I have three strings (a NSString, a NSMutableString, and another NSString) which I need to concatenate into a mutable string, in that order, to display as the source for a UIWebView. Comming from a PHP/JavaScript/HTML background, my knowledge of concatenation is pretty much this:
var concatenatedString = string1 + string2 + string3;
I presume that sort of thing won't work in Objective-C, so I'm wondering how to go about pulling them all together properly.
To give a bit of setting for this, the first string (NSString) is the header and canvas element of a web page, the second string (NSMutableString) is javascript from a text field that the user can define to manipulate the canvas element, and the third string (NSString) is the end tags of the web page.
Also, rather than initially creating the NSMutableString, should I just referance the UITextView.text to the get the user's text when concatenating the whole thing, or should I pull the text from the UITextView first?
NSMutableString *concatenatedString = [[NSString stringWithFormat:#"%#%#%#", string1, string2, string3] mutableCopy];
The other two answers are correct in that they answer the question as you asked it. But by your description of what you want to do there is a much easier way. Use a format.
Assuming string1 and string3 will always be the same and only string2 will change,which is what it sounds like you are doing you can write something like this.
static NSString *formatString = #"String1Text%#String3Text";
NSString *completeString = [NSString stringWithFormat:formatString,self.myTextFieldName.text];
NSLog(#"%#",completeString);
The %# in the format says to insert the description of the object following the format.(The description of an NSString is the stringValue.)
Assuming you have a UITextField named myTextFieldName, that currently contains the text 'String2Text' Then this will be the output:
'String1TextString2TextString3Text'
In this way you only create 1 instance of an NSString format for the whole class no matter how many times you call this code.
To me it sounds like you don't need a mutable string at all. Feel free to leave a comment if I misunderstood anything.
Response to comment:
I'm not sure how you are implementing 'moves to test it out again' but, let's say you have a button named 'testJavaScript'. The IBAction method connected to that button would have the first two lines in it. So each time you pushed the button it would make a new formatted NSString filled with the current contents of the textfield. Once this string was formed it could not be changed. But it won't matter since next time it will make another.
NSString *concatenatedString = [string1 stringByAppendingFormat:#"%#%#", string2, string3];
You can make the resulting string mutable (if you really need to) by adding mutableCopy as shown in the answer by #Vinnie.
Related
I would like to understand more about the way XCode/Objective-C handle constant strings. I found a related question, but I would like more information. Consider the following code:
NSString *a = [[NSString alloc] initWithUTF8String:[[_textFieldA stringValue] UTF8String]];
NSString *b = [[NSString alloc] initWithUTF8String:[[_textFieldB stringValue] UTF8String]];
NSString *c = [a copy];
NSString *d = [a mutableCopy];
Note that the textFields are just a way to set the strings at runtime ensuring that the compiler doesn't get too smart on me and build in a single instance.
If my text fields are empty, or contain a single character such as "x" or "$", then a == b == c == the same constant NSString instance. If I instead provide "xy", then a == c != b. d is always unique, as one might expect since it is mutable.
Now normally this wouldn't be an issue, I'm not trying to modify the contents of these strings, however, I am working on a system where I frequently use objc_setAssociatedObject. So here now I might come accross an empty string, and then set associated object data on it, and then have another empty string and collide with the first.
I have, for the moment, solved my issue by creating mutable strings instead.
So my questions:
Is this an Objective-C specification, or an XCode excentricity?
Does anyone know how the instance is determined? Why "x" get's one instance, but not "xy"? I would think some internal dictionary is involved and there's no good reason to stop at 1 character.
Is there a way to turn this off, so all empty strings are unique instances, or other suggestions?
I am using XCode 5.1.1, OSX 10.9.4, SDK 10.9.
Thank you!
Is this an Objective-C specification, or an XCode excentricity?
It is just implementation detail. Not documented any where. These kind of behaviour may changed in future without notice.
