		printf("Enter your guess.\n");
		printf("Hint: It's between %d and %d\n", low, high);
		(void)fgets(line, sizeof(line), stdin);
		(void)sscanf(line, "%d", &guess);
		if (guess == secret) {
			printf("Bingo!\n");
			break;
		}
		if (guess > secret) {
			high = guess;
			printf("Sorry, too low.\n");
		} else {
			low = guess;
			printf("Sorry, too high.\n");
		}
	}
	return(0);
}

Random numbers in computers aren't actually random, they are generated using a starting number called seed and then does som arithmetic on it to get a new value. Your program doesn't change the seed so the program will use the same value over and over and thus always producing the same result.
You can get a nice new seed by putting srand(time(0)) before using the rand() function. Don't forget to #include <time.h> either.
[edit] And don't put the srand() inside of the loop, it should only be done once, so put it before your loop.

Using the time as a seed is good because time is always changing so your seed will always be different when running the program.

Ah! Of course. I knew about the seed issue, but I didn't see the meaning. Thank you kindly. I shall fix that immediately.