How come this happens? Using the code...
- (IBAction)dismissKeyboard:(id)sender {
if(textField.text == label.text) {
NSLog(#"Correct!");
}
else {
NSLog(#"Error?");
NSLog(textField.text);
NSLog(label.text);
}
}
The statement evaulates to be false and prints...
Error?
Word
Word
The words are the same, but the if statement fails... Why is this? I have no clue why this is happening if it is printing the exact same words but the statement is comparing them to be false?
Is it something I am doing wrong in the if statement? Please help. Thank you in advance.
NSString is a class. The proper way to see if two NSString objects (or objects of any class) have the same value is to use isEqual:. In the case of NSString you can use isEqualToString:.
if ([textField.text isEqualToString:label.text]) {
}
The sign == compares the memory location. It sees if you are referring to the same object or not.
The method "isEqualToString: " should be used to compare string values inside variables.
Related
I am using switch case into my method like in screenshot
I already done with Enumeration but how can I directly pass int constant into case??
I googled it but I found it with Enumeration
Please help me with it
If that's all the functionality you want to achieve then you should write something like this:
if (index.row == value) {
// logic when it's that row
} else {
// default case
}
What you tried to do is simply not possible, see the two questions linked by Martin R in the comments.
This question already has answers here:
Should you use 'isEqual' or '=='?
(2 answers)
Comparing objects in Obj-C
(4 answers)
Closed 8 years ago.
I am reading the Programming with Objective-C . In the section of Determining Equality of Objects , it says the following words:
- When dealing with objects, the == operator is used to test whether two separate pointers are pointing to the same object:
if (firstPerson == secondPerson) {
// firstPerson is the same object as secondPerson
}
- If you need to test whether two objects represent the same data, you need to call a method like isEqual:, available from NSObject:
if ([firstPerson isEqual:secondPerson]) {
// firstPerson is identical to secondPerson
}
I get confused about the differences between == and isEqual with the above explanation, does it mean firstPerson == secondPerson is an alternative of [firstPerson isEqual:secondPerson] ?
The definition of == is correct, it checks to see that they're pointing to the actual same pointer/memory address (ie. 0xffffff)
The key to understanding what you're asking is to think about what you mean by the word "equal". "equal" typically means, from the developer's point of view, that "these two objects contain the same data in the fields that I require for all practical purposes". You can have two user objects each with the same userID property but different times in lastUpdated - would you consider them equal? Depends on your use case. Most likely you would say yes because they're the same user. They were updated from the server at different times, so some fields differ, but for your implementation, they're equal.
In the case above, are they the same object? Definitely not. They point to different memory addresses. So == would be NO, whereas if you wrote your isEqual: method to check just the userID property, it would return YES
The definition of isEqual: is entirely up to the author of the class. isEqual: can be written to use == if you wanted. All you have to do, in your class, is to override the isEqual: method which is defined by the NSObject protocol.
If you have a custom object, use isEqual: to define what your definition of equal is. In the example of a user object, you might define:
- (BOOL)isEqual:(id)otherObject {
if ([otherObject isKindOfClass:[self class]]) {
MyClass *otherObjectAfterCast = (MyClass*)otherObject;
if ([otherObjectAfterCast.userID isEqualToString:self.userID])
return YES;
}
return NO;
}
Technically you'd probably want to use caseInsensitiveCompare: or something like that but you get the drift...
isEqual: can also be used to trigger other methods - in the case of NSString - calling isEqual: when both operands are strings results in a call to isEqualToString: - which is why the documentation recommends calling isEqualToString: if you know they're both strings, since it's a bit faster.
So, isEqual: is whatever you make of it, or whatever the class author has defined it to be.
This is also a pretty clear definition in the docs (for once lol): NSObject Protocol Reference
Hope this helps! Let me know if you need any further clarification.
NSString *string1 = [[NSString alloc] initWithString:#"some string"];
NSString *string2 = [[NSString alloc] initWithString:#"some string"];
NSString *string3 = string2;
BOOL equal1 = (string1 == string2); // NO
BOOL equal2 = [string1 isEqual:string2]; // YES
BOOL equal3 = (string2 == string3); // YES
BOOL equal4 = [string2 isEqualToString:string3]; // YES
The simple version is this.
== tells you if the pointers are the same object or not.
The isEqual: family of methods do something different.
They tell you if the objects at the other end of the pointers are effectively the same based on some criteria such as the properties or ivars holding equal values or whatever logic is implemented in the method used. They may or may not be the exact same object.
I know I am missing something obvious, but I just cannot see it. This method is meant to compare the text of a label to the text of a text box, and then delete the text. So if the 1st label reads "Puppy" and the 2nd label reads "Kittens," and the text box says "Kittens," the method should delete the text of the 2nd label and leave the 1st label's text. If the 2nd label is blank, then the method should delete the text of the 1st label.
But no matter how I mess with the method, either it deletes the 2nd label but not the 1st, deletes both of them, or deletes neither of them. Here's what I've tried
(lblFabric1 is the 1st label, lblFabric2 is the 2nd label, txtType is the text box):
-(IBAction)btnDelete:(id)sender
{
if ((self.lblFabric2.text== self.txtType.text))
{
self.lblFabric2.text = #"";
}
else if ((self.lblFabric2.text != self.txtType.text))
{
self.lblFabric1.text=#"";
}
}
It deletes the 2nd label, but not the 1st label. If I try to set the "Else if" to:
else if ((self.lblFabric2.text==#""))
it gives me an error (""Direct comparison of a string literal has undefined behavior.") Am I just going about this the wrong way? What am I missing?
You should not use == or != for string comparison in Objective C. You need to use the isEqualToString or isEqual method.
if (([self.lblFabric2.text isEqualToString:self.txtType.text]))
When you use == or != you are comparing the pointers where the strings are stored.
To compare NSStrings use:
if ([myString1 isEqualToString:myString2])
Documentation
Compairing String literals using == is not guaranteed to behave as you might expect. Use isEqual: or isEqualToString: instead.
See http://nshipster.com/equality/.
When you are comparing NSStrings with == what you are actually comparing are two memory addresses and that is not what you are really intended for. You want to compare the values of two strings what == operator is not suitable for and thus you are getting the warning
Direct comparison of a string literal has undefined behavior.
To compare the values of NSStrings you should use isEqualToString: method. You could have also use isEqual: method derived from NSObject class but that is not suitable for Unicode comparison. So isEqualToString: is always the safest bet.
After using isEqualToString: your code should look something like:
-(IBAction)btnDelete:(id)sender
{
if ([self.lblFabric2.text isEqualToString:self.txtType.text])
{
self.lblFabric2.text = #"";
}
else
{
self.lblFabric1.text=#"";
}
}
I know I'm overlooking something easy, but I am using
if (textField == self.nameTextField || textField == self.ageTextField || textField == ...)
when ideally I'd like to use something that allows the textField to be compared to an array of allowed (named) textField objects.
I've tried to pseudocode what I think the solution would look like a couple of times, I just don't know what I'm doing. Can anyone point me in the right direction?
Just check like:
if([textFieldArray containsObject:textField])
{
//do stuff here
}
Here textFieldArray is the array which holds all textFields.
If the textField object is in the textFieldArray, it'll return true.
Firstly add all references of UITextFields in NSArray.
Now use containsObject in NSArray to check if exists
BOOL contains = [yourArrayofTextFields containsObject:textField];
if(contains)
// contains
else
// not contains
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"]) {