I want to replace multiple elements in my string in Objective-C.
In PHP you can do this:
str_replace(array("itemtoreplace", "anotheritemtoreplace", "yetanotheritemtoreplace"), "replacedValue", $string);
However in objective-c the only method I know is NSString replaceOccurancesOfString. Is there any efficient way to replace multiple strings?
This is my current solution (very inefficient and.. well... long)
NSString *newTitle = [[[itemTitleField.text stringByReplacingOccurrencesOfString:#"'" withString:#""] stringByReplacingOccurrencesOfString:#" " withString:#"'"] stringByReplacingOccurrencesOfString:#"^" withString:#""];
See what I mean?
Thanks,
Christian Stewart
If this is something you're regularly going to do in this program or another program, maybe make a method or conditional loop to pass the original string, and multi-dimensional array to hold the strings to find / replace. Probably not the most efficient, but something like this:
// Original String
NSString *originalString = #"My^ mother^ told me not to go' outside' to' play today. Why did I not listen to her?";
// Method Start
// MutableArray of String-pairs Arrays
NSMutableArray *arrayOfStringsToReplace = [NSMutableArray arrayWithObjects:
[NSArray arrayWithObjects:#"'",#"",nil],
[NSArray arrayWithObjects:#" ",#"'",nil],
[NSArray arrayWithObjects:#"^",#"",nil],
nil];
// For or while loop to Find and Replace strings
while ([arrayOfStringsToReplace count] >= 1) {
originalString = [originalString stringByReplacingOccurrencesOfString:[[arrayOfStringsToReplace objectAtIndex:0] objectAtIndex:0]
withString:[[arrayOfStringsToReplace objectAtIndex:0] objectAtIndex:1]];
[arrayOfStringsToReplace removeObjectAtIndex:0];
}
// Method End
Output:
2010-08-29 19:03:15.127 StackOverflow[1214:a0f] My'mother'told'me'not'to'go'outside'to'play'today.'Why'did'I'not'listen'to'her?
There is no more compact way to write this with the Cocoa frameworks. It may appear inefficient from a code standpoint, but in practice this sort of thing is probably not going to come up that often, and unless your input is extremely large and you're doing this incredibly frequently, you will not suffer for it. Consider writing these on three separate lines for readability versus chaining them like that.
You can always write your own function if you're doing something performance-critical that requires batch replace like this. It would even be a fun interview question. :)
Considered writing your own method? Tokenize the string and iterate through all of them replacing one by one, there really is no faster way than O(n) to replace words in a string.
Would be a single for loop at most.
Add the # to the start of the all the strings, as in
withString:#""
It's missing for a few.
Related
I have an app (Cocoa Touch, Web Browser), however I need to be able to compare an NSString with thousands of other strings. Here's the deal.
When a WebView loads, I get the URL. I need to compare this URL with literally thousands of results (27,847). Each of those numbers represents a line of text in a plain text file.
I would like to know the best way to go about getting the data from the text file, and comparing it with the NSString. I need to know if the URL that the WebView is loading contains any of these strings.
The app needs to be very fast, so I can't just parse through every line in the text file, turn it into an array, and then compare each and every result.
Please share your ideas. Thanks.
I think the cleanest solution is to:
Create a web service that can offload the work to a server and return a response. Since it sounds like you're building a web protection service, your database may grow to be quite substantial over time, and you can just scale your server up to increase its speed. Furthermore, you don't want to have to update your app every time the lookup data changes.
Other options are:
Use a local SQLite database. SQL databases should perform lookups relatively fast.
If you don't want to use any database, have you tried putting all the search strings into an NSDictionary or NSMutableDictionary object? This way, you would just check if the valueForKey: for the string you're searching for is nil.
Sample code for this:
NSDictionary *searchDictionary = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES], #"google.com",
[NSNumber numberWithBool:YES], #"yahoo.com",
[NSNumber numberWithBool:YES], #"bing.com",
nil];
NSString *searchString = #"bing.com";
if ([searchDictionary valueForKey:searchString]) {
// search string found
} else {
// search string not found
}
Note: if you want the NSDictionary to perform case-insensitive comparisons, pre-load all values lowercase, and make the search string lowercase when using valueForKey:.
How much memory this could take is a whole other story, but I don't see how this comparison could be made much faster locally. I strongly recommend the remove web service approach, though.
Create a string from the file and enumerate through the lines.
NSString *stringToCheck;
NSData *bytesOfFile = [NSData dataWithContentsOfFile:#"/path/myfile.txt"];
NSString *fileString = [[NSString alloc] initWithData:bytesOfFile
encoding:NSUTF8Encoding];
__block BOOL foundMatch = NO;
[fileString enumerateLinesUsingBlock:^(NSString *line, BOOL *stop){
if([stringToCheck isEqualToString:line]){
*stop = YES;
foundMatch = YES;
}
}];
This is a job for regular expressions. Take all of the substrings you're looking for/filtering against, escape them appropriately (escaping characters such as [, ], |, and \, among others, with \), and join them with a |. The resulting string is your regular expression, which you apply to each URL.
