IOS 5 | Tutorial on How-To Increment and Decrement a Global Counter Variable

This is a tutorial to demonstrate how - to implement a globally visible integer counter to step through records of an Array. This tutorial shows how to code the IBAction button to increment and decrement a counter which can be used to retrieve an objectAtIndex in an Array;

1- Create the project

Create a Single View Application, selecting ARC and Storyboard. Once the project is created, open the storyboard and add two buttons, one with a "-" label and the other with a "+" label. Also add two labels. For the first label, add the following text "Index Number" and remove the "label" text from the second. For the second label, open the Attributes inspector and select the "Center" alignment and set the background color to light blue (Figure 1).



Figure 1 - View Controller Layout
Figure 1 - View Controller Layout

Open the header file next to the Storyboard using the Assistant editor and ctrl+drag an IBAction for both buttons and an IBOutlet for the output label (Figure 2). Also declare an C int variable for our counter.


#import <UIKit/UIKit.h>

@interface klViewController : UIViewController

{

int nbr;

}

@property(nonatomic, retain) NSNumber * number;

- (IBAction)forward:(id)sender;

- (IBAction)back:(id)sender;

@property (strong, nonatomic) IBOutletUILabel *labelOutput;

@end



Figure 2 - Setting Ip the header file
Figure 2 - Setting Ip the header file

The implementation file

In the implementation file, first synthesize your instance variables:

@implementation klViewController

@synthesize labelOutput;

@synthesize number;

...

Then add the following code to the viewDidLoad method:

//if NSNumber is needed

number = [[NSNumberalloc] initWithInt:0];

nbr = 0;

NSString * nbrStr=[NSString stringWithFormat:@"%d",nbr];

[labelOutput setText:nbrStr];

The NSNumber is only there to show how to convert a primitive C int to a NSNumber. To display our initial value of our counter, set the nbr variable to 0 then create a NSString and pass the nbr value to the NSString variable, converting to a string format. Then assign it to the label.

This will be the standard coding format for the two buttons, except of course, we will add some code to increment tand decrement the counter.Run the app as it and we should see our label has an initial value of 0 (Figure 3).


Figure 3 - Initializing our Label Counter
Figure 3 - Initializing our Label Counter

Increment and Decrement buttons

The code for each of the buttons are the same, almost, as the proceeding code. The only will add is a bit of code to increment for the "Forward" button and decrement for the "back" button.

This is the code for the forward button:

- (IBAction)forward:(id)sender {

nbr ++;

NSString * nbrStr=[NSString stringWithFormat:@"%d",nbr];

[labelOutput setText:nbrStr];

/* If you want to get an object in an Array you could implement like this

[[MyArray objectAtIndex:nbr] myObject];

Do something with your object, like display the values on screen

*/

}

This is the code for the decrement button:

- (IBAction)back:(id)sender {

nbr --;

NSString * nbrStr=[NSString stringWithFormat:@"%d",nbr];

[labelOutput setText:nbrStr];

}

This code is the same as above, except for the nbr--;. Also you can retrieve the value from an NSArray or NSMutableArray as mentioned before. Here are some sample output screens for the running app:

Figure 3 : Increment the Label
Figure 3 : Increment the Label
Figure 3 : Decrement the Label
Figure 3 : Decrement the Label

Summary

There you have it, this bit of code can be easily adapted to many situations to fetch a value from an Array like a record from a database or storage facility like Core Data, a file or NSDictionary.

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Comments 10 comments

mikeydcarroll67 4 years ago

very cool! Might have to look at that for one my projects!


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klanguedoc 4 years ago from Canada Author

Great, glad to help


Fernando 2 years ago

This is exactly what i need, im pretty new to obj C so thanks a lot!


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klanguedoc 2 years ago from Canada Author

Glad to help


Pratik Shah 2 years ago

Hi,

This was immensely helpful. Previous & Next button now works perfectly.

I have one request if you can help me resolve.

When on first record, I want to hide "Previous" button and when on Last record, I want to hide "Next" button.

Can you please guide on how to get this.

Many thanks.

Pratik


klanguedoc profile image

klanguedoc 2 years ago from Canada Author

Hi Pratik

I am happy that the tutorial was useful. the hide functionality is quite straightforward. When your index is at position or the last position in your array, you can set the UIButton IBAction hidden on the corresponding view . So something like prevBtn.hidden = YES, nextBtn.hidden = NO

Hope this helps

Kevin


Pratik Shah 2 years ago

Hello Kevin,

Thank you for your reply.

My mistake. Extremely sorry.

I have been following your another tutorial, the one with Wines; I am using Tool Bar Item for navigation and not any button.


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klanguedoc 2 years ago from Canada Author

Ok, no problem. Hope everything is working...


zakaria ben jawed 19 months ago

Hi , I try to run this code , but an error message appear "initializer element is not a compile-time constant" in this line "number = [[NSNumberalloc] initWithInt:0];" . what's the problem ?


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klanguedoc 4 days ago from Canada Author

this was written with iOS sdk 5. What version are you using.

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