I have a list of contacts retrieved from Address book stored inside a MutableArray contactList. Each contact is an object which has properties like "contactName, contactImage.... etc".
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),^{
//getAllContacts is a method which returns a Mutable array of Objects
self.contactList = [NSMutableArray arrayWithArray:[instance getAllContacts]];
//groupLetterToLoad could be "DEF"
for(int j=0; j<self.groupLetterToLoad.length;j++) {
//1st iteration D, 2nd iteration E and 3rd iteration F
NSString *testChar = [NSString stringWithFormat:#"%c",[self.groupLetterToLoad characterAtIndex:j]];
//check D,E,F with contact name property's first letter of the contact list array
for(int i=0;i<self.contactList.count;i++) {
NSString *firstChar =[[[self.contactList objectAtIndex:i] contactName] substringToIndex:1];
if([testChar isEqualToString: firstChar]) {
pos=i; //retrieve the index of the matched position
break;
}
}
if(pos!=-1) break;
}
});
Now this has two for loops (Time O(n^2)).. The disadvantage here is, if the groupLetterToLoad is "WXYZ", then comparison will start from W with A to W with Z.. How can I optimise it?
Ordering your array by contactName and performing a half interval search will reduce your complexity greatly if can avoid sorting every time you search (hint: keep [instance getAllContacts] sorted).
http://rosettacode.org/wiki/Binary_search#Objective-C - that's a starting point. you could replace the compare: with your first character comparison.
This isn't an algorithmic improvement, but the way you're handling characters is about the slowest way possible. If your group letters are really ASCII letters as you indicate, try this (I include the "if" in my answer because doing correct comparison of non-ASCII is really best left up to NSString):
1) Instead of using -substringToIndex to get the first character, use -characterAtIndex:0 and store a unichar
2) Instead of using +stringWithFormat:#"%c" to make a single character string, just use -characterAtIndex: and store it in a unichar
3) Instead of using -isEqualToString:, use == on the unichars
Unrelated, I'm pretty suspicious of the thread-safety of this. Are all those properties on self and instance you're accessing really not accessed on any other queue or thread?
Related
New to objective-c, need help to solve this:
Write a function that takes two parameters:
1 a String representing a text document and
2 an integer providing the number of items to return. Implement the function such that it returns a list of Strings ordered by word frequency, the most frequently occurring word first. Use your best judgement to decide how words are separated. Your solution should run in O(n) time where n is the number of characters in the document. Implement this function as you would for a production/commercial system. You may use any standard data structures.
What I tried so far (work in progress): ` // Function work in progress
// -(NSString *) wordFrequency:(int)itemsToReturn inDocument:(NSString *)textDocument ;
// Get the desktop directory (where the text document is)
NSURL *desktopDirectory = [[NSFileManager defaultManager] URLForDirectory:NSDesktopDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:NO error:nil];
// Create full path to the file
NSURL *fullPath = [desktopDirectory URLByAppendingPathComponent:#"document.txt"];
// Load the string
NSString *content = [NSString stringWithContentsOfURL:fullPath encoding:NSUTF8StringEncoding error:nil];
// Optional code for confirmation - Check that the file is here and print its content to the console
// NSLog(#" The string is:%#", content);
// Create an array with the words contain in the string
NSArray *myWords = [content componentsSeparatedByString:#" "];
// Optional code for confirmation - Print content of the array to the console
// NSLog(#"array: %#", myWords);
// Take an NSCountedSet of objects in an array and order those objects by their object count then returns a sorted array, sorted in descending order by the count of the objects.
NSCountedSet *countedSet = [[NSCountedSet alloc] initWithArray:myWords];
NSMutableArray *dictArray = [NSMutableArray array];
[countedSet enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
[dictArray addObject:#{#"word": obj,
#"count": #([countedSet countForObject:obj])}];
}];
NSLog(#"Words sorted by count: %#", [dictArray sortedArrayUsingDescriptors:#[[NSSortDescriptor sortDescriptorWithKey:#"count" ascending:NO]]]);
}
return 0;
}
This is a classic job for map-reduce. I am very familiar with objective-c, but as far as I know - these concepts are very easily implemented in it.
1st map-reduce is counting the number of occurances.
This step is basically grouping elements according to the word, and then counting them.
map(text):
for each word in text:
emit(word,'1')
reduce(word,list<number>):
emit (word,sum(number))
An alternative for using map-reduce is to use iterative calculation and a hash-map which will be a histogram that counts number of occurances per word.
After you have a a list of numbers and occurances, all you got to do is actually get top k out of them. This is nicely explained in this thread: Store the largest 5000 numbers from a stream of numbers.
In here, the 'comparator' is #occurances of each word, as calculated in previous step.
The basic idea is to use a min-heap, and store k first elements in it.
Now, iterate the remaining of the elements, and if the new one is bigger than the top (minimal element in the heap), remove the top and replace it with the new element.
At the end, you have a heap containing k largest elements, and they are already in a heap - so they are already sorted (though in reversed order, but dealing with it is fairly easy).
