Finding value of 'i' in a for loop?

Question

I have this question here:

What is i after the following for loop? The given code is:

 int y= 0; 
for (int i= 0; i<10; ++i) 
{  
  y+= i; 
}

I put that the answer is 9, but that is incorrect according to the grader. I even printed 'i' and it came out as 9.

`i` doesn't exist after the for-loop because it goes out of scope. — Kon, Jul 15 '15 at 22:58
Are you sure you don't mean the value of y? Because that certainly won't be 9... — James, Jul 15 '15 at 23:00
The last value of i before it cease to exist is 10. That is the value that makes the loop break. — Olivier Grégoire, Jul 15 '15 at 23:01

score 2 · Answer 1 · answered Jul 15 '15 at 22:59

2

The answer is that i is undefined after the loop. It is 9 at the last iteration, though.

answered Jul 15 '15 at 22:59

Alain

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`i` is 10 at the last iteration, but everything else is spot on. – Makoto Jul 15 '15 at 23:00
Guess it depends on how we define "last iteration". If we were to print the value of `i` inside the loop the highest value you'd see is 9, but `i` is assigned the value 10 before the program exits the loop. I think. Now I have to go try it. – Alain Jul 15 '15 at 23:07

UnknownOctopus · Answer 2 · 2015-07-15T23:17:35.170

1

Think of it this way. Using y+=i is constantly adding the current value of y to the value of i. So, in turn, you're not getting the true value of i, but rather the cumulative value.

This is what is actually happening in y+=i

1+2+3+4+5+6+7+8+9

Also, just printing out i after the loop would be invalid since outside that for loop, i no longer exists.

You could just do this:

int y=0;
for(int i = 0; i<10;i++)
  y=i;
System.out.println(y);

edited Jul 15 '15 at 23:17

answered Jul 15 '15 at 22:58

UnknownOctopus

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`y+=i` is `y= y + i`, isn't it? – Vallabh Patade Jul 15 '15 at 23:09
Correct, `y+=i` is basically `y` constantly adding itself to `i` – UnknownOctopus Jul 15 '15 at 23:10

Vallabh Patade · Accepted Answer · 2015-07-15T23:08:30.977

Value of i would be 10. As of now, if you try to print the value of i outside the loop, i is undefined.

When i was 9, you continued with the loop, for next iteration i became 10 and condition fails and causes to break the loop. So value of i is 10. Keep it in mind having stpe statement as ++i or i++ is not different w.r.t the values that i would attend. step statement always executes before start of the next iteration.

Following small change would help you to prove the output.

 int i,y= 0; 
 for (i= 0; i<10; ++i) 
 {  
      y+= i; 
 }
 printf("%d\n",i);

That being said if you are printing the value of i within the loop, then you'll get maximum value of i as 9 on the output. Probably this is what you were doing to conclude the answer as 9.

 int y= 0; 
 for (int i= 0; i<10; ++i) 
 {  
     y+= i; 
     printf("%d\n",i);
 }

Yes, thank you that is exactly what I was doing. I understand now. — ecain, Jul 15 '15 at 23:13

Finding value of 'i' in a for loop?

3 Answers3