Quick question on programming a multiplication

start is a string. You can't multiply strings. You have to convert the string to a numeric type, such as long or double, then perform the multiplication.

You must then reverse the conversion, and convert you numeric value back into a string before assigning it to output.text.

After solving that, if you still think you need to release start, you should reread the memory management guide and carefully review the rules of ownership. You should be able to answer the question "Do I own this?" and decide about release without having to ask anyone else. This is fundamental.

 

The problem is that an NSString is just an ordered sequence of symbols with no inherent additional meaning. You also can't apply direct mathematical operators to objects — Objective-C differentiates between primitive types (all the ones from C, such as float, short, int) and objects (which are always kept track of by pointers, so have names beginning with an asterisk such as your object 'start'). Operators like *, /, + and - work on primitive types but not on objects.

What you can do is use either the 'floatValue' or 'doubleValue' method to get the Objective-C runtime to inspect the string and figure out what value it would represent as a number. Then you can multiple that by 1.1. Then you can use the runtime's ability to create strings that represent how numbers would look to a human to output the value.

That'd be (in very expressive code):

Code:

	NSString *start = input.text;
	double startValue = [start doubleValue];

	double endValue = startValue * 1.1;
	NSString *endText = [NSString stringWithFormat:@"%0.2f", endValue];

	output.text = endText;

Because start is an object, you'd expect it to know how to do operations on itself, so it performs doubleValue. Because endValue is a primitive type, not an object, it doesn't have any intelligence and you have to ask some other object (in this case NSString) to apply the intelligence to operate on it.