You could loop through an entire array full of substrings, doing rangeOfString:options: with each one, but that's the slow way. A good regular expression implementation is built for this sort of thing, and I would hope that Apple's implementation is suitable.
That said, profile the hell out of it. I've seen some regex implementations choke on the | operator, so you'll want to make sure that Apple's is not one of them.
If you need to compare each string in your text file, you are going to have to compare it, no way around it.
What you can do however is do it on a background thread while showing some loading or something, and it won't feel as if the app got stuck.
I would suggest you try with NSDictionary first. You can load up all your URLs into this, and internally it will use some sort of hash table/map for very quick (O(1)) lookup.
You can then check the result of [dictionary objectForKey:userURL], and if it returns something then the URL matched one in the dictionary.
The only problem with this is that it requires an exact string match. If your dictionary contains http://server/foobar and the user enters http://server/FOOBAR (because it's a case-insensitive server), you are going to get a miss on your lookup. Similarly, adding ?foobar queries to the end of URLs will result in a miss. You could also add an explicit port with server:80, and with %XX character encoding you can create hundreds of variations of the same URL. You will have to account for this and canonicalize both the URLs in your dictionary, and the URL entered by the user prior to lookup.
I'm learning Objective C and iOS development. I'm trying to recreate some of the projects we did in my Java class (I know they're completely different) but I'm running into trouble in one of the projects. We were doing a caesar shift in a lab one day. A string manipulation lab. It was a really basic deal in Java... a for loop through the string and change each character. I can't seem to find any way to change individual characters in Objective C. I've looked through the NSMutableString documentation and NSString documentation and I know I can do a
[NSString stringByReplacingCharactersInRange:(NSRange *) withString:(NSString *)
but that doesn't really help because I don't know what I'm going to be replacing with. I need to find a way to grab a character at a specific index and change it. Any ideas?
Sounds like you are looking for the [NSString characterAtIndex:(NSUInteger)] method
E.g.
NSString *string = #"abcde";
NSString *character = [NSString stringWithFormat:#"%C",[string characterAtIndex: 0]];
NSLog(#"%#", character);
Result: a
Using this, and an NSMutableString, you can build the string you need.
Ussing appendString you can add to the end of an NSMutableString
Probably the best way to do this would be using a good old C-string, as that allows you to change the bytes without the overhead of reallocating a different string every time:
NSString *ceasarShift(NSString *input)
{
char *UTF8Str = strdup([input UTF8String]);
int length = [input length];
for (int i = 0; i < length; i++)
{
UTF8Str[i] = changeValueOf(UTF8Str[i]); // some code here to change the value
}
NSString *result = [NSString stringWithUTF8String:UTF8Str];
free(UTF8Str);
return result;
}
This reduces overhead, and although you have to free the data you allocated when you are done, it gives you the advantage of not relying on a high level API, improving performance drastically. (The difference between an array set and a dynamic method lookup is ~5 CPU cycles, which means a lot if you are doing any major sort of encryption)
Maybe also look into NSMutableData for this kind of task, instead of NSString, as the random \0 may per chance appear in the result string.
I've been trying to read sections of a string into an array and encountered some problems. Though my task is more complicated, even a simple example case like the one below gives the same problem: the code compiles and runs but the the size of the strings array is always one and the only thing that's contained in strings[0] is "__NSArrayM".
NSString *string = #"John, Bob, Jane";
NSArray *strings = [string componentsSeparatedByString: #", "];
Thanks in advance for any ideas!
When you say "the size of the strings array", do you mean what you get from [strings count] or something else? Also, Objective-C doesn't let you access NSArray elements with the square bracket notation, so if you're literally calling strings[0] you're doing some pointer math on the Array object that you don't intend to be doing.
Well, I've been learning Objective-C for a while now, and I don't get why creating an NSArray would be beneficial for you. It's just a collection of some stuff right? Why can't you just use them without making an NSArray. Or can you use the objects in it in the implementation of every one of your methods (even if it's a local ivar).
So, any help would be appreciated. Thanks guys!
You could make instance variables for every “item” you need:
NSString *str1;
NSString *str2;
NSString *str3;
…but that’s hard to work with, an array is simply more convenient. What would you do if you wanted to print all these strings?
NSLog(#"%#", str1);
NSLog(#"%#", str2);
NSLog(#"%#", str3);
Wouldn’t it be easier to loop over an array?
NSArray *strings = [NSArray arrayWithObjects:#"one", #"two", #"three", nil];
for (NSString *str in strings)
NSLog(#"%#", str);
How about if you have ten, twenty strings? And what if you don’t know how many strings you will need? What if you want to pass all these items to somebody else? Are you going to pass them one by one?
