I use [items indexOfObject:items.lastObject] to get the last index, but this code returns nil. Why does this happen?
The first and last object in your array are both bar button items created with the system item of "fixed space".
The result of calling indexOfObject: is 0, not nil. This means that the object is being found at index 0. indexOfObject: can't return nil. If an object isn't found, it returns the special value NSNotFound which is the unsigned value for -1.
From the documentation for indexOfObject::
Starting at index 0, each element of the array is passed as an argument to an isEqual: message sent to anObject until a match is found or the end of the array is reached. Objects are considered equal if isEqual: (declared in the NSObject protocol) returns YES.
The implementation of UIBarButtonItem isEqual: will return YES if two bar button item instances are created with the same system item (and probably a few other properties as well).
indexOfObject: is not based on the instance of the object, it's based on isEqual:.
If you want to find the index of an object based on the identity (its address) of the object instead of isEqual:, use indexOfObjectIdenticalTo:.
p [items indexOfObjectIdenticalTo:items.lastObject]
will give you 6 instead of 0.
Related
I only know method addObject, which is add object to the next element in the array. I want to be able to add / set / update object to an arbitrary position at the NSMutableArray, ex:
arr[105] = #(true);
arr[709] = #(30);
arr[1010] = #"Hello world!";
NSLog (#"%#", arr[1010]);
I have been trying something like this, but the next time I tried to retrieve the value, it says nil. How to do this? Thanks.
EDIT: last time I tried, it gave me error: index 1010 beyond bounds for empty array.
you specify a size when you create an array, the specified size is regarded as a “hint”; the actual size of the array is still 0. This means that you cannot insert an object at an index greater than the current count of an array. For example, if an array contains two objects, its size is 2, so you can add objects at indices 0, 1, or 2. Index 3 is illegal and out of bounds.
read more at https://developer.apple.com/library/prerelease/ios/documentation/Cocoa/Reference/Foundation/Classes/NSMutableArray_Class/index.html
below method use for replace existing object at index with new object
[self.arr replaceObjectAtIndex:<#(NSUInteger)#> withObject:<#(nonnull id)#>]
below method use for insert new object at index
[self.arr insertObject:<#(nonnull id)#> atIndex:<#(NSUInteger)#>]
Ex:
if object already available at index and replace with new object
[self.arrayBuyers replaceObjectAtIndex:1010 withObject:#"Hello world!"]
add new object at index
[self.arr insertObject:1 atIndex:#"Hi"];
I am using the following code to store "sqlite3_last_insert_rowid" into NSMutableArray,I am getting the rowid but nothing is stored in the array. It is giving me null.
NSUInteger rowIDNum=sqlite3_last_insert_rowid(myDatabase);
NSNumber *xWrapped = [NSNumber numberWithInt:rowIDNum];
[_rowID insertObject:xWrapped atIndex:0];
NSLog(#"row ID array %#",[_rowID objectAtIndex:0]);
Could you please tell me the correct way to store rowid into an array?
I suspect the array is not allocated, else you would get an NSRangeException if the array was allocated but empty (i.e. the first time you called that method).
From the NSMutableArray reference:
index
The index in the array at which to insert anObject. This value
must not be greater than the count of elements in the array.
Important: Raises an NSRangeException if index is greater than the
number of elements in the array.
You would normally allocate the array in the class init method or viewDidLoad method, depending on what the class is. Once you've allocated the array, use:
NSUInteger rowIDNum=sqlite3_last_insert_rowid(myDatabase);
[_rowID insertObject:#(rowIDNum)];
I've read a text file into an array of strings. In the below code I'm creating objects from that array and adding them into an NSMutableArray.
NSMutableArray* metaphors = [[NSMutableArray alloc] init];
unsigned int i, cnt = [allLinedStrings count];
for(i = 0; i < cnt-3; i+=5)
{
Metaphor *newMetaphor = [[Metaphor alloc] init];
[newMetaphor setMetaphorTitle: allLinedStrings[i]];
[newMetaphor setCorrectAnswer: allLinedStrings[i+1]];
[newMetaphor setLiteralAnswer: allLinedStrings[i+2]];
[newMetaphor setWayOffAnswer: allLinedStrings[i+3]];
[metaphors addObject:newMetaphor];
}
As for the problem, when I access any item via index ([metaphors objectAtIndex:3] for example) every other element (odd numbered ones) are nil elements. All of the objects are added to the array, though. My guess is that addObject is adding an element to the array as well as a new nil sentinel every time? Should this be happening/should I manually go through and remove these elements?
Also a side note, as I'm new to Objective-C, my Metaphor class contains the 4 instance fields you can see within the body: I'm sure there is quicker syntax to initialize one of these objects if anyone could point me the right way. Thanks.
NSArray can't contain nil. It's invalid. If you are getting nil back in a call to an array, the array pointer itself is almost certainly nil. (You CAN send messages to a nil object pointer in Objective C. It simply returns nil/zero.)
Trying to add a nil to an NSArray will cause a crash, and trying to index past the end of an NSArray will also crash.
There is a special class NSNull that provides a singleton placeholder object that can take the place of a nil entry in an NSArray.
The nil sentinel is only a way to know the last element of a variable length array has been reached. The nil sentinel ISN'T added to the NSMutableArray;
NSMutableArray addObject method doesn't need a nil sentinel as you only add ONE object, not a variable length array.
