stringByReplacingOccurrencesOfString not working as expected

Question

Having a problem. Here's my code:

Latitude = [TBXML textForElement:lat]; //Latitude & Longitude are both NSStrings
Longitude= [TBXML textForElement:lon];
NSLog(@"LAT:%@ LON:%@",Latitude,Longitude);
NSString *defaultURL = @"http://api.wxbug.net/getLiveWeatherRSS.aspx?ACode=000000000&lat=+&long=-&unittype=1";
newURL = [[defaultURL stringByReplacingOccurrencesOfString:@"+" 
                                                        withString:Latitude]
                                    stringByReplacingOccurrencesOfString:@"-" 
                                                        withString:Longitude];
NSLog(@"%@",newURL);

And here's the output:

LAT:-33.92 LON:18.42 
http://api.wxbug.net/getLiveWeatherRSS.aspxACode=000000000&lat=18.4233.92&long=18.42&unittype=1

As you can see, something strange is happening to the appending code. Am I doing something wrong here?

score 7 · Accepted Answer · answered Feb 17 '11 at 18:19

Before replacing the longitude, the string is

http://....&lat=-33.92&long=-&...
                ^           ^

The system sees that there are two -, and thus both of them will be replaced by the latitude.

You should use a more descriptive string to replace with, e.g.

NSString *defaultURL = @"http://....&lat={latitude}&long={longitude}&unittype=1";
newURL = [defaultURL stringByReplacingOccurrencesOfString:@"{latitude}" 
                                               withString:Latitude];
newURL = [newURL stringByReplacingOccurrencesOfString:@"{longitude}" 
                                           withString:Longitude];

or simply use +stringWithFormat:.

NSString* newURL = [NSString stringWithFormat:@"http://....&lat=%@&long=%@&...",
                                              Latitude, Longitude];

Thank you, it all comes clear now. :) I will do as you suggested and change the placeholder to a more clear value. Thanks! — sudo rm -rf, Feb 17 '11 at 18:22

score 3 · Answer 2 · answered Feb 17 '11 at 18:20

Here's where we started:

url = @"http://...?ACode=000000000&lat=+&long=-&unittype=1"
Latitude = @"-33.92"
Longitude = @"18.42"

Then you replaced all occurrences of @"+" with @"-33.92":

url = @"http://...?ACode=000000000&lat=-33.92&long=-&unittype=1"

Then you replaced all occurrences of @"-" with @"18.42". Note that there are two '-' characters; one after lat= and one after long=. The one after 'lat' is there because the string you pasted in had a - in it.

url = @"http://...?ACode=000000000&lat=18.4233.92&long=18.42&unittype=1"

Thus, your final result.

score 2 · Answer 3 · edited May 23 '17 at 12:13

@KennyTM, BJ Homer, and madmik3 are correct. Your value is getting replaced twice.

However, you should technically be building your URL in a totally different manner:

NSMutableDictionary *query = [NSMutableDictionary dictionary];
[query setObject:@"000000000" forKey:@"ACode"];
[query setObject:Latitude forKey:@"lat"];
[query setObject:Longitude forKey:@"long"];
[query setObject:@"1" forKey:@"unittype"];

NSMutableArray *queryComponents = [NSMutableArray array];
for (NSString *key in query) {
  NSString *value = [query objectForKey:key];
  key = [key stringByAddingPercentEscapesUsingEncoding:NSASCIIStringEncoding];
  value = [value stringByAddingPercentEscapesUsingEncoding:NSASCIIStringEncoding];

  NSString *component = [NSString stringWithFormat:@"%@=%@", key, value];
  [queryComponents addObject:component];
}

NSString *queryString = [components componentsJoinedByString:@"&"];

NSString *fullURLString = [NSString stringWithFormat:@"http://api.wxbug.net/getLiveWeatherRSS.aspx?%@", queryString];
NSURL *newURL = [NSURL URLWithString:fullURLString];

(ignoring the efficacy of -stringByAddingPercentEscapesUsingEncoding: for now)

The reason this is better is that according to the HTTP specification, the keys and values in the query of the URL should be URL encoded. Granted, you're only encoding numbers for simple keys. But if that ever changes, you URL might break. (The flaw with this method is that it only allows a single value per key, and the HTTP spec allows you to specify multiple values. For the sake of simplicity, I've left that out)

There are also some issues on using -stringByAddingPercentEscapesUsingEncoding:. For more information on that, check out Objective-c iPhone percent encode a string?.

Dave, awesome advice. For now I don't think I need to implement it, but I'll keep it in mind. Thanks for sharing. — sudo rm -rf, Feb 17 '11 at 18:51

score 1 · Answer 4 · answered Feb 17 '11 at 18:21

1

Your LAT is negative. So the - gets replaced twice.

answered Feb 17 '11 at 18:21

madmik3

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stringByReplacingOccurrencesOfString not working as expected

4 Answers4