how to update a string variable with another string variable - iphone

Question

i have a string declare as such
NSString *str = [[NSString alloc] initWithFormat:#"I require an average GPA of at least %.2f to achieve my Goal of %# this semester - NTU GPA Calculator", pgagoal,(NSString *)[myPickerDelegate.myGoal objectAtIndex: [myPicker selectedRowInComponent:0]]];
i declared a global variable
NSStrinng *tweetString
and wants to copy the the string in str to tweetString. how should i copy it? since both are pointers, i tried:
tweetString = str;
or
tweetString = [NSString stringWithFormat:#"%#", str];
but it doest work.
EDIT:
my code:
-(void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex1{
NSLog(#"buttonindex 1 clicked");
NSString *str2;
NSLog(#"tweetString before if: %#", tweetString);
if (pgagoal < 0) {
NSString *str2 = [[NSString alloc] initWithFormat:#"Confirm, Guarantee, Chop and Stamp! I can achieve my Goal of %# this semester - NTU GPA Calculator", (NSString *)[myPickerDelegate.myGoal objectAtIndex: [myPicker selectedRowInComponent:0]]];
NSLog(#"tweetString: < 0 %#", str2);
}
else if (pgagoal > 5){
NSString *str2 = [[NSString alloc] initWithFormat:#"Its impossible!, i need an average GPA of at least %.2f to achieve %# this semester - NTU GPA Calculator", pgagoal,(NSString *)[myPickerDelegate.myGoal objectAtIndex: [myPicker selectedRowInComponent:0]]];
NSLog(#"tweetString: >5 %#", str2);
}
else{
NSString *str2 = [[NSString alloc] initWithFormat:#"I require an average GPA of at least %.2f to achieve my Goal of %# this semester - NTU GPA Calculator", pgagoal,(NSString *)[myPickerDelegate.myGoal objectAtIndex: [myPicker selectedRowInComponent:0]]];
NSLog(#"tweetString with else: %#", str2);
}
//did i update tweetString correctly?
tweetString = [NSString stringWithString:str2]; <-- stop working from this point EXC_BAD_ACCESS
NSLog(#"tweetString after if else: %#", tweetString);
[self sendEasyTweet:tweetString];
NSLog(#"tweetString: %#", tweetString);
[str2 release];
}
- (void)sendEasyTweet {
// Set up the built-in twitter composition view controller.
TWTweetComposeViewController *tweetViewController = [[TWTweetComposeViewController alloc] init];
// Set the initial tweet text. See the framework for additional properties that can be set.
[tweetViewController setInitialText:tweetString];
// Create the completion handler block.
[tweetViewController setCompletionHandler:^(TWTweetComposeViewControllerResult result) {
switch (result) {
case TWTweetComposeViewControllerResultCancelled:
// The cancel button was tapped.
NSLog(#"Tweet cancelled");
break;
case TWTweetComposeViewControllerResultDone:
// The tweet was sent.
NSLog(#"Tweet done");
break;
default:
break;
}
// Dismiss the tweet composition view controller.
[self dismissModalViewControllerAnimated:YES];
}];
// Present the tweet composition view controller modally.
[self presentModalViewController:tweetViewController animated:YES];
}
EDIT2:
Debbuger output:
2011-12-29 09:54:22.963 GPA[487:707] buttonindex 1 clicked
2011-12-29 09:54:22.966 GPA[487:707] tweetString before if: NTU GPA Calculator <-- i init the string at viewDidLoad
2011-12-29 09:54:22.968 GPA[487:707] tweetString with else: I require an average GPA of at least 1.56 to achieve my Goal of Third Class Honors this semester - NTU GPA Calculator
(gdb)
EDIT3:
my tweetString is declared in view controller.h as
#interface GPAMainViewController : UIViewController <GPAFlipsideViewControllerDelegate>{
UIPickerView * myPicker;
GPAAppDelegate * myPickerDelegate;
IBOutlet UITextField *txtGPA;
IBOutlet UITextField *txtTotalAU;
IBOutlet UITextField *txtRemainingAU;
double pgagoal;
NSString *tweetString;
}
#property (nonatomic, retain) IBOutlet UIPickerView * myPicker;
#property (nonatomic, retain) IBOutlet GPAAppDelegate *myPickerDelegate;
#property (nonatomic, retain) UITextField *txtGPA;
#property (nonatomic, retain) UITextField *txtTotalAU;
#property (nonatomic, retain) UITextField *txtRemainingAU;
#property (nonatomic, retain) NSString *tweetString;
-(IBAction)finishEditing:(id)sender;
-(IBAction)calculateGoal: (id) sender;
-(IBAction)showInfo:(id)sender;
-(IBAction)nextField:(id)sender;
-(IBAction)resetField:(id)sender;
-(void)sendEasyTweet:(id)sender;

Answer 1

The reason that it doesn't work (it is probably crashing with an EXC_BAD_ACCESS) is because the scope of the variable str is only within the block in which it is declared, the block of the else part of your if/else statement. Try something like this:
-(void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex1 {
NSString* str; //declare string here so it is in scope the entire method
.
. //your code
.
.
if(yourConditionHere) {
//make sure you initialize str here as well so if the else part of the statement
// isn't executed, you aren't trying to access an uninitialized variable
} else {
str = [[NSString alloc] initWithFormat:#"I require an average GPA of at
least %.2f to achieve my Goal of %# this semester - NTU GPA Calculator",
pgagoal,(NSString *)[myPickerDelegate.myGoal objectAtIndex: [myPicker
selectedRowInComponent:0]]]; //give str a value
NSLog(#"tweetString with else: %#", str);
} //Variable str is going out of scope here the way you have your code set up now
tweetString = [str copy];
NSLog(#"tweetString after if else: %#", tweetString);
[self sendEasyTweet:tweetString];
NSLog(#"tweetString: %#", tweetString);
[str release];
}

Answer 2

If you want to copy the string, or use the string after you assign it, you either need to copy it or retain it.
NSString *someString = #"This is a string";
NSString *copiedString = [NSString stringWithFormat:#"%#", someString"];
Within a few seconds, both strings will be nil or some other non value. What you must do is :
NSString *someString = #"This is a string";
NSString *copiedString = [NSString stringWithFormat:#"%#", someString"] retain];
By doing this, you will keep both variables in memory as long as they are viable. But in my opinion a better way, especially when dealing with strings is to use copy, like this :
NSString *someString = #"This is a string";
NSString *copiedString = [NSString stringWithFormat:#"%#", someString"] copy];
This will make someString just go away in a few seconds or clock ticks, but copiedString will live on until the function is finished or the class released.
I suspect that you are not getting the string value inside tweetString because both variables have gone from memory when you want to use it.
If you need a variable to stay around, you must copy or retain it.