Edit individual chars in Objective-C - objective-c

Question

I'm learning about Cocoa/Objective-C, and I'm trying to be able to modify a NSString and edit individual chars in the string. I've managed to be able to change it to an array of chars, and change it back, but when I try to do c[0] = 'b', it says, "read-only variable is not assignable". I've included the .m file and .h file.
This is the .m file:
import "AppDelegate.h"
#implementation AppDelegate
#synthesize textField, myLabel;
-(IBAction)changeLabel:(id)sender {
NSString *message = [[NSString alloc] initWithFormat:#"Hello, %#!", [textField stringValue]];
NSString *message2 = [[NSString alloc] initWithFormat:#"%# This part was added on afterwards.", message];
const char *c = [message2 UTF8String];
c[0] = 'b';
NSString *output;
output = [NSString stringWithCString:c encoding:NSASCIIStringEncoding];
[myLabel setStringValue:output];
}
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
}
#end
This is the .h file:
#interface AppDelegate : NSObject <NSApplicationDelegate>
#property (assign) IBOutlet NSWindow *window;
#property (weak) IBOutlet NSTextField *textField;
#property (weak) IBOutlet NSTextField *myLabel;
-(IBAction)changeLabel:(id)sender;
#property (weak) IBOutlet NSTextField *changeButtonText;
#end
I'm assuming the .h file isn't relevant, but I though it'd include it.

Answer 1

NSString is a container for an immutable string. Taken from official reference:
The NSString class declares the programmatic interface for an object that manages immutable strings. An immutable string is a text string that is defined when it is created and subsequently cannot be changed.
Basically, by using just cocoa, you can generate new NSStrings by replacing characters in existing strings, for example with:
- (NSString *)stringByReplacingCharactersInRange:(NSRange)range withString:(NSString *)replacement
but you can't modify the same object. Exactly for this reason UTF8String returns a const char* which is an immutable pointer to a sequence of character. That's why you get the error.
You can use the provided NSString methods to generate new strings or copy the const char* C string to a char* (through, for example, strdup).

Answer 2

c is a const char* -> its initialising value is its value throughout the whole runtime. Maybe you should erase the const keyword.

Answer 3

In Objective-C an NSString instance is not modifiable, for a modifiable string you need to NSMutableString. Alternatively you can create a new string from an old one. Unfortunately NSMutableString/NSString does not have the inverse of characterAtIndex:, to change a single character you need to use stringByReplaceCharactersInRange:withString:/replaceCharactersInRange:withString: where you supply a range - location & length - of the characters to change. Here is part of your code re-written to use these, first creating a new string:
NSString *message2 = [[NSString alloc] initWithFormat:#"%# This part was added on afterwards.", message];
NSString *output = [message2 stringByReplacingCharactersInRange:NSMakeRange(0,1) withString:#"b"];
This first option creates a new string. Using a modifiable string:
NSMutableString *message2 = [[NSMutableString alloc] initWithFormat:#"%# This part was added on afterwards.", message];
[message2 replaceCharactersInRange:NSMakeRange(0,1)
This second option changes the string referenced by message2.
Whether you create new strings or use a modifiable string will depend on how many changes you need to make and whether you need to keep the original string.