Comparing to null

Posted on 2003-11-07
Last Modified: 2010-04-15
how do i say

x != null in C?

and what is the equivilant of the correct statement in C#?

Question by:mwmiller78
LVL 45

Expert Comment

ID: 9702823

C is quite versatile here:

if (x != NULL)

is the same as saying

if (x)


Author Comment

ID: 9702852
what if x = 0?

Expert Comment

ID: 9703022
If your question is
"How will I check if x's value is zero?"
then adding an else construct to what Kdo had written would handle it.

If your question is
"What if i just say   if ( x = 0) ?"
Then 0 will be assigned to the variable x and the expression would be evaluated as the value of x, which will become zero in this case and the statements falling under if will not get executed.
How to improve team productivity

Quip adds documents, spreadsheets, and tasklists to your Slack experience
- Elevate ideas to Quip docs
- Share Quip docs in Slack
- Get notified of changes to your docs
- Available on iOS/Android/Desktop/Web
- Online/Offline

LVL 10

Expert Comment

ID: 9703177
Here's is the explanation

As Kent said

if ( x != NULL )  {
  // some code

is same as saying

if ( x ) {
  // some code

Now it depends on your requirement. If u want that only if x has some non-zero value should some code be executed, then
the above 'if' would handle both the conditions [for NULL as well for 0] i.e. the above 'if' would execute the //some code part only if x has some value other than 0 [non-zero] and it is not NULL

If u want to handle a separate case for 0, then u would have to use the comparison operator ==
U can say

if ( x == 0 ){
// some code

this would check if x is 0, if yes, then it would execute some code part

U can obviously combine the above two conditions

LVL 45

Accepted Solution

Kdo earned 75 total points
ID: 9703702

For all intents and purposes, NULL is zero.  (There may be size differences or casting differences but NULL is zero.)

So adding one more "is the same as"

if ( x != NULL )  {
  // some code

is the same as

if (x != 0) {
  // some code

is same as

if ( x ) {
  // some code

You needn't worry about this, but on some compilers the last example is the most efficient.  The compiler will simply test whether the value of x is zero.

The other examples all produce LOGICAL expressions.  The test in "if (x == NULL)" is a logical test.  A logical test always produced a 1 or 0 answer so the compiler will first evaluate "x == NULL" as a logical expression and convert it to 1 or 0.  The if () will then test whether the value is zero.  This process requires an extra step and a couple more instructions.

Of course, if your compile options include code optimization, then the compiler may reduce all of the tests above to a single instruction.

LVL 23

Expert Comment

ID: 9707641
> what if x = 0?

NULL is usually defined as (void *)0

Featured Post

Find Ransomware Secrets With All-Source Analysis

Ransomware has become a major concern for organizations; its prevalence has grown due to past successes achieved by threat actors. While each ransomware variant is different, we’ve seen some common tactics and trends used among the authors of the malware.

Join & Write a Comment

Preface I don't like visual development tools that are supposed to write a program for me. Even if it is Xcode and I can use Interface Builder. Yes, it is a perfect tool and has helped me a lot, mainly, in the beginning, when my programs were small…
Windows programmers of the C/C++ variety, how many of you realise that since Window 9x Microsoft has been lying to you about what constitutes Unicode ( They will have you believe that Unicode requires you to use…
The goal of this video is to provide viewers with basic examples to understand how to use strings and some functions related to them in the C programming language.
The goal of this video is to provide viewers with basic examples to understand how to create, access, and change arrays in the C programming language.

762 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

22 Experts available now in Live!

Get 1:1 Help Now