conventions - Using constants instead of numbers - c

Question

I'm currently learning c, and our teacher told us we should never use plain numbers in code, and always use constants.

For example:

Don't do this:

if (age >= 18) {...}

Do this:

#define MIN_AGE 18
// ...
if (age >= MIN_AGE) {...}

They did not give us any reasoning for why to do this, and I'm left confused. Is this actually recommended? And why?

Answer 1

The reason to use variables is that is much easier for maintenance and visualization. In the code without the variable that you showed, you would not have much of a problem changing the value directly in the if statement because you're using it only once.

if (age >= 18) {...}

So if you need to change the value to 17 (as an example) you could just do this:

if (age >= 17) {...}

But imagine if you had a lot more if statements in your code, like in the example below:

if (age >= 18) {...}
if (age >= 18) {...}
if (age >= 18) {...}

You would need to change it in every statement, one by one. Using a variable would be a lot easier because you could just change the value assigned to the variable:

#define MIN_AGE 17

And all the other MIN_AGE variables would be already correct:

if (age >= MIN_AGE) {...}
if (age >= MIN_AGE) {...}
if (age >= MIN_AGE) {...}

Besides, that is a lot easier to understand the meaning of MIN_AGE, the code will better to read and understand.

Sorry for my bad English btw!