Does anyone know how the instance is determined? Why "x" get's one instance, but not "xy"? I would think some internal dictionary is involved and there's no good reason to stop at 1 character.
No until someone able to access source code want to share the details with us.
Is there a way to turn this off, so all empty strings are unique instances, or other suggestions?
No way to turn it off. Don't use objc_setAssociatedObject with NSString
As #Ken Thomases said in comment
In general, it probably doesn't make sense to use objc_setAssociatedObject() with any value class.
Some other examples are NSNumber, NSData and NSValue. They are often cached and reused.
I have a web server which I used to fetch some data in my iOS application. The data include a field as the itemId let say '48501' (with no quotation). I read item JSON data into my itemObject in which itemId is defined as a NSString and not a NSInteger.
Everything works until this point but I have problems where I want to compare itemObject.itemId using isEqual: function with another NSString filled with 48501.
In other words both string are exactly the same and include 48501 when I print them. No space and hidden things is there. All isEqual: and isEqualToString: and == report false on comparison.
On the hand when I convert NSStrings to NSIntegers and compare them it works but not always! sometime TRUE sometime CRASH with no error to catch and just pointing to the line! I see them printed exactly the same but the if statement does not go through.
I showed the code to someone with far more experience than me and he was like this could be a bug! Anyone has ever exposed to this?
If your itemId is 48501 without any quotation in the JSON, then it's deserialized as NSNumber. Probably that's the problem in the first place. Try logging the type of your itemId and use appropriately -isEqualToString: for NSString and -isEqualToNumber: for NSNumber.
I have a list of words like: hello hi bonjour etc.
I'm literally adding #"" around every word manually. It's not bad, but when you have thousands of words it can get really tedious. Is there a method, where you input a bunch plain text like hello hi and it turns it into: #"hello" #"hi", which I can then copy into code, or do I have to do this manually?
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What I've done in similar cases is use an editor's text replacement feature to replace all blanks with (in your case) " #". You may have to get a bit fancier if you don't have blanks at the start and end of each line, or you have multiple adjacent blanks, etc, but the general technique works.
It sounds like what you want is NSString's componentsSeparatedByString: method. You can enter in a bunch of plain text into some text field or text view, get the raw string, and then push that through "componentsSeparatedByString:#" "" (make sure there's one space in there) and out comes an array of words.
You could try loading the strings from a Plist -- inserting them into code is kind of ugly. Something like [NSDictionary dictionaryWithContentsOfURL:urlToMyPlist] or [NSArray arrayWithContentsOfURL:urlToMyPlist]
Look at the docs for NSString and you'll find the -initWithCharacters:length: method, which you can use to create strings. If you have a string that you want to break up into smaller strings, use -componentsSeparatedByString:.
I'm new to Objective-C and need help with the concept of pointers. I've written this code:
//myArray is of type NSMutableArray
NSString *objectFromArray = [myArray objectAtIndex:2];
[objectFromArray uppercaseString];
I assumed that this would change the string at myArray[2] since I got the actual pointer to it. Shouldn't any changes to the dereferenced pointer mean that the object in that location changes? Or does this have something to do with 'string immutability'? Either way, when I use NSLog and iterate through myArray, all the strings are still lowercase.
Shouldn't any changes to the dereferenced pointer mean that the object in that location changes?
Yes, they would. But if you read the documentation for uppercaseString, you see that it does not modify the string in place. Rather, it returns a new uppercase version of the original string. All methods on NSString work like that.
You would need an instance of NSMutableString to be able to modify its contents in place. But NSMutableString does not have a corresponding uppercase method, so you would have to write it yourself (as a category on NSMutableString).
of course!! no string in the array will be converted to uppercase as the statement [objectFromArray uppercaseString]; would have returned the uppercase string which was not collected in any object though. "uppercaseString" does not modify the string object itself with which is is called...!!
I'm getting a EXEC_BAD_ACCESS when concatenting a large string.
