Confusing output after use of increment operator

Question

#include <stdio.h>

main()
{   
    int a[5] = {5,1,15,20,25};
    int i,j,m;
    i = ++a[1];
    j = a[1]++;
    m = a[i++];
    printf("%d %d %d\n",i,j,m);
}

Okay now the program compiles and runs fine. But the output which i get is 3 2 15 i.e i =3 , j=2, m = 15. I don't understand how come the value of i becomes 3 as a[1] i.e '1' when incremented must become 2. This is a 'trace the output' type of question. I'm using gcc.

ajay · Accepted Answer · 2014-04-20 05:02:12Z

2

I have written the values of the variables before and after the statement gets executes and all side effects have taken place.

int a[5] = {5, 1, 15, 20, 25};
int i, j, m;

// before : i is indeterminate, a[1] = 1
i = ++a[1];  
// after: i = 2, a[1] = 2

// before: j indeterminate, a[1] = 2
j = a[1]++;  
// after: j = 2, a[1] = 3

// before: m indeterminate, i = 2
m = a[i++];  
// after: m = a[2] = 15, i = 3

Therefore, the final values are

i = 3
j = 2
m = 15

edited Apr 20, 2014 at 5:02

answered Apr 20, 2014 at 4:56

ajay

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Comments

R Sahu · Accepted Answer · 2014-04-20 04:53:03Z

2

When the following line is executed,

m = a[i++];

i is incremented again, making its value 3.

answered Apr 20, 2014 at 4:53

R Sahu

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2 Comments

user3553190 Over a year ago

oh yes. Thank you and thts why m return a[2] and thats 15 as i++ is post increment. m is assigned the value before i is incremented. But then j must be equal to '1' as value will be assigned first and then it will increment isnt it ?

R Sahu Over a year ago

The line j = a[1]++; can be interpreted as j=a[1];a[1] = a[i]+1;. The value of the expression a[1]++, which is assigned to j, is the value of a[1] before a[1] is incremented.

user3553031 · Accepted Answer · 2014-04-20 04:57:45Z

0

On line 7, you have

i = ++a[1];

This increments a[1] (to 2), then assigns that to i.

Then on line 9, you have

m = a[i++];

This increments i (to 3), then assigns a[3] (15) to m.

answered Apr 20, 2014 at 4:57

user3553031

Comments

Shahidul · Accepted Answer · 2014-04-20 05:00:15Z

0

i = 3 because

i = ++a[1] => i = ++1 and so i,a[1] are both 2
m = a[i++] => i = 3 because of i++

answered Apr 20, 2014 at 5:00

Shahidul

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