Check If a Variable Is Empty

check if variable empty

If you want to test whether a variable is really NULL, use the identity operator:

$user_id === NULL  // FALSE == NULL is true, FALSE === NULL is false
is_null($user_id)

If you want to check whether a variable is not set:

!isset($user_id)

Or if the variable is not empty, an empty string, zero, ..:

empty($user_id)

If you want to test whether a variable is not an empty string, ! will also be sufficient:

!$user_id

How can I determine if a variable is 'undefined' or 'null'?

You can use the qualities of the abstract equality operator to do this:

if (variable == null){
    // your code here.
}

Because null == undefined is true, the above code will catch both null and undefined.

Is there a standard function to check for null, undefined, or blank variables in JavaScript?

You can just check if the variable has a truthy value or not. That means

if( value ) {
}

will evaluate to true if value is not:

• null
• undefined
• NaN
• empty string ("")
• 0
• false

The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.

Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance

if( typeof foo !== 'undefined' ) {
    // foo could get resolved and it's defined
}

If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.

fish shell. How to check if a variable is set/empty?

• set -q var (note the missing "$" - this uses the variable name) can be used to check if a variable has been set.

• set -q var[1] can be used to check whether the first element of a variable has been assigned (i.e. whether it is non-empty as a list).

• test -n "$var" [fn0] (or [ -n "$var" ]) can be used to check whether a variable expands to a non-empty string (and test -z is the inverse - true if it is empty).

These will be true/false in slightly different circumstances.

When no set var has been performed at all (and it has not been inherited from the parent process), set -q var, set -q var[1] and test -n "$var" will be false, test -z "$var" will be true.

When something like set var has been done (without any additional arguments), set -q var will be true, set -q var[1] will be false.

When something like set var "" has been done, both set versions will be true.

When something like set var "somestring" (or even set var "" "" [fn1]) has been done, the sets will be true and test -z "$var" will be false.

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[fn0]: You never want to use test (or [) without quoting the variable. One particularly egregious example is that test -n $var will return true both if the variable contains something and if it is list-empty/unset (no set at all or set var without arguments). This is because fish's test is one of the few parts that follow POSIX, and that demands that test with any one argument be true. Also it does not handle lists properly - test -n $var will have weird results if var has more than one element.

[fn1]: This is because a list will be expanded as a string by joining the elements with spaces, so the list consisting of two empty strings will expand to " " - one space. Since that isn't empty, test -z returns false.

I'm am trying to check to see if a variable is empty or not in C

A variable resides in RAM. RAM is never empty. Each cell has a value. Either the value is known by the programmer or not, depends whether the programmer has initialized that variable or not.

I know two ways of signaling the "emptyness" of a variable. One is to have a value that means "empty", like "0", or MAX_INTEGER, or whatever you like. This works if the logic of your algorithm imposes your variable to have only certain values, so you can know when the value is valid or not. If not, you can say the variable is not valid, or empty.

If your variable can hold any value (within the limits of your type), then a solution may be to use a small struct, like this:

typedef struct
{
  int value;
  int is_empty;
} tVar;

So declaring variable i this way...

tVar i;

You can initialize as empty, like this:

i.is_empty = 1;

So, your program becomes:

int main( void )
{
  tVar someVar = {0,1};  // declaring and initializing it as empty

  ...
  ...
  ...    
  if (!someVar.is_empty) // if someVar is not empty...
  {
     someVar.value = -1;
     someVar.is_empty = 0; // is not empty any more
  }
  else if (someVar.is_empty && (some_other_condition))
  {
     someVar.value = 1;
     someVar.is_empty = 0;
  }

  return 0;
}

How to check whether a str(variable) is empty or not?

You could just compare your string to the empty string:

if variable != "":
    etc.

But you can abbreviate that as follows:

if variable:
    etc.

Explanation: An if actually works by computing a value for the logical expression you give it: True or False. If you simply use a variable name (or a literal string like "hello") instead of a logical test, the rule is: An empty string counts as False, all other strings count as True. Empty lists and the number zero also count as false, and most other things count as true.

how to check if the variable is null?

you can use .equals("") or .isEmpty()

check check if the variable is null

Check if variable is the empty string

'' is an empty character. It does not mean “completely empty” – that is indeed NULL.

To test for it, just check for equality:

if (variable == '') …

However, the error you’re getting,

missing value where TRUE/FALSE needed

means that there’s a missing value in your variable, i.e. NA. if cannot deal with missing values. An NA occurs as a result of many computations which themselves contain an NA value. For instance, comparing NA to any value (even NA itself) again yields NA:

variable = NA
variable == NA
# [1] NA

Since if expects TRUE or FALSE, it cannot deal with NA. If there’s a chance that your values can be NA, you need to check for this explicitly:

if (is.na(variable) || variable == '') …

However, it’s normally a better idea to exclude NA values from your data from the get-go, so that they shouldn’t propagate into a situation like the above.

How to find whether or not a variable is empty in Bash

In Bash at least the following command tests if $var is empty:

if [[ -z "$var" ]]; then
   # $var is empty, do what you want
fi

The command man test is your friend.

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