function returning optional value

Whenever we call a function returning value why it is not required to catch the value? consider the following C code,

int main()
{
      int i;
      scanf("%d",&i);
      printf("Value of i is: ",i);
      return 0;
}

Here scanf() returns value 1, bu开发者_运维百科t as it is not catched in anywhere why didn't the error pops up? What is the reason to allow such programming?

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Primarily because expressions in C also yield values. For example: x = 1; yields the value 1. Sometimes you use that for multiple assignment like x = y = 1;, but more often you don't.

In early C, the void return type hadn't been invented either, so every function returned some value, whether it was generally useful or not (for example, your call to printf also returns a value).

The rules of the language don't make this an error (doing so would lose compatibility with virtually existing code) and since this is common and rarely indicates a problem, most compilers don't warning about it either. A few lint tools do, which has led a few misguided programmers to write things like (void)printf("whatever"); (i.e., casting the unused return to void to signal that it really, truly was intentional when it was ignored. This, however, rarely does any good, and frequently does quite a bit of harm, so (thankfully) it's rarely seen.

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Functions that return a value have that functionality for the use and convenience of the programmer. If you aren't interested in the return value, then don't use it.

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Do you need the return value in this example? NO. So you have not used that in this case. But in another situation the return value might be important. For example if you want to read as long as some integer in available in input stream, then you can do something like this:

while (scanf("%d", &i) == 1) {
    // do something
}

If there is an EOF then this loop will break. Here return value is needed.

So the summary is use return value when needed, and don't use when not needed. Such programming is allowed because both scenario is possible.

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A lot of library functions return values you might not think about. Imagine having to use the return value of every printf call! (printf returns the number of characters printed.)

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Pretty much every native c function returns a int. scanf and printf included. It would be really annoying if you always had to "capture" it to satisfy the compiler; In a large program you would end up creating thousands of variables just for storing return values that you never look at.

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The reason is that C has no other established way of handling errors than by return value. These returned values, that is to say those return to report success or failure, should almost always be checked (unless you're just doodling around or you have a proof that the function will not fail).

Now since return values are also used for other things than returning success/failure information there might be, and are, situations where you will not be interested in the value a function returns, but just the side effects of executing it. In this case forcing the programmer to inspect/bind the returned value would become quite tideous.

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Simple: if you don't use the return value, you don't use it.

It is not mandated that you do.

(That said, in a case like this, you should: you have no idea at present whether scanf encountered an error.)

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It is not the case that everytime a value is evaluated,& it must be stored or returned,because the value you have obtained may be used by some other functions,to evaluate different kind of things.A value may be a measure...for example consider the following simple programm,where we want to check that the number entered is even or not

'int main()
{
 int a;   
 printf("Enter a number");
 scanf("%d",&a);
 if(a%2==0)
 printf("even number");
  else
 printf("odd no");
  return 0;
  }'

here the variable 'a' is not necessarily to be returned,because we just want to check that the number is even or odd...no need of returning