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Our DNA is written in Objective-C

Category Archive for ‘Q&A’ rss

The Best Way to Save Data

BebopSong asks:

Can anyone tell me the best way to create and store data on the iPhone so that the app can read the data but also write to it?

Having tried out all the possibilities that the iPhone platform holds in stock, here’s my opinion.

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NSString Category for Compressing Whitespace

Anonymous users keep asking:

How do I remove whitespace characters from a string?

A quick way to do that is to first split the words in the string by whitespace characters and then join them back together without seperator. But there is a more elegant way to achieve this, that’s reusable at the same time: extend NSString by adding a category to compress whitespace to an arbitrary seperator.

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OpenStreetMap in a UIWebView

Nicholas asks:

I’ve made an attempt recently to incorporate OpenStreetMap into my app. I’ve failed miserably. The result is a blank webpage. Any help/hints would be appreciated.

I immediately suspected that there was wrong with how he constructed the URL for the UIWebView, because I did run into a similar problem before.

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Minimum/Maximum of Multiple Values

MadIvad asks:

Is there some sort of math function for the minimum of a set of values? I have searched the docs and not found one reference to math in iPhone OS2.2, and min only returns the like of ‘minimum’ for different control values. or for stating what the minimum of the integer or NSUInt class etc…

Does a function/class/anything exist that would simply return the lowest value of 2 or more values?

Boy, that was easy. There are compiler macros defined that work on any scalar datatype. MIN(a,b) and MAX(a,b). Note the case.

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Variable Number of Decimal Places

Trapper asks:

I have one integer holding a variable number of decimal places that another variable needs to be rounded to when I stringWithFormat it. What is the correct way to do this?

Trapper is not content with just specifying %.2f in a stringWithFormat, but he wants the number of decimal places to be dependent on a second variable.

Here is the shortest method I came up with.

int decimals = 3;
double d = 3.1415;
NSString *format = [NSString stringWithFormat:@"%%%0.1ff", decimals/10.0];
NSString *formattedString = [NSString stringWithFormat:format, d]; // e.g. %0.3f 

I got confused at first because the NSLog would always output a strange value when wanted to output the formatting string. Then I remembered that the first parameter of NSLog itself is also interpreting formatting information. NSLog combines stringWithFormat into the output.

That’s good to know in case you want to add an NSLog statement for debugging floating point variables.

double d = 3.1415;
NSLog("%0.2f", d);  // formatting directly here

Don't Quit Your Day Job

Rob asks:

I have decided to start writing apps as a full time job. Assuming I can master this, and assuming I can get 1 app per month accepted in the App Store, can anyone give me some guidance on how much income I am likely to make.

Here are some numbers from my data that might help you:

  • A general purpose tool app like GeoCorder might sell between 1 and 5 copies a day.
  • Something interesting or unusual like iFR Cockpit can expect to sell around 5-10 copies a day.
  • A niche market tool like iWoman might to do well at 10-20 copies a day.
  • A game like LuckyWheel would sell around 20 copies a day IF you also have a LITE Version that has about 900-1000 downloads a day. Without a LITE version it could only be 5-10 copies a day.;-)

So assuming you concentrate on niche apps and games and calculating from $25 a day per such app you might make around $2000 a month if you manage to land 3 of those in the store. NOT taken into account additional cost like taxes or hardware. And not considering that Apple has the painful final word. Does that sound easy enough for you to immediately quit your day job?

For me it didn’t and it took me 8 months to get where I am today. It’s ok to see it as a lucrative hobby or even second income, but to stake your existance solely iPhone development you have to be extremely disciplined. Or even better: to know how to build teams of bright minds who can bring skills to the table that your don’t possess yourself.

How to Start Developing

I keep getting questions about how to best get into coding apps for the iPhone. Do I recommend books? How should one go about this?

I have one recommendation: Only steel good stuff. Download all the samples you can find on the Apple site and look them through. For most of the basic tasks you can find code to steal there, well commented.

