"Dynamic" display of floats?

I'm trying to write an app for iphone in ObjectiveC. I need to display a statustext that tells the user the value of a few variables. These values are 开发者_开发技巧changeable by dragging a marker with your finger in a diagram. The problem I have is that the value range is quite wide. If you want to set values between 0 and 1 you probably want 3 decimals (like 0.345). But if the range is 0 to 10000, you don't need any decimals at all.

I now have about 20 diffrent messages that can be displayed and if I want all of them to display values "dynamically" there is going to be a lot of code like this:

float start,stop;    // Defined earlier...
switch ( numberOfDecimals ) {
    case 0:
        lblStatus.text = [NSString stringWithFormat:@"Start: %.0f  Stop :%.0f", start, stop];  break;
    case 1:
        lblStatus.text = [NSString stringWithFormat:@"Start: %.1f  Stop :%.1f", start, stop];  break;
    case 2:
        lblStatus.text = [NSString stringWithFormat:@"Start: %.2f  Stop :%.2f", start, stop];  break; 
    default:  break;
} 

Is there not a better way to do it?

Use an NSNumberFormatter. This is what it's for.

Build your format string with a format string.

NSString* MyFormatString = [NSString stringWithFormat:@"Start :%%.%df Stop :%%.%df",
        numberOfDecimals,
        numberOfDecimals];
[NSString stringWithFormat:MyFormatString, start, stop];  break; 

In case anyone cares, this is what I came up with after consulting the docs for NSNumberFormatter:

float start,stop; 
NSNumberFormatter *formatter;

// Setup the formatter
formatter = [[NSNumberFormatter alloc] init]; 
[formatter setNumberStyle:NSNumberFormatterDecimalStyle ];
[formatter setMaximumFractionDigits: numberOfDecimals ];

lblStatus.text = [NSString stringWithFormat:@"Start: %@  Stop:%@", 
                  [formatter stringFromNumber: [NSNumber numberWithFloat: start]], 
                  [formatter stringFromNumber: [NSNumber numberWithFloat: stop ]]];  

Not sure if it's the most elegant or efficient solution but it works and looks better.

Simple. Replace the switch statement with:

lblStatus.text = [NSString stringWithFormat:@"Start: %.*f  Stop :%.*f", numberOfDecimals, start, numberOfDecimals, stop];