I've read from a feed and to create my webview I build up my string like:
NSString *pageData = #"<h1>header</h1>";
pageData = [pageData stringByAppendingFormat#"<p>"];
pageData = [pageData stringByAppendingFormat#"self.bodyText"];
pageData = [pageData stringByAppendingFormat#"</p>"];
etc
The problem I've got is self.bodytext is 21,089 characters with spaces when I do a count on word.
Is there a better method for doing this?
Thanks
You would definitely want to use NSMutableString for something like this:
NSMutableString * pageData = [NSMutableString stringWithCapacity:0];
[pageData appendFormat:#"<h1>header</h1>"];
[pageData appendFormat:#"<p>"];
...
NSMutableString is designed for this kind of sequential concatenation, where the basic NSString class is really not meant to be used in this manner. Your original code would actually allocate a new NSString every time you called stringByAppendFormat:, and then procede to copy into it all of the thousands of characters you had already appended. This could easily result in an out of memory error, since the size of the temporary strings would be growing exponentially as you add more and more calls.
Using NSMutableString will not re-copy all of the string data when you call appendFormat:, since the mutable string maintains an internal buffer and simply tacks new strings on to the end of it. Depending on the size of your string, you may want to reserve a huge chunk of memory ahead of time (use a meaningful number for the ...WithCapacity: argument). But there is no need to go that route unless you actually run into performance issues.
There are a few problems with your sample code:
You should be using a NSMutableString to build up an output string by appending multiple parts. NSString is an immutable class which means that each time you call stringByAppendingFormat: you are incurring the overhead of creating an additional new NSString object which will need to be collected and released by the autorelease pool.
NSMutableString * pageData = [NSMutableString stringWithCapacity:0];
You should use appendString: on your NSMutableString to append content, instead of stringByAppendingFormat: or appendFormat:. The format methods are intended for creating new strings based on a format specifier which includes special fields as placeholders. See Formatting String Objects for more details. When you're using stringByAppendingFormat: with just a literal string like your code has, you are incurring the overhead of parsing the string for the non-existant placeholders, and more importantly, if the string happens to have a placeholder (or something that looks like one) in it, you'll end up with the EXEC_BAD_ACCESS crash that you are getting. Most likely this happening when your bodyText is appended. Thus if you simply want to append a '' to your NSMutableString do something like this:
[pageData appendString:#"<p>"];
If you want to append the contents of the self.bodyText property to the string, you shouldn't put the name of the property inside of a string literal (i.e. #"self.bodyText" is the literal string "self.bodyText", not the contents of the property. Try:
[pageData appendString:self.bodyText];
As an example, you could actually combine all three lines of your sample code by using a format specification:
pageData = [pageData stringByAppendingFormat:#"<p>%#</p>", self.bodyText];
In the format specification %# is a placeholder that means insert the result of sending the description or descriptionWithLocale: message to the object. For an NSString this is simply the contents of the string.
I doubt the length of the string is really a problem. A 50,000-character string is only about 100 KB. But you want to be very careful about using format strings. If your string contains something that looks like a formatting specifier, there had better be a corresponding argument or you'll get garbage if you're lucky and a crash if you're not. I suspect this is the error, since there is no other obvious problem from your description. Be careful about what you put in there, and avoid ever putting dynamic text in a format string — just put a %# in the format string and pass the dynamic text as an argument.
Use appendString: instead of appendFormat: when dealing with arbitrary strings.
pageData = [pageData stringByAppendingString:#"<p>"];
pageData = [pageData stringByAppendingString:#"self.bodyText"];
pageData = [pageData stringByAppendingString:#"</p>"];
or do not use an arbitrary string as the format:
pageData = [pageData stringByAppendingFormat:#"<p>%#</p>" , #"self.bodyText"];
If you are building the string up in pieces, use NSMutableString instead of several stringBy calls.
Remember that % is a special character for formatted strings and for url escapes, so if bodyText contains a url it could easily cause a crash.