如何更新一個字符串變量與另一個字符串變量

Question

我有一個字符串聲明爲這種

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]]]; 

我宣佈一個全局變量

NSStrinng *tweetString 

，並希望將字符串str中複製到tweetString 。我應該如何複製它？因爲兩者都是三分球，我想：

tweetString = str; 

或

tweetString = [NSString stringWithFormat:@"%@", str]; 

但它合乎理工作。

---

編輯： 我的代碼：

-(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輸出：

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： 我tweetString聲明鑑於或者Controller.h作爲

@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

它不起作用的原因（它可能與EXC_BAD_ACCESS崩潰）是因爲變量str的範圍只在聲明的塊內，if/else語句的else部分的塊。試試這樣：

-(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

如果你想複製字符串，或者在分配字符串後使用字符串，你需要複製它或者保留它。

NSString *someString = @"This is a string"; 
NSString *copiedString = [NSString stringWithFormat:@"%@", someString"]; 

在幾秒鐘內，這兩個字符串將是零或其他非值。你必須做的是：

NSString *someString = @"This is a string"; 
NSString *copiedString = [NSString stringWithFormat:@"%@", someString"] retain]; 

通過這樣做，你會保持這兩個變量在內存中，只要他們是可行的。但在我看來，一個更好的辦法，用繩子尤其是在處理時是使用複製，就像這樣：

NSString *someString = @"This is a string"; 
NSString *copiedString = [NSString stringWithFormat:@"%@", someString"] copy]; 

這將使someString走開在幾秒鐘或時鐘滴答，但copiedString將生活，直到功能完成或班級發佈。

我懷疑你沒有得到tweetString中的字符串值，因爲當你想使用它時，這兩個變量都從內存中消失了。

如果你需要一個變量留下來，你必須複製或保留它。