Error code randomly appears when trying to run slot machine program

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I am writing a program for a basic slot machine, randomly generating numbers and displaying the corresponding "symbols" for them from an array. The program executes most of the time but randomly I get an error code -1073741819 (0xC0000005). I'm extremely new to C so I'm unfamiliar with a lot of things, including what exactly can be causing this error in my code.

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

int main()

{
    srand(time(NULL));

    char *symbols[] = {"Pineapple", "Kiwi", "Orange", "Lime", "Peach", 
    "Lemon", "Pear", "Banana", "Cherry", "Grape", "Blueberry", 
    "Blackberry", "Apricot"};


    int num1, num2, num3;
    int balance;

    printf("Enter initial balance (in cents): ");
    scanf("%d", &balance);


    num1 = (rand() % (13 - 0 + 1)) + 0;
    num2 = (rand() % (13 - 0 + 1)) + 0;
    num3 = (rand() % (13 - 0 + 1)) + 0;
    printf("%s ", symbols[num1]);
    printf("%s ", symbols[num2]);
    printf("%s ", symbols[num3]);
    printf("\n");

    if ((num1 == num2) && (num1 == num3))
    {
        printf("Congratulations! You have won $1");
        balance += 100;
        printf("\n");
    }

    balance -= 5;
    printf("Remaining Balance: %d", balance);

return 0;
}

The error as I said happens randomly and I haven't been able to reliably reproduce it. Sometimes only 1/3 fruit names prints and I get the error code, sometimes it's 2 and sometimes none prints.

c
asked on Stack Overflow Sep 13, 2019 by (unknown user)

3 Answers

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The statement (rand() % (13 - 0 + 1)) + 0; is equivalent to rand() % 14 which will generate random numbers from 0-13 INCLUSIVE. The problem is that you only have 13 strings, so when the rng spits out a 13, the fruit at index 13 doesn't exist.

This can be fixed by either adding another fruit or by changing your rng to rand() % 13

answered on Stack Overflow Sep 13, 2019 by Kevin Montambault
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This:

num1 = (rand() % (13 - 0 + 1)) + 0;

Is the same as:

num1 = rand() % 14;

Which means you're generating a number from 0 to 13. Your symbols array has 13 elements with indexes from 0 to 12. So when you randomly generate the value 13, you're reading past the end of the array. Doing so invokes [undefined behavior]9https://en.wikipedia.org/wiki/Undefined_behavior), which in this case causes your program to crash.

Change that line and the two that follow to:

num1 = (rand() % 13;
num2 = (rand() % 13;
num3 = (rand() % 13;

That way you get a number from 0 to 12.

answered on Stack Overflow Sep 13, 2019 by dbush
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You are randomly accessing an invalid 14th element of symbols array (which has 13 elements).

When you write 

 num1 = (rand() % (13 - 0 + 1)) + 0;

you are trying to calculate symbols index. But this could lead to index 13... and symbols[13] does not exist, and it will translate in accessing a memory location you are not supposed to access.

Write simply

num1 = rand() % 13;

and everything will be fine.

answered on Stack Overflow Sep 13, 2019 by Roberto Caboni • edited Jan 19, 2020 by Roberto Caboni

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