If you need to add something "nil" to an array, you might use [NSNull null] which is an object "equivalent to nil".
I have an NSArray and I need to change the order of the items within it. I have written a method that will determine the new order:
+(NSArray*)sortProxyForms:(NSArray*)arrayOfForms
{
NSArray* sortedForms = [arrayOfForms sortedArrayUsingComparator:^(HPSModelFormProxy* a, HPSModelFormProxy* b) {
return [#(a.ordinal) compare:#(b.ordinal)]; // #(a.ordinal) aka Boxing turns int into NSNumber
}];
arrayOfForms = [sortedForms copy]; // DOES NOT WORK
return sortedOfForms; // WORKS IF ASSIGNED IN THE CALLER
}
So, I can pass the NSArray to be sorted into the method. I call the method like this:
[HPSModelUtilities sortProxyForms:_formProxies];
If I actually try setting arrayOfForms (a reference to _formProxies) within the method then once I have returned from the method then the array is unchanged.
However, if I return the sorted array from the method and assign it to the NSArray in the calling method then the assignment works:
_formProxies = [HPSModelUtilities sortProxyForms:_formProxies]; // _formProxies NSArray is changed
_formProxies is declared in the calling class, and "HPSModelUtilities" is a different class.
How come the NSArray can be changed in the caller, but not changed in the called method, even though it is passed by reference?
When you pass a value into a method it is copied. This is called "pass by value". The arrayOfForms you are passing in is a pointer to an NSArray. This means that the pointer is copied when passed in. Redirecting this pointer to another instance of an NSArray does not change where the original pointer is pointing.
I would rename your method to (NSArray*)sortedArrayFromProxyForms:(NSArray*)proxyForms
If you really want to change where your NSArray reference is pointing in the method. Do it like this.
+ (void)sortProxyForms:(NSArray**)proxyForms {
*proxyForms = sortedForms;
}
You are passing a copy of the array reference (subtly different than passing by reference), but then you are changing where that reference points with this line:
arrayOfForms = [sortedForms copy];
arrayOfForms no longer points to the array instance you passed, but to a different array. You could pass a pointer of pointer, and change where the caller's pointer is pointing, but for what you are doing, I think the reassignment is fine.
If you'd really like here's what your function would look like with pointer of pointer:
+(void)sortProxyForms:(NSArray**)arrayOfForms {
NSArray* sortedForms = [arrayOfForms sortedArrayUsingComparator:^(HPSModelFormProxy* a, HPSModelFormProxy* b) {
return [#(a.ordinal) compare:#(b.ordinal)]; // #(a.ordinal) aka Boxing turns int into NSNumber
}];
*arrayOfForms = [sortedForms copy];
}
but I'll add the caveat that this isn't a pattern you see often in objective-c, so I'd avoid it when there are other alternatives available.
Also note when calling this function you need to add the & to get the extra level of indirection:
[HPSModelUtilities sortProxyForms:&_formProxies];
I have a MutableArray on this view called array and the object in question is detailItem, which has a property of rank (int). On this view, there's a text field displaying the rank and I want to be able to move the detailItem up and down the MutableArray by changing the rank.
So, for example let's say the detailItem has a rank of 3, which is index value of 2. If I change this in the text field to 3, I want the array to adjust and move it down one place. However, as I type in the value of rankField (the text field), it crashes the app since it automatically updates the value before I'm done editing. So, if I click on the text field and write 23 (planing on deleting the 2) or just press delete (now the value is nil) the app crashes with an uncaught exception.
Here's the code:
- (IBAction)rankFIeldTextChanged:(id)sender {
QueueMember *member = self.detailItem;
[self.array removeObjectAtIndex:self.detailItem.rank];
if (0<= [self.rankField.text intValue]<= self.array.count) {
[self.array insertObject:member atIndex:[self.rankField.text intValue]-1];
}
}
The if condition of making the text value in-between the array size and 0 seems to have no effect.
btw this is all in the detailsViewController which is connected to the main view controller via push segue. does it make more sense(or more better coding) to just set the new rank value in details and actually make the array changes in the mainviewcontroller.m?
The problem is that you are trying to do two boolean statements at once (which doesn't work). Change your if statement to something like:
if (0< [self.rankField.text intValue] && [self.rankField.text intValue] < self.array.count) {
//Insert your object here
}
else
{
//Add object here
}
Your current setup check to see if 0<= [self.rankField.text intValue], which will return true for all values greater than or equal to 0. Then it checks the result of that (YES:1, NO:0) if it's less than or equal to your array count. That will always return true if your array has anything in it. So basically your check will always return true.
Since it always returns true I could check for array object number 1000, your if statement says go for it, then I check and the array says "No way in heck!" and crashes your app.
EDIT: Updated my code snippet to take into account your array insertion line.
I'd just do this
- (IBAction)rankFIeldTextChanged:(id)sender {
QueueMember *member = self.detailItem;
[self.array removeObjectAtIndex:self.detailItem.rank];
if ((0<= [self.rankField.text intValue])&&([self.rankField.text intValue]<= self.array.count)) {
if (self.array.count >([self.rankField.text intValue]-1)){
[self.array insertObject:member atIndex:[self.rankField.text intValue]-1];
}
}
}
you are probably trying to insert at an index greater than the count of the array.