But most importantly I recommend you get a mentor. Somebody who is slightly ahead of you in the objC programming game. But not too far ahead to feel annoyed by your questions… ;-)

And that’s the last recommendation: learn to ask the right questions. Because if you ask the right questions you will find that …

40% get answered by Apple documentation
40% get answered by Google: somebody has answered the question in his blog or in a forum
15% get answered by somebody more experienced than you directly if you ask
4% you have to discover for yourself … and then hopefully you document your discovery on your blog
1% are bugs that you have discovered by accident :-) Those you please submit to Apple for them to fix.

When I got started with coding objC for iPhone there was next to no useful literature. And now that I have written half a dozen apps and managed to get 4 into the store I don’t think I would ever need a book. Once you know how to read and understand objC code there is nothing really that you need for reference except the sources I mentioned above.

One more thing: try to develop a network of friends who all are sharing coding for iPhone as the same hobby. Of 10 questions I asked in various forums I only got around 3-5 answered usefully. If you remember the SDK 3.0 presentation, you know that more than half of current iPhone developers are new to the platform! So you are in the same boat as most of us. Be strong! Don’t give up!


Simon asks:

I would like to loop through variables whose names end with numbers 0 to 5. What is the correct syntax for the i? I have tried variable[i], variable + i, without success

I know two methods to achieve this:

1. Put your objects into an array and then enumerate or for-loop over those

// e.g. in awakeFromNib
playerNameLabels = [[NSArray arrayWithObjects:player1NameLabel, player2NameLabel, player3NameLabel, nil] retain];
for (i=0;i<3;i++)
     UILabel *nameLabel = [playerNameLabels objectAtIndex:i];
     // do something the the nameLabel

2. Use the Key-Value-Coding approach.

// in this object's header the strings are defined, here we intialize them
string1 = @"string1";
string2 = @"string2";
string3 = @"string3";
int i;
for (i=1;i<3;i++)
     NSString *aName = [NSString stringWithFormat:@"string%d", i];
     NSString *oneString = [self valueForKey:aName];
     NSLog(@"string %d is '%@' and has length %d", i, oneString, [oneString length]);

The dangerous thing about the latter might be that you have construct precisely the right name for the value. Or else you will get an exception:

*** Terminating app due to uncaught exception ‘NSUnknownKeyException’, reason: ‘[<selectortestAppDelegate 0x524e80> valueForUndefinedKey:]: this class is not key value coding-compliant for the key string0.’

But actually the second approach is how Cocoa does most KVC internally.

Localization Workaround

Trebor asks:

“When I add a new localization, there are just a few choices:





I thought that the system needed “fr” to know it is French and “de” to know it is German?

Will it recognize the word “German” and know that is the same as a de.lprog?

If the answer is yes, does it know the full names of all languages?

For example, if I want to add Chinese, can I call the localization “Chinese” or do I need to call it zh?”

I believe the following to be true. Xcode is still putting in English as first localization if you make a file localizable. This works, but the “modern way” to do things (or so I read) is to use the two letter iso codes. The problem is, that if you remove the English then you can never add any new localization. That’s a bug in Xcode.

So I do it like this:

  • create the file
  • make it localizable
  • add localizations for all locales you require, including English: en, de, it, es, fr, nl
  • LEAVE English empty, but put English into the “en” locale

Do not remove the “English”, because then XCode will not allow you to add new localizations in the future.

String to Array of Characters

Michael asks:

“How do I split the characters in an NSString into an NSArray? In BASIC I could split a string with empty character, but in Object C this does not work”

One way to do it is to just get one character substrings:

NSString *s = @"Hello World";
int i;
NSMutableArray *m = [[NSMutableArray alloc] init];
for (i=0;i&lt; [s length]; i++)
	[m addObject:[s substringWithRange:NSMakeRange(i, 1)]];
NSLog([m description]);  // most NS* objects have a useful description
[m release];  // don&#039;t forget

In Objective C every time you need a starting point and then a length from there it’s call an NSRange. Conveniently there is also a *Make macro for most structures like this. So in this case you just NSMakeRange(position,length) and pass this to a substringWithRange.