Need help in Understanding an output problem of C++

Sanyam Khurana · 2013-07-03T20:25:21+00:00

Hey, I saw a problem to know the output of the below mentioned code.. ‎#include<stdio.h> int fun() { static int num = 40; return num--; } int main() { for(fun(); fun(); fun()) { printf("%d ", fun()); } return 0; }Now my problem is that, I've never seen that we can use a function call in initialisation, updation and conditional expression of a for loop. So, first of all, is this correct? and how we judge the output..?

Need help in Understanding an output problem of C++

Sanyam Khurana
@sanyam-Nl7Zqc

Updated: Jul 3, 2013

Views: 1.1K
Hey,

I saw a problem to know the output of the below mentioned code..
```
‎#include<stdio.h>
int fun()
{
static int num = 40;
return num--;
}
 
int main()
{
for(fun(); fun(); fun())
{
printf("%d ", fun());
}
 
return 0;
}
```
Now my problem is that, I've never seen that we can use a function call in initialisation, updation and conditional expression of a for loop.

So, first of all, is this correct?

and how we judge the output..?
0

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Sanyam Khurana

Member • Jul 3, 2013

#-Link-Snipped-# and #-Link-Snipped-#

Can you help in this..?

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Kaustubh Katdare

Administrator • Jul 3, 2013

Tagging #-Link-Snipped-#

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rahul69

Member • Jul 3, 2013
Sanyam Khurana
#-Link-Snipped-# and #-Link-Snipped-#

Can you help in this..?
OK , let me answer your questions one by one

So, first of all, is this correct?
Yes this is technically correct, i.e it will not give any errors.
Reason being, we can write any statement inside for loop initialisation . We can even leave it empty, see for example:
```
for(;;;);
```
is a valid statement. 😀

and how we judge the output..?

For judging the output, let me explain the working of a for loop in a general manner:
1. Initialization condition;
2. loop(test condition)
3. {
4. body statements.....
5. ...
6. updation (ie increment etc.);
7. }
which is written in a for loop as :

for(Initialization; test; updation)

{

body statements...

}

Notice above that initialization statement is executed only once

Now in the code u mentioned, there can be many doubts which u may face as this code uses some tricky concepts as static variable , post increment operator etc, so try solving the output, it will be along the lines of :

38,35,32.... upto 2 .

If u face any problem do mention it, I'll be happy to help 😀
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Sanyam Khurana

Member • Jul 3, 2013

I'm not getting it, what is initialized by the function, how it is updated, and what condition is being checked..??

Please help 😕

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Sanyam Khurana

Member • Jul 3, 2013

Ok, let's take it step by step here

in Initialisation, we have fun(); so, it would return 40 and then in memory it would have value 39 due to post increment, but where is this value returned by the function going to be stored, I mean, we have to store the value in a variable which is returned by the function so as to use it, but here no variable is there..?

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Jeffrey Arulraj

Member • Jul 4, 2013

The fun() returns a value 39 and that is assigned to the control variable of the for loop

I think you can look into the function of return() statement for info #-Link-Snipped-#

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Sanyam Khurana

Member • Jul 4, 2013
Conqueror
The fun() returns a value 39 and that is assigned to the control variable of the for loop

I think you can look into the function of return() statement for info #-Link-Snipped-#
Wouldn't it first return the value of num and then decrement it in the memory, due to post decrement operator..?

As in, it would give value of 40 only, but in memory the value would be 39.

Now, whatever value may the function has given, but in order to run the for loop, we must have some initialisation condition and test and update expression as in
```
.
//some header files
void main(){
//assume some code
for(int i=1;i<10;i++){
//assume some code 
}
}
```
Now, whatever value is returned, but shouldn't that be stored in a variable ?

because we have to continuously use it in iteration?
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rahul69

Member • Jul 4, 2013
Sanyam Khurana
Wouldn't it first return the value of num and then decrement it in the memory, due to post decrement operator..?
As in, it would give value of 40 only, but in memory the value would be 39.
Yes u are right 😀.
Sanyam Khurana
Now, whatever value may the function has given, but in order to run the for loop, we must have some initialisation condition and test and update expression as in
```
.
//some header files
void main(){
//assume some code
for(int i=1;i<10;i++){
//assume some code
}
}
```
Now, whatever value is returned, but shouldn't that be stored in a variable ?

because we have to continuously use it in iteration?
See, as I told earlier these three expressions viz. initialization, test and update are not mandatory conditions, u may not initialize if u don't want to, same goes for other two, we write these conditions as per the need of our program.
Lets take each condition one by one:

We test to come out of loop, test means either true (non zero) or false (zero value). Here whenever function will return zero, loop will end, so no need to store it😀.

We update the value so that test condition can be fulfilled, now as the value is changing by means of fun() function. So no need of update too.

And, no need to initialize here as the variable is initialized to 40 in the fun() function.
Hope it clears your doubts 😀
-Rahul
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Sanyam Khurana

Member • Jul 4, 2013

rahul69
Yes u are right 😀.

See, as I told earlier these three expressions viz. initialization, test and update are not mandatory conditions, u may not initialize if u don't want to, same goes for other two, we write these conditions as per the need of our program.
Lets take each condition one by one:

We test to come out of loop, test means either true (non zero) or false (zero value). Here whenever function will return zero, loop will end, so no need to store it😀.

We update the value so that test condition can be fulfilled, now as the value is changing by means of fun() function. So no need of update too.

And, no need to initialize here as the variable is initialized to 40 in the fun() function.
Hope it clears your doubts 😀
-Rahul

Yes, I got it, hows it executed, in each iteration, when update expression is executed it call the fun function , again value of num decrements, and then in test condition, it again decrements, and finally when output is given the function is called and value of num is one less than that...

Yes, I got it now..

Thanks man..!!! 🎉

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simplycoder

Member • Jul 4, 2013

I think I am slightly late here, but anyways, this was a good example.

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Vishal Sharma

Member • Jul 4, 2013

me too! returned home now 😛

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Sanyam Khurana

Member • Jul 4, 2013

simplycoder
I think I am slightly late here, but anyways, this was a good example.
Vishal0203
me too! returned home now 😛
Don't get disappointed if I have or someone else have more doubts they will ask you here.. 👍

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