For example, I have #"John", #"Peter", ..., #"May" and need to construct NSArray:
[#"John", #"Peter", ..., #"May"]
The number of NSString is unknown and is taking from an import text file. As NSArray does not support appending new element, how can I create NSArray?
Thanks
UPDATE, let me rephrase the question. How can I create the dynamic array paremeter required by the follow function call?
[segmentedAttributes attributesWithTitlesArray:[NSArray arrayWithObjects:#"John", #"Peter", #"May", nil]]
You don't.
You misunderstand the library behavior.
It is true that there is a convenience constructor arrayWithObjects: which is used thus:
NSArray* array=[NSArray arrayWithObjects:#"Low", #"Medium", #"High", nil];
But this does not create an array with nil at the end. This nil is just to signify the end of the variable-length argument list. It just creates an NSArray with three elements, not four with the last one nil.
You just need to create an NSArray containing the required elements, and pass it to the library function. For example:
NSMutableArray*array=[NSMutableArray array];
while(...){
... get a string ...
[array addObject: string];
}
SCSegmentedAttributes*attributes=[SCSegmentedAttributes attributesWithSegmentTitlesArray:array];
should work, without adding a nil or [NSNull null].
You can't store nil in a Foundation collection class. Instead, you can use [NSNull null]. Use an NSMutableArray, then when you want to add your 'nil' object, use [NSNull null].
Note that when you want to see if an object is [NSNull null] later on, that method will return the same instance every time, so you can do a direct point equality test, like this:
for (id anObject in myArray) {
if (anObject == [NSNull null]) {
NSLog(#"object is nil");
}
else {
NSLog(#"object is not nil: %#", anObject);
}
}
create mutable array then just use
NSString *myString = [NSString stringWithFormat:#"ur new string"];
[myArray addObject:myString];
you have to add object type to array when adding new abject in mutable array.
hope this will help
Related
How can we identify the object is available for a particular key. I have tried following:
for(NSDictionary *item in jsonArray){
if([item objectForKey:#"EventDate"])
NSLog([item objectForKey:#"EventDate"]);
}
This is getting crash the code with error:
-[__NSCFString objectForKey:]: unrecognized selector sent to instance 0x6a567b0
I have also find many posts that is showing objectForKey will return nil if a key doesn't exists. Than my question is there is also a method in NSDictionary class that is "setNilValueForKey". How is this possible that we cannot specify the NSDictionary key with nil object and also we have the method to set nil value for object in NSDictionary.
Please Suggest on first and also make me clear on second query.
1) Your jsonArray contains other types of objects than NSDictionaries, including at least one NSString. NSString doesn't respond to objectForKey: so it throws an exception when you try to call it. You'll have to look at the JSON to determine how to proceed with whatever you were doing.
2) There is an NSObject method setNilValueForKey: which is related to key-value coding. This isn't really related to NSDictionary. If you really need to represent nil in your dictionary, set [NSNull null] as the object for your key that represents nil.
Hope this helps!
Not all the objects in your array are dictionaries. You need to check what kind of data you're processing before you work on it. Something like this:
for(NSObject* item in jsonArray) {
if ([item isKindOfClass:[NSDictionary class]]) {
// do dictionary stuff
}
else if ([item isKindOfClass:[NSString class]]) {
// do string stuff
}
}
item is not a NSDictionary its a String. So check your jsonArray it may be contains only strings not dictinaries
Answer to second query
There is a non-nil object called NSNull that is built specifically to represent nils in situations where "plain" nil is not acceptable. If you replace your nils with [NSNull null] object, NSDictionary will take them. You would need to check for NSNull on the way out, though.
Refer more here
Answer to first query
for(id *item in jsonArray)
{
if([item isKindofClass:[NSDictionary class])
{
NSArray *allKeys = [item allKeys];
for(NSString *strKey in allKeys)
{
if([strKey isEqualToString:#"EventDate"])
{
// EventDate key has object
NSLog([item objectForKey:#"EventDate"]);
}
}
}
}
i wanted to know how to add strings into an array.
I used the following methods but it is showing null.
1) [arrData addObject:[NSString stringWithString:strlast]];
2) [arrData addObject:strlast];
Thanks in advance
You can't add anything to an NSArray once it's created. You need to use an NSMutableArray if you want to make changes to it.
Update: You may actually have two problems.
Using an NSArray instead of an NSMutableArray when mutability is needed.
Not initializing the array object (either kind). If arrData is nil, you can happily send as many messages as you want to nil. Nothing will happen.
If it is showing null (nil) you need to make sure you set arrData somewhere in your code before trying to addObject:.
arrData = [[NSMutableArray alloc] init];
Also strlast is a string so use your second example, the first example is pointless.
[arrData addObject:strlast];
Did you allocate an array and assign it to arrData?
Try:
NSMutableArray *arrData = [NSMutableArray array];
NSString *string = #"My string";
[arrData addObject:string];
NSLog(#"%#", [arrData objectAtIndex:0]); //logs "My string"
If you're using a non-mutable array, you can also use arrayByAddingObject:
arrData = [arrData arrayByAddingObject: strlast];
but a mutable array is probably a better idea.
I have a mutable array that is retained and storing several objects. At some point, one object may become nil. When this happens the app will crash, because arrays cannot have nil objects. Imagine something like
[object1, object2, object3, nil];
then, object2 = nil
[object1, nil, object3, nil];
that is not possible because nil is the end of array marker. So, how can I solve that? thanks for any help.
Use [NSNull null] if you have to store an empty placeholder object.
