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parameters of a function in C

Posted on 2008-10-21
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Last Modified: 2012-05-05
if say I have :

can I do such thing in C? Not in C++ or C#
int foo(int a, int b){
  a = 2;   ============> is this possible?
  return a; ===========> is this possible?
}

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Question by:kuntilanak
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Expert Comment

by:Frosty555
ID: 22773448
I don't see anything wrong with the code you posted. Parameters aren't really anything special. They're just variables that get pre-filled by the caller just before the function runs.

Keep in mind that the variable "a" will be passed by value from the caller. As a result, changing it's value to 2 will not cause anything to happen to the variables on the caller's side.
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Expert Comment

by:jfmador
ID: 22773459
you should use const to avoid modifying a parameter

int foo(const int a, int b) {
   a = 2;  // should not compile
}

but if you don't use const you should be able to modify the value

for the return a;  I don't see why it could not possible
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Author Comment

by:kuntilanak
ID: 22773478
so you're saying that return a; although I assigned a = 2 won't give me anything?
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LVL 31

Expert Comment

by:Frosty555
ID: 22773493
What jfmador is suggesting is a good programming practice - not mandatory.

Your code will work fine, but it is bad programming style to modify a paramter. In theory it should have no impact because the parameter is passed by value, but if you get into the habit of directly modifying parameters, one day you'll write a program, and the parameter will be passed by REFERENCE, and you'll cause bugs you will spend hours trying to fix.

Putting "const" into the parameter forces C to fail with a compile error if you attempt to modify the parameter's value, thereby forcing you to follow this good programming style.

But, syntactically, I don't think there's anything wrong with what you're doing.
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by:sunnycoder
ID: 22773494
return a would return value 2
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Expert Comment

by:Anurag Thakur
ID: 22773511
i dont see anything wrong in you function in C or in C#
you are passing a and b as parameters and they are being passed as normal stack variables and not as references, even if you assign 2 to a and then return it will pose no issues with the compiler and logic.
if you are expecting that the value of the passed variable a changes in the caller function then you might need to use a ref or out (in C#) or use pointers in C

int foo(int a, int b)
{
  a = 2;   ============> is this possible?
  return a; ===========> is this possible?
}
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Author Comment

by:kuntilanak
ID: 22773523
okay.. all I need to know here is just whether this kind of statement is allowed or not.. some statements in C are tricky... having int a in a parameter, is it basically the same as declaring a variable inside the function body?
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Expert Comment

by:sunnycoder
ID: 22773528
Yes it is allowed
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Expert Comment

by:Frosty555
ID: 22773531
Yes. the parameter is no different from any other variables in the function. The only difference is that it's value is filled for you.
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Author Comment

by:kuntilanak
ID: 22773540
ok, if that's the case then doing something like:

will cause a multiple declaration of identifier right?
int foo(int a, int a, int b){
  a = 2;   
  return a; 
}

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Expert Comment

by:Frosty555
ID: 22773548
the compiler should complain that you have two variables named "a", and that is not allowed.

Similarey this is not allowed and should fail at compile time, because "a" has already been defined.
int foo(int a, int b){
  int a = 2;
  return a; 
}

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Expert Comment

by:sunnycoder
ID: 22773552
Yes it would. How would compiler know which a to use?
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Expert Comment

by:Anurag Thakur
ID: 22773560
the code in ID:22773540 will not compile
the variables have to have different names at least
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Author Comment

by:kuntilanak
ID: 22773563
how about in function definition:

int foo(int a, int a);
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Accepted Solution

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Anurag Thakur earned 2000 total points
ID: 22773586
no i will not
try it in your compiler
how can two variables have same name in the same scope.
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Author Comment

by:kuntilanak
ID: 22773591
they are just a definition though :), not the actual function declaration...
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Expert Comment

by:sunnycoder
ID: 22773632
int foo(int , int );
would compile
int foo(int a, int a); wont
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Author Comment

by:kuntilanak
ID: 22773655
ok, thanks all
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