how does if (i++) condition work?

Question

Possible Duplicate:
Post Increment and Pre Increment concept?

I cant understand how the "if condition" works with the increment/decrement operator in this scenario:

#include<stdio.h> 
void main() 
{ 
  int n=0; 
  if(n++) 
  { 
    printf("C-DAC"); 
  } 
  else if(n--) 
  { 
    printf("ACTS"); 
  } 
}

Its output is ACTS.

what is happening in the IF condition?

please indent your code - all in column 1 makes it very hard to read - thanks. — Levon, Jun 15 '12 at 11:50

score 24 · Accepted Answer · edited Jun 15 '12 at 17:06

24

if (n++) It checks if n is not equal to zero and then increments n

else if (n--) It checks if n is not equal to zero and then decrements n

Your first if statement is not true (because n is zero), then n is incremented, and else if statement is checked (n is equal to 1 at this point), if (1) is true and printf("ACTS") is called

edited Jun 15 '12 at 17:06

tomlogic

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answered Jun 15 '12 at 11:50

Ribtoks

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I am sorry, but it is not specified anywhere in the program that it has to check whether n!=0... how does it understand this ?? – Anvay Jun 15 '12 at 11:57
Statement `if (n)` in `c++` is equal to `if (n != 0)` statement – Ribtoks Jun 15 '12 at 11:57
1

There is actually no if condition at all, at least not ouside a debug build. As the program is written, the compiler can trivially prove at compile time that the first test is always false and the second test is always true, and the variable is not otherwise used. Thus it will reduce the program to `int main() { printf(ACTS"); return 0;}` – Damon Jun 15 '12 at 12:01
@Damon true, but we're talking about c++ itself, but not about compiler optimizations – Ribtoks Jun 15 '12 at 12:02
@Anvay If Ribtok's answer solved your problem you should [**accept it**](http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work/5235#5235) by clicking the checkmark next to this answer. This will give you both some rep points and also mark this problem as solved – Levon Jun 15 '12 at 12:16
Thanx @Levon .. I didn't know this..!!! Accepted his answer now..! – Anvay Jun 15 '12 at 12:22

Levon · Answer 2 · 2012-06-15T12:05:15.917

Both n++ and n-- are post-increment/decrement operations.

That means the value of the variable is used for evaluation in the current expression first and only then changed, (i.e., incremented/decremented afterward).

0 means FALSE in a Boolean expression. Since n is initially 0, the test in your first if fails and the else clause is evaluated.

At that point n has become 1 due to the n++ in the first test, so this results in this test becoming true and

printf("ACTS");

to be executed.

score 3 · Answer 3 · answered Jun 15 '12 at 12:23

3

if(0++) ==> if(0) else if(1--) ==> else if(1)

so ACTS is printed

answered Jun 15 '12 at 12:23

Elavarasan

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score 2 · Answer 4 · answered Jun 15 '12 at 11:52

2

int n=0; 
if(n++)

Meaning: check and then increment by 1.

int n=0; 
if(++n)

Meaning: increment by 1 and then check.

answered Jun 15 '12 at 11:52

Dzha

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score 2 · Answer 5 · answered Jun 15 '12 at 11:54

2

An int always gives true, unless it is zero. The postincrement operator, i++, increments the i variable after the check. If you had used the preincrement operator, ++i, the incrementation would take place before the check.

answered Jun 15 '12 at 11:54

EvenLisle

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score 2 · Answer 6 · answered Jun 15 '12 at 11:55

2

while checking your IF part, value of n is '0'. And because of the i++ , 'n' increments by 1. So, now the value of n is 1. So, while entering ELSE part, as value of n is 1 , it executes ELSE part. And you are getting the output of the ELSE part.

answered Jun 15 '12 at 11:55

Tirumudi

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score 1 · Answer 7 · answered Jun 15 '12 at 11:51

1

It first checks the condition that n != 0 and after that it will increment the value of n

answered Jun 15 '12 at 11:51

mathematician1975

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score 1 · Answer 8 · answered Jun 15 '12 at 11:53

1

The ++ and -- operator not only modify the value, they also have a return value. Behaviour is different depending on the position: ++var increases and returns the increased value, whereas var++ increases but returns the old value.

answered Jun 15 '12 at 11:53

mensi

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I'm pretty sure that `++` and `--` do not 'have a return value'. They are modifiers, not functions. – KevinDTimm Jun 15 '12 at 14:30

score 1 · Answer 9 · answered Jun 15 '12 at 13:04

1

because i is 0. ++i would increment before evaluation, i++ increments after

answered Jun 15 '12 at 13:04

SetSlapShot

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score 0 · Answer 10 · answered Jun 15 '12 at 11:52

0

The compiler reads the command in the left to right fashion. Meaning first it checks if the condition is true of false

if (n)

and then increments/decrements

 n = n +/- 1

answered Jun 15 '12 at 11:52

Zain Khan

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how does if (i++) condition work?

10 Answers10