C Program: Why does this array have this output?

Question

Ok so I have never taken a C programming course and I have a general idea of what Arrays are, but I cant seem to figure out why this array has a certain output.

#include <stdio.h>
#include <time.h>
#include <stdlib.h>  // include srand function
#pragma warning(disable : 4996) 
#define Size 15
main() {
    int a[Size];
    int i;
    double b1[Size], b2[Size];
    srand( (unsigned)time( NULL ) );  // Use current time as seed
    // apply (start) timestamp 
       for (i = 0; i < Size; i++)
    {
        a[i] = rand() % 100; 
              // initialize the array using random number between 0 and 99
        b1[i] = a[i]/5;
        printf("a[%d] = %d\n", i, a[i]);
        printf("b1[%d] = %f\n", i, b1[i]);
        b2[i] = a[i];
        b2[i] = b2[i]/5;
        printf("b2[%d] = %f\n", i, b2[i]);
    }
    // apply (end) timestamp 
    //compute the time elapsed and display time in seconds
}

For my homework I need to run this code and explain why b1 and b2 have same or different values. I cant seem to wrap my head around what is happening here:

       for (i = 0; i < Size; i++)
    {
        a[i] = rand() % 100; 
              // initialize the array using random number between 0 and 99
        b1[i] = a[i]/5;
        printf("a[%d] = %d\n", i, a[i]);
        printf("b1[%d] = %f\n", i, b1[i]);
        b2[i] = a[i];
        b2[i] = b2[i]/5;
        printf("b2[%d] = %f\n", i, b2[i]);
    }

If someone could explain what the code is doing here I would very much appreciate it. Thanks!

Edit- This is the output once I run the program:

a[0] = 8
b1[0] = 1.000000
b2[0] = 1.600000
a[1] = 74
b1[1] = 14.000000
b2[1] = 14.800000
a[2] = 78
b1[2] = 15.000000
b2[2] = 15.600000
a[3] = 64
b1[3] = 12.000000
b2[3] = 12.800000
a[4] = 53
b1[4] = 10.000000
b2[4] = 10.600000
a[5] = 6
b1[5] = 1.000000
b2[5] = 1.200000
a[6] = 71
b1[6] = 14.000000
b2[6] = 14.200000
a[7] = 4
b1[7] = 0.000000
b2[7] = 0.800000
a[8] = 7
b1[8] = 1.000000
b2[8] = 1.400000
a[9] = 57
b1[9] = 11.000000
b2[9] = 11.400000
a[10] = 55
b1[10] = 11.000000
b2[10] = 11.000000
a[11] = 13
b1[11] = 2.000000
b2[11] = 2.600000
a[12] = 55
b1[12] = 11.000000
b2[12] = 11.000000
a[13] = 96
b1[13] = 19.000000
b2[13] = 19.200000
a[14] = 25
b1[14] = 5.000000
b2[14] = 5.000000

What values do you see printed out?

Peter M
– Peter M

2015-01-31 21:45:38 +00:00
Commented Jan 31, 2015 at 21:45 — Peter M
– Peter M, Commented Jan 31, 2015 at 21:45

myaut · Accepted Answer · 2015-01-31 21:54:59Z

3

Because you implicitly convert int to double and both times do that different ways.

b1[i] =   a[i]   / 5   ;
/*     ^  ^ int    ^ 
 *     |           +-- int
 *     +-- implicit convertion to double */

b2[i] =  b2[i]     /  5   ;
/*        ^ double    ^
 *                    + int implicitly converted
 *                      to double */

In first expression you divide int by int where you lose precision (i.e. 7 / 2 gives you 3, not 3.5),
In second expression you divide double by double. That doesn't lead to that.

edited Jan 31, 2015 at 21:54

answered Jan 31, 2015 at 21:48

myaut

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1 Comment

Rakeen Huq Over a year ago

Aha! Yes thank you that makes sense to me. I can understand why the values printed out are different. b1 only prints out values as int whereas b2 prints out values as a double which is why they are all in decimal. It divides the value in array a[] at the certain index by 5 and thats how b1 and b2 get their values. Is my thought process here right?

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