How to check the NULL value in NSString in iOS?

Question

I have an NSString and I want to check if it has a NULL value. If it does, then the if condition should execute. Else it should execute the else condition.

Below is the code which I am using:

if ([appDelegate.categoryName isEqual:[NSNull null]])
{
    select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID where ContentMaster.ContentTagText='%@'", appDelegate.tagInput];
}
else
{
    select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'", appDelegate.tagInput, appDelegate.categoryName, appDelegate.topicName];
}

It always executes the else condition, and never the if condition, even when the value is NULL.

Since NSNull is a singleton, you could actually compare pointers directly: if (appDelegate.categoryName == [NSNull null]) — Kumar KL
– Kumar KL, Commented Oct 24, 2013 at 5:13
rather than comparing pointers use to check it is isKindOfClass[NSNull Class]. see my ans — Satish Azad
– Satish Azad, Commented Oct 24, 2013 at 5:22
possible duplicate of What is the right way to check for a null string in Objective-C? — Gabriele Petronella
– Gabriele Petronella, Commented Oct 24, 2013 at 5:23

Jason Coco · Accepted Answer · 2013-10-24 05:14:27Z

36

In Objective-C and Cocoa, the property may not be set—that is, it's nil—or it may be set to the object representation of nil, which is an instance of NSNull. You probably want to check for either of these conditions, like this:

NSString* categoryName = appDelegate.categoryName;
if (categoryName == nil || categoryName == (id)[NSNull null]) {
  // nil branch
} else {
  // category name is set
}

This will execute the nil branch if the categoryName property is set to nil (the default for all properties), or if it's been explicitly set to the NSNull singleton.

answered Oct 24, 2013 at 5:14

Jason Coco

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5 Comments

Queen Solutions Over a year ago

thanks it worked but can you please tell me one thing in else i have given querry it is fine but it does not return anything

Queen Solutions Over a year ago

SQL is select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID= Category.CategoryID LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='Good' && Category.CategoryName='Product' && Topic.TopicName='M2M Operations'

Queen Solutions Over a year ago

this is the log of querry which is executing in else

Jason Coco Over a year ago

@QueenSolutions What do you mean it doesn't return anything? The SQL query doesn't yield results, or are you saying that select is nil?

Jason Coco Over a year ago

@QueenSolutions I suggest positing a new question and put more of the code in the loop in the new question. Ask why the value is not being set there, since this question is about a nil check.

Abhinav · Accepted Answer · 2013-10-24 05:14:45Z

The NULL value for Objective-C objects (type id) is nil.

While NULL is used for C pointers (type void *).

(In the end both end up holding the same value (0x0). They differ in type however.)

In Objective-C:

nil (all lower-case) is a null pointer to an Objective-C object.
Nil (capitalized) is a null pointer to an Objective-C class.
NULL (all caps) is a null pointer to anything else (C pointers, that is).
[NSNull null] (singleton) for situations where use of nil is not possible (adding/receiving nil to/from NSArrays e.g.)

So to check against NSNull one can either use:

if ((NSNull *)myString == [NSNull null])

or if one wants to omit the need of casting to NSNull:

if ([myString isKindOfClass:[NSNull class]])

Satish Azad · Accepted Answer · 2013-10-24 05:20:57Z

Try to use this. Check your value is kind of NULL class or not rather than comparing Pointers value.

if ([appDelegate.categoryName isKindOfClass:[NSNull class]]){

        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID where ContentMaster.ContentTagText='%@'",appDelegate.tagInput];

        }

    else {


      select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID  LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'",appDelegate.tagInput,appDelegate.categoryName,appDelegate.topicName];


    }

It's much more efficient and easier to read to just do a pointer comparison with the NSNull singleton. You also need to check if the property is simply not set (almost certainly the problem here). Avoid doing class comparisons, especially when the majority of the time they're going to fail—that is, this value is not going to be the NSNull singleton.

