Last element of array changes when finding minimum

Question

I have written a program to input 3 integers and find the mean, largest and smallest value:

#include <stdio.h>

int main(void) {
    int array[2];   /* declares array */
    int min = 99;
    int max = -99;
    float mean;
    int i;

    printf( "Please enter three numbers seperated by spaces: " );
    scanf( "%d %d %d", &array[0], &array[1], &array[2] );

    mean = ( array[0] + array[1] + array[2] ) / 3; /* calculates mean */

    for( i = 0; i < 3; i++ ) {
        /* displays array elements at the start of each loop */
        printf ( " i = %d begin \t %d %d %d \n", i, array[0], array[1], array[2]); 

        /* finding the maximim */
        if ( array[i] > max ) {
            max = array[i];
        }

        /* displays array elements in the middle of each loop */
        printf ( " i = %d mid \t %d %d %d \n", i, array[0], array[1], array[2]);

        /* finding the minimum */
        if ( array[i] < min ) {
            min = array[i];
        }

        /* displays array elements at the end of each loop */
        printf ( " i = %d end \t %d %d %d \n", i, array[0], array[1], array[2]);
    }

    printf( " final =  %d %d %d \n", array[0], array[1], array[2] );
    printf(" min =  %d \t max = %d \t mean = %.1f \n",  min, max, mean );

    return 0;
}

I have written it so that the array elements are displayed at beginning and end of each iteration of the loop.

This is the output I get:

Please enter three numbers seperated by spaces: 5 6 7
 i = 0 begin     5 6 7
 i = 0 mid       5 6 7
 i = 0 end       5 6 5
 i = 1 begin     5 6 5
 i = 1 mid       5 6 5
 i = 1 end       5 6 5
 i = 2 begin     5 6 5
 i = 2 mid       5 6 5
 i = 2 end       5 6 5
 final =  5 6 5
 min =  5        max = 6         mean = 6.0

Notice that the last element of the array changes when

/* finding the minimum */
if ( array[i] < min ) {
    min = array[i];
}

is executed. This therefore affects the smallest value and largest value output. Any idea why this happens?

Stephen Docy · Accepted Answer · 2018-03-16 05:00:06Z

1

You declare your array to hold only 2 elements:

int array[2];   /* declares array */

The array needs to be able to hold 3 elements

int array[3];   /* declares array */

Trying to write past the end of the array results in undefined behavior.

Array indices can be a little confusing at first.

int array[2];

means two elements, and since indices start at [0], valid indices you can access are [0] and [1]

answered Mar 16, 2018 at 5:00

Stephen Docy

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