Objective c converter app - ios

Question

I am working on a small converter app for my school project, an app that won't be published in App Store, just for school grade. I am wondering if anyone knows how did this developer in this app
made connections for converting values? The simplest way i can imagine is with switch cases but that would make too much unnecessary code:
if (first column is distance) then if(second column is metres)
case0:third column is yards -> do something
case1:third column is kilometers ->do something
case2:third column is metros ->do nothing
etc....
Thanks for answering.
EDIT:
Thanks for fast answer, this is what i made so far, iOS simulator is displaying correctly three column picker in which second two are displayed depending on the row of the first one. Can u tell me is this any good? Maybe some things are uneccesary complicated?
#import "konverterViewController.h"
#interface konverterViewController ()
#end
#implementation konverterViewController
- (void)viewDidLoad
{
[super viewDidLoad];
self.fizVelicine = [[NSArray alloc] initWithObjects:#"distance",#"mass" , nil];
self.udaljenostJedinice = [[NSArray alloc] initWithObjects:#"meter", #"kilometer", #"yard", #"inch", nil];
self.masaJedinice = [[NSArray alloc] initWithObjects:#"kilogram",#"dekagram",#"gram",#"tone" , nil];
}
- (UIView *)pickerView:(UIPickerView *)pickerView viewForRow:(NSInteger)row forComponent:(NSInteger)component reusingView:(UIView *)view
{
UILabel *retval = (id)view;
if (!retval) {
retval = [[UILabel alloc] initWithFrame:CGRectMake(0.0f, 0.0f,
[pickerView rowSizeForComponent:component].width,
[pickerView rowSizeForComponent:component].height)];
}
retval.opaque = NO;
retval.backgroundColor = [UIColor orangeColor];
retval.adjustsFontSizeToFitWidth = YES;
retval.font = [UIFont systemFontOfSize:12];
if (component == 0) {
retval.textAlignment = NSTextAlignmentCenter;
retval.text = [self.fizVelicine objectAtIndex:row];
}
else if ([self.odabir isEqualToString:#"distance"]){
retval.textAlignment = NSTextAlignmentCenter;
retval.text = [self.udaljenostJedinice objectAtIndex:row];
}
else {
retval.textAlignment = NSTextAlignmentCenter;
retval.text = [self.masaJedinice objectAtIndex:row];
}
return retval;
}
// returns the number of 'columns' to display.
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView
{
return 3;
}
// returns the # of rows in each component..
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent: (NSInteger)component
{
if(component== 0)
{
return [self.fizVelicine count];
}
else
{
if([self.odabir isEqualToString:#"distance"]){ return [self.udaljenostJedinice count];}
else {return [self.masaJedinice count];}
}
}
-(NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component
{
if(component == 0)
{
return [self.fizVelicine objectAtIndex:row];
}
else {
if([self.odabir isEqualToString:#"distance]){
return [self.udaljenostJedinice objectAtIndex:row];}
else {
return [self.masaJedinice objectAtIndex:row];
}
}
}
- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component
{
if (component == 0) {
self.odabir=[[NSString alloc] initWithFormat:#"%#" , [self.fizVelicine objectAtIndex:row]];
[pickerView reloadAllComponents];}
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end

Answer 1

It sounds like you're puzzling over how to initialize the picker sections at the top. I think you'll need a data structure that is an array of arrays:
#[#"distance", #[#"meters", #"inches", #"cubits", #"whatevers"],
#"weight", #[#"grams", // etc
Tell the picker that you have three segments. The first segment can be manufactured (and cached, if you want) as the first elements of each sub-array. The second two segments get their values from the data structure, indexed by the value of the first section.
EDIT; You'll probably want a data structure that helps you compute the conversion. For each kind of measure, imagine an NxN table, where N is the number of units on the wheels. A row let's you look up the left wheel setting and the column lets you look up the right wheel. The table should contain what you need to do the calculation. If all of the measures linearly related, like inches-meters, the table can simply contain floats (wrapped as NSNumber) conversion factors.
// conversion factors for distance, units as the first value, conversion factors in a subarray
// meters = 1*meters, 39.37*inches, 2.19*cubits, etc
NSArray *conversions =
#[#"meters", #[#1.0, #39.37, #2.19, //etc
#"inches", // etc. conversions to meters inches cubits etc.
You might notice that this array contains some waste. Only half the table is needed, since inches->meters is just the reciprocal of meters->inches) but I think the space small enough to not worry about this. You might also notice that the table is good only for simple linear relationships between units. Centigrade to Fahrenheit will require a different treatment. The most general treatment for arbitrary conversions might be an array of blocks that carry out the computation. But I suggest starting more simply.
EDIT AGAIN:
Now, to get a conversion formula, get the current picker selections and look up a conversion factor:
NSInteger fromUnitsIndex = [self.picker selectedRowInComponent:1];
NSArray *fromArray = self.conversions[fromUnitsIndex];
// the second member of this array is an array of conversion factors
NSArray *conversionNumbers = fromArray[1];
NSInteger toUnitsIndex = [self.picker selectedRowInComponent:2];
NSNumber *conversionNumber = conversionNumbers[toUnitsIndex];
It sounds like a fun project. Good luck!