#define function

Hello everyone. I started learning C and everything till now is perfect. But there is something which i couldn't completely understand.
Why do we use the #define function ?
what is the difference between these two :

age = 20;

and

#define AGE 20;

i couldnt understand what is the use of #define function. I hope anyone could explain it to me.
Thanks!

It's not a function. It's a "preprocessor instruction". They can be function-esque.* The preprocessor goes through the code before it is compiled. What it does is literally replace an instance of a #define label with it's output. So in your example, anywhere the label "AGE" occurs in the code, the preprocessor replaces it with the number 20. This is very handy if you want to say, use a file in a number of different places:

Code:

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#define INPUT_FILE "thatdata.txt"

---

Now, if you want to change the file used, rather than search and replace the name everywhere, you use the define label in it's place, and all you have to do is change that one #define.

There are also defines that work like functions, called macros. So for example:

Code:

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#define MAX(a, b)  (a)>(b) ? a : b

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This uses the ternary operator -- of the form condition ? true : false -- to output the higher value. So if you use this macro somewhere:

Code:

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int x = 5, y = 6, z = MAX(x,y);

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The preprocessor will replace that with 6.

* actually you can write more or less literal C functions into a define but they have some logical limitations.

To follow up on MK27's post, something I consider important. Take his example:

Code:

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#define MAX(a, b)  (a)>(b) ? a : b

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And the following piece of code:

Code:

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c = MAX(a++, b++);

---

This actually becomes:

Code:

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c = (a++)>(b++) ? a++ : b++;

---

So you'll get incorrect and unexpected results ;-). So best avoid using macro's, most of the time.

Quote:

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Originally Posted by EVOEx

So best avoid using macro's, most of the time.

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Between you & Elysia we'll all be avoiding programming soon -- it is just too error prone :p

Quote:

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Originally Posted by MK27

Between you & Elysia we'll all be avoiding programming soon -- it is just too error prone :p

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Not at all. Programming is finding the right tool for the right problem. In case of the max function, it may sometimes be useful using a macro (if it needs to work with any type). I'd prefer using C++ with a template function, however. But for pure C, yeah, MAX should probably be a macro.
But still; be careful with macro's.

Quote:

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Originally Posted by EVOEx

I'd prefer using C++ with a template function

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Heck, I'd prefer to just use a dynamically typed language with a nice foolproof built-in. But that is not very useful advice for someone trying to learn C.

Quote:

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I'd prefer to just use a dynamically typed language with a nice foolproof built-in.

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O_o

Really?

Soma

Quote:

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Originally Posted by phantomotap

O_o

Really?

Soma

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Yeah, I'm not religious so have no commitment to any particular church. Altho to be honest a macro for MAX will do me just fine. Danger be damned!

Quote:

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I'm not religious so have no commitment to any particular church.

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O_o

Oh? This has taken a weird turn. So all those "C is better than C++" posts of yours, those before and after trying to learn C++, are hyperbole? I never realized. You defend hyperbole with a strange aggression.

Soma

Quote:

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Originally Posted by phantomotap

all those "C is better than C++" posts of yours, those before and after trying to learn C++,

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There aren't any. Read more carefully. I have made some grouchy observations about specific details of C++, such as the defanging of void*, and tried to provide some perspective on "C++ salemanship", but I have never said anything to the effect that one language is ultimately "better" than the other.

True. You probably didn't ever use the word "better" or any other comparative. Since when does one need to use a comparative to proselytize?

Soma

Code:

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int age = 20

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is declaring a variable.

Code:

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#define age 20

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simply replaces every occurance of "age" with "20".

#define code helped me a lot :) Thanks everyone

C is getting harder now. I copy the same code that i read from the book but it doesn't work for me.

Then you have to show us the code you're copying. Otherwise we can't help.