im building a chart view and I'm giving it a datasource to which it can ask for a Y value that corresponds to a given X index. In some cases, there is no Y value for a given X index. I can't send nil in this circumstance, as 0 may be a perfectly valid Y value. What is the best way in objective C to communicate that there is No Value at all?
It seems to me that you're sending the data between the model and the view using some sort of object, perhaps NSNumber, that's holding a value, and you are retrieving the value by sending it a message, and you know that if the object is nil, the message send will return 0.
That's correct. In this case, simply check if the object is nil, and proceed accordingly:
- (void)plotValue:(NSNumber *)number
{
if (number == nil) {
// no value
} else {
// Value may be 0, but you can now safely plot it.
}
}
Don't use primitives in your data source, instead use NSNumber. Then you can differentiate between nil and an NSNumber storing the value 0.
nil is not 0. They are different.
If you are using NSNumber, then nil is clearly disjoint from the set of NSNumber.
If you are using int, then you can't use nil.
If you don't want to use an object (NSNumber), you can return a negative number (if possible) or the maximum/minimum integer that is unlikely to be returned by the datasource.
I'm assuming you're talking about some scalar like an NSInteger or something. If literally any integer is valid for the Y value, then there is no way to do this. However, most likely there is some practical range, so you can just choose some value outside this range to be "special". For example, Cocoa has the value NSNotFound, which NSArray will return if you ask for the index of an object that is not in the array. It is defined as the largest possible NSInteger. You could reuse this yourself if your Y value will never be NSIntegerMax.
The other option, assuming you don't want to create a special type just for this, would be to have your method return both Y and whether or not Y was actually found. For example, some rough pseudocode:
- (NSInteger)yForX:(NSInteger)x wasFound:(BOOL *)found {
if (yWasFound) {
if (found)
*found = YES;
return y;
} else {
if (found)
*found = NO;
return 0;
}
}
You could use NSNumber, as some have suggested, but I find it a little awkward to deal in NSNumbers just to get nil.
Related
I'm doing some CoreBluetooth tutorial and got to this obj-c code:
if (request.offset > myCharacteristic.value.length) {
// stuff
}
value is of type NSData. Since my code is in Swift 3, value is of type Data. However, a Data object does not have the property length. Do you know what's the equivalent in Swift?
You can use the property count.
https://developer.apple.com/reference/foundation/data/1779648-count
Should use the property .isEmpty
While .count replaces .length in Swift, it is inefficient if all you want to know is does .count == 0.
Using .count will for loop through the entire collection, whereas '.isEmpty' will stop after one.
Also '.isEmpty' returns a Bool which is easier to read and understand...
if data.count == 0 {
vs
if data.isEmpty {
https://developer.apple.com/documentation/foundation/data/1780284-isempty
ok so I am trying to return nil if a certain type is passed into my function. In this case im passing in an instance of my class "BlogPost" and a type within this blogpost. I also have an array called "types" and I have assigned the variable Videos to the last index of that array. If this type is passed into my function I would like to return nil (so assuming im going to need an optional here for returning a possible nil) this is what I have so far :-
so all in all I need to pass in an instance of my blog post but always return nil if a certain type is passed in. Hope this makes sense
Update:
The types array is defined as follows:
let types : [String] = ["technology", "Fashion", "Animals"]
this is the array I am referring to in the function. Basically if that last entry of the array is entered into the function I need to return nil
sure this is blogpost it does actually have an empty string for type
great so im getting there what Ive done now is change the blogpost.type to choose one at random. So now if the specfic type is chosen from this array how would I do that still getting an error. This is what I have updated to
so now all I need to do is access the 2 type in that array and if I do access it return nil. Any thoughts on that? so to drag it on thanks
I don't think you can. You can create failable initialisers which does what you need but you cannot use it with normal function.
The best solution for you would be return optional Int or String and when you call the function just check the result for nil and do what you need to do, otherwise ignore it:
func randomViews(blog : BlogPost.Type) -> Int? {
case 10:
return nil
case 10, 20 :
return 0
default:
random
}
if (randomViews(parameter) == nil) {
//function returned nil
}
You have displayed error because you compare optional blog to Videos, you have to unwrap it first, for example if you are sure the blog has always have a value use:
if blog! == Videos
if not sure is safer to use:
if let blg = blog {
if blg == Videos {
}
else {
// blog has not have a value
}
You are passing blog as a BlogPost.Type parameter. That is not correct. You should have either just passed it the String parameter, or you could pass it the BlogPost itself:
func randomViews(blog: BlogPost) {
let videos = types[2]
if blog.type == videos {
// do whatever you want
}
// carry on
}
Unrelated to your question at hand, but notice that I use let instead of var when defining videos. Always use let if the value will not (and cannot) change.
Also note that I use lowercase letter v in videos, because Cocoa naming conventions dictate that variables generally start with lowercase letters, whereas types, classes, structs, and enums generally start with uppercase letters.
I am currently studying objective-c and the basic c programming language.
I have a questions about a particular line of code:
if (!balance)
Balance is an object that is created. I understand that this code is checking to see if the object balance is nil or not, is this correct?
Could somebody please explain how exactly the code checks for nil? Does it return 0 if the value of balance is nonzero and 1 if the value is 0?
Thanks in advance.
In Objective-C, nil is roughly analogous to 0, NULL or false, but for object pointers. In an if statement, it will behave the same as one of the aforementioned scalar values. For example, the following two if statements should produce the same results:
NSNumber *balance = nil;
if (!balance) {
// do something if balance is nil
}
if (balance == nil) {
// do something if balance is nil
}
NSLog should return (null) (which probably is description for nil), not NULL in console. Your check should look like this:
if (!controller)
{
// do some stuff here
}
If balance is nil then it will be a pointer to 0x0. That address is never used for a valid object.
In C anything within an if that evaluates to zero is considered a negative response, anything that evaluates to non-zero is a positive response.
Pointers evaluate to their address — exactly as if you cast them to an integral type. The ! means "NOT".
So the test is if(address of balance is not zero).
First you need to understand how if works. Basically, any non-zero value is treated as true and a zero value is treated as false.
Something as simple as if (10) will be treated as true while if (0) is treated as false.
Any expression evaluates to either a value of zero or a non-zero value.
An object pointer is just a number - a memory address. A nil pointer is simply an address of 0. Any non-nil pointer will have a non-zero address.
The ! operator negates that state. A non-zero value will be treated as a zero value and visa-versa.
So now combine all of this.
Foo *bar = nil;
if (bar) // false since bar is nil (or zero)
Foo *bar = [[Foo alloc] init]; // some non-nil value
if (bar) // true since bar is non-zero
Foo *bar = nil;
if (!bar) // true since !bar mean "not bar" which is "not zero" which is "non-zero"
Foo *bar = [[Foo alloc] init]; // some non-nil value
if (!bar) // false since !bar mean "not bar" which is "not non-zero" which is "zero"
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.
I'm using a label to display the string result of function. However I have a class variable that stores the previous result and I need to update that variable in different ways depending on different conditions. The code I wrote is
if(displayPassword.text == #"Memorable")
{
prevpass = [newPassword returnPassword];
}
else
{
prevpass = displayPassword.text;
}
However it always jumps to the else as it seems to show under debugging that displayPassword.text is always empty depsite it showing a value.
You can only use == to compare scalar values. A string is an object. You need to use the isEqual: or isEqualToString: method instead.
if([displayPassword.text isEqualToString:#"Memorable"]) {