For example:
NSArray * myArray = [NSArray arrayWithObjects:obj1, [NSNull null], obj3, nil];
myArray will contain 3 objects. When you retrieve the object, you can do a simple pointer equality test to see if it's the Null singleton:
id object = [myArray objectAtIndex:anIndex];
if (object == [NSNull null]) {
//it's the null object
} else {
//it's a normal object
}
EDIT (responding to a comment)
#Mike I think you're getting confused with what's actually going on.
If you have:
id obj = ...;
Then obj contains an address. It does not contain an object. As such, if you do NSLog(#"%p", obj), it'll print something like 0x1234567890. When you put obj into the array, it's not copying the object, it's copying the address of the object. So the array actually contains 0x1234567890. Therefore, when you later do: obj = nil;, you're only affecting the pointer outside of the array. The array will still contain 0x1234567890.
I have an array of NSDictionaries. How can I pull out the first element in the dictionary?
NSArray *messages = [[results objectForKey:#"messages"] valueForKey:#"message"];
for (NSDictionary *message in messages)
{
STObject *mySTObject = [[STObject alloc] init];
mySTObject.stID = [message valueForKey:#"id"];
stID = mySTObject.stID;
}
There is no "first" element in an NSDictionary; its members have no guaranteed order. If you just want one object from a dictionary, but don't care which key it's associated with, you can do:
id val = nil;
NSArray *values = [yourDict allValues];
if ([values count] != 0)
val = [values objectAtIndex:0];
NSDictionaries are unordered, meaning that there are not first or last element. In fact, the order of the keys are never guaranteed to be the same, even in the lifetime of a specific dictionary.
If you want any object, you can get one of the keys:
id key = [[message allKeys] objectAtIndex:0]; // Assumes 'message' is not empty
id object = [message objectForKey:key];
NSArray has a selector named firstObject that simplifies the code and makes it more readable:
id val = [[yourDict allValues] firstObject];
If yourDict is empty val will be nil, so is not necessary to check the dictionary/array size.
Simplest:
[[dict objectEnumerator] nextObject];
According to Apple, calls to allKeys or allValues incur the cost of creating new arrays:
A new array containing the dictionary’s values, or an empty array if
the dictionary has no entries (read-only)
So, an alternative option that does not incur such cost could look like this:
NSString* key = nil;
for(key in yourDict)
{ // this loop will not execute if the dictionary is empty
break; // exit loop as soon as we enter it (key will be set to some key)
}
id object = yourDict[key]; // get object associated with key. nil if key doesn't exist.
Note: If the dictionary is empty, the key will remain nil, and the object returned will also be nil, we therefore don't need special handling of the case where the dictionary is actually empty.
If someone is still looking for answer for this type of situation then can refer this:
// dict is NSDictionary
// [dict allKeys] will give all the keys in dict present
// [[dict allKeys]objectAtIndex:0] will give from all the keys object at index 0 because [dict allKeys] returns an array.
[[dict allKeys]objectAtIndex:0];
If you have NSDictionary named message,
It's pretty simple:
message[[[message allKeys] objectAtIndex:0]];
But you have to be sure (or better check) that your dictionary has at least one element.
Here is how you can check it:
if ([message allKeys] > 0) NSLog(#"%#", message[[[message allKeys] objectAtIndex:0]]);
But NSDictionary has no guaranteed order, so you probably should use this code only if your dictionary has only one element.
[UPDATE]
It's also good idea to use this if you need to get ANY element of dictionary
Try this:
NSDictionary *firstValue = [responseObject objectAtIndex:0];
NSArray *array = [[NSArray alloc] initWithObjects:#"ΕΛΤΑ",
#"ΕΛΤΑ COURIER", #"ACS", #"ACS ΕΞΩΤΕΡΙΚΟ",
#"DHL", #"INTERATTICA", #"SPEEDEX",
#"UPS", #"ΓΕΝΙΚΗ ΤΑΧΥΔΡΟΜΙΚΗ", #"ΜΕΤΑΦΟΡΙΚΕΣ ΕΞΩΤΕΡΙΚΟΥ", nil];
This is working because it has nil at the end.
But I add objects like this: addObject:name etc...
So at the end I have to add nil I do this addObhect:nil but when I run the app it still crashes at cellForRowAtIndexPath:
how can I do this work?
Ok, I dont have to add nil
What is the reason that my app crashes then?
If you must add a nil object to a collection, use the NSNull class:
The NSNull class defines a singleton object used to represent null values in collection objects (which don’t allow nil values).
Assuming "array" is of type NSMutableArray:
....
[array addObject:[NSNumber numberWithInt:2];
[array addObject:#"string"];
[array addObject:[NSNull null]];
You don't need to call [addObject:nil]
The nil in initWithObjects: is only there to tell the method where the list ends, because of how C varargs work. When you add objects one-by-one with addObject: you don't need to add a nil.
You can't add nil when you're calling addObject.
If you really want a Null-ish item in your collection, NSNull is there for that.
You need to add NSNull class and the best way to do it is like this:
NSArray *array = #[ #"string", #42, [NSNull null] ];
I personally recommend to use a specific value like 0 instead of null or nil in your design of your code, but sometimes you need to add null.
There is a good explanation from this Apple reference.
nil is used to terminate the array
nil is not an object that you can add to an array: An array cannot contain nil. This is why addObject:nil crashes.
pass your object through this method when adding to array to avoid attempt to insert nil object from objects crashes.
-(id) returnNullIfNil:(id) obj {
return (obj == nil) ? ([NSNull null]) : (obj);
}
[NSNull null] returns an null object which represents nil.
You can't add an object to an NSArray because that class is immutable. You have to use NSMutableArray if you want to change the array after it is created.