Darshit Shah · Accepted Answer · 2015-01-27 12:27:01Z

0

#define SAFESTRING(str) ISVALIDSTRING(str) ? str : @""
#define ISVALIDSTRING(str) (str != nil && [str isKindOfClass:[NSNull class]] == NO)
#define VALIDSTRING_PREDICATE [NSPredicate predicateWithBlock:^(id evaluatedObject, NSDictionary *bindings) {return (BOOL)ISVALIDSTRING(evaluatedObject);}]

SAFESTRING("PASS_OBJECT_HERE");

answered Jan 27, 2015 at 12:27

Darshit Shah

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Comments

coreDeviOS · Accepted Answer · 2015-12-29 07:59:43Z

0

Its better to be on safer side in checking null values , as it can lead to crash.

if (![string isKindOfClass:[NSNull class]] && string && string != NULL)

answered Dec 29, 2015 at 7:59

coreDeviOS

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Comments

KSR · Accepted Answer · 2016-08-04 04:48:35Z

Use the following code:

-(void)viewDidLoad {
  [super viewDidLoad];
  //Example - 1
    NSString *myString;
    if([[self checkForNull:myString] isEqualToString:@""]){

        NSLog(@"myString is Null or Nil");

   }
   else{

     NSLog(@"myString contains %@",myString);

   }

 //Example - 2
   NSString *sampleString = @"iOS Programming";
    if([[self checkForNull:sampleString] isEqualToString:@""]){

        NSLog(@"sampleString is Null or Nil");

   }
   else{

     NSLog(@"sampleString contains %@",sampleString);

   }


}

-(id)checkForNull:(id)value{
    if ([value isEqual:[NSNull null]]) {

            return @"";
   }

    else if (value == nil)

            return @"";
    return value;

}

In Example -1, myString contains nothing. So the output is:

         myString is Null or Nil

In Example -2, sampleString contains some value. So the output is:

    sampleString contains iOS Programming

Janak Thakkar · Accepted Answer · 2017-01-07 13:57:04Z

0

+(BOOL)isEmpty:(NSString *)str{
    if (str == nil || str == (id)[NSNull null] || [[NSString stringWithFormat:@"%@",str] length] == 0 || [[[NSString stringWithFormat:@"%@",str] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0){
        return YES;
    }
    return NO;
}

Just pass your string in Method :)

answered Jan 7, 2017 at 13:57

Janak Thakkar

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Comments

TheTravloper · Accepted Answer · 2018-12-22 06:13:54Z

You can do it by using - [NSNull class]

if ([appDelegate.categoryName isEqual:[NSNull class]])
    {
        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID where ContentMaster.ContentTagText='%@'", appDelegate.tagInput];
    }
    else
    {
        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'", appDelegate.tagInput, appDelegate.categoryName, appDelegate.topicName];
    }

Itesh · Accepted Answer · 2013-10-24 05:17:52Z

Add an additional check for length also. This will definitely work.

if ([appDelegate.categoryName isEqual:[NSNull null]] && appDelegate.categoryName.length>0){


        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID where ContentMaster.ContentTagText='%@'",appDelegate.tagInput];

        }        else {
          select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID  LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'",appDelegate.tagInput,appDelegate.categoryName,appDelegate.topicName];

    }

[appDelegate.categoryName isEqual:[NSNull null]] && appDelegate.categoryName.length > 0 you may want to freshen up on boolean arithmetic.
Yeah, that statement as written will cause the app to crash, since by definition if categoryName is the NSNull singleton, it doesn't respond to a length message.

Darshan Kunjadiya · Accepted Answer · 2015-11-18 10:13:56Z

-2

  NSString *str;

  if ([[str stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]]          isEqualToString:@""] || str==nil) 
  {

  }

edited Nov 18, 2015 at 10:13

Darshan Kunjadiya

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answered Oct 24, 2013 at 5:19

Varsha Vijayvargiya

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