CODEWITHSUNDEEP
cfor-loopscanf
Rohit Saluja
Rohit Saluja

Reputation: 1517

Will this for loop run infinitely?

I have some experience with the c programming but sometimes the author of some books just pose such strange syntactical question which we will never use in actual programming and similar to that I stuck on one such question though the answer is given but I am not able to understand the answer.Here is the code below

int main
{
int i;
for(;scanf("%d",&i);printf("%d",i)){
     ;
    }
}

The question is how many times this for loop will run and the answer given is indefinite Can anyone explain how the loop will get executed

Upvotes: 3

Views: 160

Answers (5)

chux
chux

Reputation: 154065

After accept answer

Yes this very well may be an infinite loop.

scanf() returns the number of successfully scanned fields (0 or 1 ) here or EOF on IO error or end-of-file.

The only way to exit the loop is to enter a non-numeric input like "xyz".

Should user close stdin, scanf() will return EOF. EOF is a negative value (not 0). So the loop will continue calling scanf(), continue receiving EOF until the program is killed.

#include <stdio.h>

int main(void) {
  int i;
  for (; scanf("%d", &i); printf("%d", i)) {
    ;
  }
  return 0;
}

Upvotes: 0

Sourav Ghosh
Sourav Ghosh

Reputation: 134356

No. This is not an infinite for loop.

As per the syntax of for loop, from section 6.8.5.3 of the n1256 TC3 C99

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....

In your case, expression-2 is scanf("%d",&i), and the return value of scanf() will be considered for evaluation of expression-2.

As per the man page of scanf()

Return Value

... return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.

So, in your case,

  • till time scanf() is success, it will number of input items successfully matched, which is 1 and a TRUE value, for loop will continue execution.

  • if the scanf() fails [example: input char value], the return value of scanf() will be 0 [matching failure] and the loop will terminate.

Upvotes: 1

Venkata Krishna
Venkata Krishna

Reputation: 1776

Here, indefinite means that it is cannot be defined(unknown), it does not mean infinite (just to clear things out). Because, here the no of times the loop is run is dependent on the input given. If a correct input is given, which is an integer, the loop keeps running. If a wrong input is given, that is a non integer, the control jumps out of the loop. So you cant have a single answer for all scenarios, hence it is indefinite.

Upvotes: 2

Karthikeyan.R.S
Karthikeyan.R.S

Reputation: 4041

Scanf will return the no of successful input get. If you give the input correctly then the condition will be true so it keep on running as indefinite.

To make the condition false you can give the input other than the integer value. That time scanf will return zero. So the loop will end.

Upvotes: 3

Gopi
Gopi

Reputation: 19874

scanf returns number of elements successfully read and in this case it will be 1 if it reads a integer and keeps printing it.

Let's say you pass a character here then scanf() fails and exits from a for loop

Upvotes: 6

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