- (void) doSomethingWithString1: (NSString*) str1 andString2: (NSString*) str2…;
Or would you rather pass an array?
- (void) doSomethingWithMyStrings: (NSArray*) strings;
Very often, the NSArray you see is a front for an NSMutableArray working behind the scenes.
Billing it in the interface as an NSArray is just a way to ensure that it cannot be manipulated from without, which could have moderately disastrous consequences. (Thus, when asking for an NSArray from an object, you'll usually get a copy of the NSMutableArray used internally.)
Understanding why creating an NSMutableArray is beneficial is left as an exercise to the reader.
Different contexts.
When you use an NSString it's generally used for a single value. It's quite simple.
NSArray
If you want to loop through many keys/values, use an NSArray. It is not possible to loop through NSStrings because they don't have the order structer that NSArray's do.
Why use NSString
It would be silly to call myArray[0] and foo[0] every time you saved just a value. NSString is simpler than NSArray, since you can easily set a value on a GUI element (without having to call myArray[0])
Why use NSArray
It is more flexible than NSString because NSArray can hold multiple values, which may be needed in certain situations.
Weighing pros and cons
Pros (NSArray):
Can make your code cleaner (call myArray[1] and myArray[0] instead of mystring and foo)
Simplified memory management (easier to release one NSArray than many NSStrings)
Cons (NSArray):
Can make your code very complicated (was that on myArray[0] or 1?)
Might be difficult to know what type of data is in the array (if it's an NSString, it's always going to be a string, an array can hold many different types of data). You wouldn't use 50 tissues to dry yourself off from the shower but use a towel.
So it boils down to one thing. What context are you using?
I want to create an NSArray with objects of the same value (say NSNumber all initialized to 1) but the count is based on another variable. There doesn't seem to be a way to do this with any of the intializers for NSArray except for one that deals with C-style array.
Any idea if there is a short way to do this?
This is what I am looking for:
NSArray *array = [[NSArray alloc] initWithObject:[NSNumber numberWithInt:0]
count:anIntVariable];
NSNumber is just one example here, it could essentially be any NSObject.
The tightest code I've been able to write for this is:
id numbers[n];
for (int x = 0; x < n; ++x)
numbers[x] = [NSNumber numberWithInt:0];
id array = [NSArray arrayWithObjects:numbers count:n];
This works because you can create runtime length determined C-arrays with C99 which Xcode uses by default.
If they are all the same value, you could also use memset (though the cast to int is naughty):
id numbers[n];
memset(numbers, (int)[NSNumber numberWithInt:0], n);
id array = [NSArray arrayWithObjects:numbers count:n];
If you know how many objects you need, then this code should work, though I haven't tested it:
id array = [NSArray arrayWithObjects:(id[5]){[NSNumber numberWithInt:0]} count:5];
I can't see any reason why this structure in a non-mutable format would be useful, but I am certain that you have your reasons.
I don't think that you have any choice but to use a NSMutableArray, build it with a for loop, and if it's really important that the result not be mutable, construct a NSArray and use arrayWithArray:
I agree with #mmc, make sure you have a valid reason to have such a structure (instead of just using the same object N times), but I'll assume you do.
There is another way to construct an immutable array which would be slightly faster, but it requires creating a C array of objects and passing it to NSArray's +arrayWithObject:count: method (which returns an autoreleased array, mind you) as follows:
id anObject = [NSNumber numberWithInt:0];
id* buffer = (id*) malloc(sizeof(id) * anIntVariable);
for (int i = 0; i < anIntVariable; i++)
buffer[i] = anObject;
NSArray* array = [NSArray arrayWithObjects:buffer count:anIntVariable];
free(buffer);
You could accomplish the same thing with even trickier pointer math, but the gains are fairly trivial. Comment if you're interested anyway.
Probably the reason there is no such method on NSArray is that the semantics are not well defined. For your case, with an immutable NSNumber, then all the different semantics are equivalent, but imagine if the object you were adding was a mutable object, like NSMutableString for example.
There are three different semantics:
retain — You'd end up with ten pointers to the same mutable string, and changing any one would change all ten.
copy — You'd end up with ten pointers to the same immutable string, or possibly ten different pointers to immeduable strings with the same value, but either way you'd not be able to change any of them.
mutableCopy — You'd end up with ten different mutable string objects, any of which you could change independently.
So Apple could write three variants of the method, or have some sort of parameter to control the semantics, both of which are ugly, so instead they left it to you to write the code. If you want, you can add it as an NSArray category method, just be sure you understand the semantic options and make it clear.
The method:
-(id)initWithArray:(NSArray *)array copyItems:(BOOL)flag
has this same issue.
Quinn's solution using arrayWithObjects:count: is a reasonably good one, probably about the best you can get for the general case. Put it in an NSArray category and that's about as good as it is going to get.