Blank 'For' Loop Giving Output

Question

I encountered a strange FOR LOOP question in the book. Here is the code for the loop -

#include<stdio.h>

int main()
{
int i=1,j=1;
for(;;)
  {
    if(i>5)
        break;
    else
        j+=i;
    printf("%d\n",j);
    i+=j;
  }
return 0;
}

The program prints 2 & 5 as the output. Now could anyone please explain how this for loop is executing?

You could also use a search engine. http://en.wikipedia.org/wiki/For_loop#Use_as_infinite_loops and http://en.wikipedia.org/wiki/C_syntax#Iteration_statements ("Any of the three expressions in the for loop may be omitted") — Karoly Horvath, Dec 24 '13 at 14:48

David Heffernan · Accepted Answer · 2013-12-24T14:53:04.220

That for loop is a classic idiom for a non-terminating loop. It's what you use when you want a loop that never terminates, or whose termination is controlled by control flow statements inside the loop. The latter is the case here.

for(;;)

All parts of the for loop are empty. They key to understanding why it is a non-terminating loop is the empty controlling expression in the for statement.

The standard (6.8.5.3 The for statement) says, with my emphasis:

The statement
for ( clause-1 ; expression-2 ; expression-3 ) statement
behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. If clause-1 is a declaration, the scope of any identifiers it declares is the remainder of the declaration and the entire loop, including the other two expressions; it is reached in the order of execution before the first evaluation of the controlling expression. If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression.158)

Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.

And this means that your loop will never terminate due to the controlling expression part of the for statement.

simonc · Answer 2 · 2013-12-24T15:13:04.500

2

for(;;)

is a for loop which performs no initialisation, has no exit condition and performs no post-action.

It will loop forever unless code inside the loop contains a condition that can result in break being called.

Your loop is equivalent to

int i,j;
for(i=1, j=1 ;i<=5 ;i+=j) {
    j+=i;
    printf("%d\n",j);
}

edited Dec 24 '13 at 15:13

answered Dec 24 '13 at 14:47

simonc

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Fair Enough. Does it mean that the condition part has some non zero value so that it is running infinitely? – Pranav Jituri Dec 24 '13 at 14:50
2

@PranavJituri Yes it does. The standard says that. – David Heffernan Dec 24 '13 at 14:51

score 2 · Answer 3 · edited May 23 '17 at 12:11

2

for(;;) is equivalent to while(true) (or while(1) in old-school C), both of whose terminations are only controlled by the statements inside them.

Edit: To rehash a couple of old jokes (borrowed from here):

Don't use for (;;) {} — it makes the statement cry.
Unless, of course, you #define EVER ;;.

edited May 23 '17 at 12:11

Community

1
1

answered Dec 24 '13 at 14:49

herohuyongtao

49,413
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@PranavJituri In other words, `while (true)` is a bit more readable than `for (;;) {}`. :) – herohuyongtao Dec 24 '13 at 15:09
Aah. That is true mate. – Pranav Jituri Dec 24 '13 at 15:16
On the other hand, in old-school C, there is no true and you'd write while (1) – David Heffernan Dec 24 '13 at 20:47
@DavidHeffernan Could you tell me why is this in old-school C? I don't find any resource about this. – herohuyongtao Dec 25 '13 at 05:45
@hero Early versions of C did not have a boolean data type. – David Heffernan Dec 25 '13 at 07:15

Blank 'For' Loop Giving Output

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