CODEWITHSUNDEEP
iosobjective-cmathnsuserdefaultsnsnumber
user2329093
user2329093

Reputation:

Use NSNumber and NSUserDefaults to do a simple computation

I have a view where a user enters an amount. Using NSUserDefaults, this amount is transferred to the next view, where I wish to do a mathematical computation with it. In this next view, I have a mutable array that is used for adding data to a table. What I want is:

Label in view 2 = NSUserDefaults - (all array objects added together)

I figured out how to add the array objects together: NSNumber *sum = [transactions valueForKeyPath:@"@sum.self"];.

I tried to do the math, but no luck: self.amountLeftInBudget.text = [[[NSUserDefaults standardUserDefaults] objectForKey:@"AmountToSpend"] - sum];. I got this error: Arithmetic on pointer to interface 'id', which is not a constant size for this architecture and platform.

Anyone have any ideas? Thanks.

Edit: My code (using an alert view) 

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    //Only do the following actions if the user hit the done button
    if (buttonIndex == 1)
    {
        NSString *amountSpentTextField = [alertView textFieldAtIndex:0].text;

        if (!transactions)
        {
            transactions = [[NSMutableArray alloc]init];
        }

        [transactions insertObject:amountSpentTextField atIndex:0];

        NSIndexPath *indexPath = [NSIndexPath indexPathForRow:0 inSection:0];

        [self.mytableView insertRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];

        //Add all the objects in the transactions array
        NSDecimalNumber *sum = [transactions valueForKeyPath:@"@sum.self"];

        //Get the user defaults from the amountToSpend field
        NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
        NSDecimalNumber *amountToSpend = [defaults objectForKey:@"AmountToSpend"];

        // Set the amountLeftInBudget label THE PROBLEM! the subtracting does not work
        [self.amountLeftInBudget setText:[amountToSpend decimalNumberBySubtracting:sum]];

        //Set the amountSpent label
        [self.amountSpent setText:[sum stringValue]];
    }
}

The problem in trying to subtract the two strings.

Upvotes: 1

Views: 549

Answers (2)

spinacher
spinacher

Reputation: 549

When you set the text on a UILabel you have to give it an NSString. So the line that you flag as a problem won't work...

// Set the amountLeftInBudget label THE PROBLEM! the subtracting does not work
[self.amountLeftInBudget setText:[amountToSpend decimalNumberBySubtracting:sum]];

...because you're giving UILabel an NSDecimalNumber. Convert the NSDecimalNumber into a string first:

[self.amountLeftInBudget setText:[[amountToSpend decimalNumberBySubtracting:sum] stringValue]];

Upvotes: 0

CRD
CRD

Reputation: 53010

Examine your expression, on the left-hand-side of - you have:

[[NSUserDefaults standardUserDefaults] objectForKey:@"AmountToSpend"]

What is the value of that expression? It is some reference to an object. Now the rhs is:

sum

and the value of that expression is some reference to an NSNumber. So what is the value of:

<some reference to an object> - <some reference to an `NSNumber`>

You are asking for the difference between two references, hence the error: Arithmetic on pointer to interface 'id', which is not a constant size for this architecture and platform.

You need to obtain the numeric value your objects represent, do your arithmetic on those, and then convert the numeric result to a string representation to assign it to your text property.

NSNumber has various methods to obtain the numeric value the object represents; e.g. doubleValue, intValue, etc.; and you need to select the appropriate one depending on your needs.

HTH.

Upvotes: 1

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