Thread: Check if a variable is null

Simple question, I have a variable "x".
I want to know if that variable has nothing in it or null, to show a message that "You should add something".

I've googled a lot but I didn't find something to help me with that.
Thank you.

To answer your question generically (and hopefully clearly enough):

C doesn't have the general concept of null meaning a/any variable is unset or invalid. Only pointers can be null (the C language defines it as all upper case: NULL), where null is a special address used to signify that the pointer is not pointing to any valid address in memory. However, pointers are not initialized to null by default, you must do so if you want them initialized.

There are more involved ways to create a systems where every variable keeps track of whether it's been used or not (e.g. reference counters), however it requires a lot of work and diligence on the programmers part, and a bit more advanced (or at least intermediate) skills.

However, I think you have a very specific use case in mind, in which case you may be able to just have an extra flag variable you use that keeps track of whether variable x has a value yet. You initialize that flag to false, and once x gets a value, you set it to true. You check that flag, rather than x itself, to display the message.

Originally Posted by iMalc

It depends on the type. What type is x?

Integer, I need something like, if some user wants to print a null variable, print that "Hey! You should add something before showing results!" or something like that.
Few words,

Code:

int x;
if(x = NULL){
printf("Hey! You should add something before showing results!");
}
else {
/* whatever */
}

Thanks for your responses.

try this?

Code:

#include <stdio.h>


int main ()
{
	int x;
	if(x != NULL)
	{
	printf("Hey! You should add something before showing results! \n");
	printf("Input a value");
	scanf("%d",x);
	
	}
	else 
	{
	/* whatever */
	}
	return (0);
}

Originally Posted by Moody Barrawi

try this?

Code:

#include <stdio.h>


int main ()
{
    int x;
    if(x != NULL)
    {
    printf("Hey! You should add something before showing results! \n");
    printf("Input a value");
    scanf("%d",x);
    
    }
    else 
    {
    /* whatever */
    }
    return (0);
}

Thanks for the response, just one question, why is with "!=" instead of "=", it's like if 'x' is different from NULL, if it's different, it means that the variable holds something. In that case, else should have the printf message. That's the way I understand it, I'm right or just speaking non-sense? D:

Originally Posted by c99tutorial

The result of this program is undefined, you must initialize x before testing for its value. Also to the OP, read the answers to the post first for ideas on how to accomplish your goal here. See post #2

So...

Code:

int x = NULL;

?

Right, now I understand, I'll test it and if goes wrong I'll post it.
Thanks.

I initialized the x in 0, and an "if", something like this.

Code:

int x = 0;
if(x == 0){
printf("You should add something.");
}
else {
*/ shows data stored in the other variables /*
}

If it's different than 0 it means that the variables got data when the program asked for it in the other "case" statement.
Thanks for the idea.

Originally Posted by CatBrownie

I initialized the x in 0, and an "if", something like this.

Code:

int x = 0;
if(x == 0){
printf("You should add something.");
}
else {
*/ shows data stored in the other variables /*
}

If it's different than 0 it means that the variables got data when the program asked for it in the other "case" statement.
Thanks for the idea.

Yes, but it would be more clear if you gave your variables meaningful names, like this:

Code:

int x_has_value = 0;  // initialize to 0/false
int x;  // no initial value
// do some stuff that may or may not give x a value
if (!x_has_value)
    print "give x a value"
else
    more things
// do some other stuff that gives x a value
scanf("%d", &x);
x_has_value = 1;  // set to 1/true
if (!x_has_value)
    print "give x a value"
else
    even more things

See, that makes it much easier to read and figure out what the code is doing/supposed to do.