Complexity is O(nlogK)
To achieve O(n + klogk) you may use selection algorithm instead of the min-heap solution to get top-k, and then sort the retrieved elements.
I created an array called NSArray citiesList from a text file separating each object by the "," at the end of the line. Here is what the raw data looks like from the text file.
City:San Jose|County:Santa Clara|Pop:945942,
City:San Francisco|County:San Francisco|Pop:805235,
City:Oakland|County:Alameda|Pop:390724,
City:Fremont|County:Alameda|Pop:214089,
City:Santa Rosa|County:Sonoma|Pop:167815,
The citiesList array is fine (I can see count the objects, see the data, etc.) Now I want to parse out the city and Pop: in each of the array objects. I assume that you create a for loop to run through the objects, so if I wanted to create a mutable array called cityNames to populate just the city names into this array I would use this kind of for loop:
SMutableArray *cityNames = [NSMutableArray array];
for (NSString *i in citiesList) {
[cityNames addObject:[???]];
}
My question is what is what query should I use to find just the City: San Francisco from the objects in my array?
You can continue to use componentsSeparatedByString to divide up the sections and key/value pairs. Or you can use an NSScanner to read through the string parsing out the key/value pairs. You could use rangeOfString to find the "|" and then extract a range. So many options.
Many good suggestions in the answers here in case you really want to construct an algorithm to parse the string.
As an alternative to that, you can also look at it as a problem of declaring the structure of the data and then just have the system do the parsing. For a case like yours, regular expressions will do that nicely. Whether you prefer to do it one way or the other is largely a question of taste and coding standards.
In your specific case (if the city name is all you need to extract from the string), then also notice that there is a bit of a shortcut available that will turn it into a one-line solution: Match the whole string, define a single capture group and substitute that one to make a new string:
NSString *city = [i stringByReplacingOccurrencesOfString: #".*City:(.*?)\\|.*"
withString: #"$1"
options: NSRegularExpressionSearch
range: NSMakeRange(0, row.length)];
The variable i is the same that you have defined in your for-loop, i.e. a string containing a string representing a line in your input file:
City:San Jose|County:Santa Clara|Pop:945942,
I have added the initial .* to make the pattern robust to future new fields added to the rows. You can remove it if you don't like it.
The $1 in the substitution string represents the first capture group, i.e. the parenthesis in the regex pattern. In this specific case, the substring containing the city name. Had there been more capture groups, they would have been named $2-$9. You can check the documentation on NSRegularExpression and NSString if you want to know more.
Regular expressions are a topic all of their own, not confined to the Cocoa, although all platforms use regex implementations with their own idiosyncrasies.
You want to use componentsSeparatedByString: as below. (These lines do no error checking)
NSArray *fields = [i componentsSeparatedByString:#"|"];
NSString *city = [[[fields objectAtIndex:0] componentsSeparatedByString:#":"] objectAtIndex:1];
NSString *county = [[[fields objectAtIndex:1] componentsSeparatedByString:#":"] objectAtIndex:1];
If you can drop the keys, and a couple delimiters like this:
San Jose|Santa Clara|945942
San Francisco|San Francisco|805235
Oakland|Alameda|390724
Fremont|Alameda|214089
Santa Rosa|Sonoma|167815
Then you can simplify the code (still no error checking):
NSArray *fields = [i componentsSeparatedByString:#"|"];
NSString *city = [fields objectAtIndex:0];
NSString *county = [fields objectAtIndex:1];
for (NSString *i in citiesList) {
// Divide each city into an array, where object 0 is the name, 1 is county, 2 is pop
NSArray *stringComponents = [i componentsSeparatedByString:#"|"];
// Remove "City:" from string and add the city name to the array
NSString *cityName = [[stringComponents objectAtIndex:0] stringByReplacingCharactersInRange:NSMakeRange(0, 5) withString:#""];
[cityNames addObject:cityName];
}
How do I concatenate the int length to the string I'm trying to slap into that array so it is "C10" given length == 10, of course. I see #"%d", intVarName way of doing it used else where. In Java I would of done "C" + length;. I am using the replaceObjectAtIndex method to replace the empty string, "", that I have previously populated the MSMutableArray "board" with. I am getting an error though when I add the #"C%d", length part at the end of that method (second to last line, above i++).
As part of my homework I have to randomly place "Chutes" (represented by a string of format, "C'length_of_chute'", in this first assignment they will always be of length 10 so it will simply be "C10") onto a game board represented by an array.
-(void)makeChutes: (int) length {// ??Change input to Negative number, Nvm.
//??Make argument number of Chutes ??randomly?? across the board.
for(int i = 0; i < length;){
int random = arc4random_uniform(101);
if ([[board objectAtIndex:random] isEqual:#""]) {
//[board insertObject:#"C%d",length atIndex:random];
[board replaceObjectAtIndex:random withObject:#"C%d",length];
i++;
}
}
}
Please ignore the extra and junk code in there, I left it in for context.
In Objective-C the stringWithFormat method is used for formatting strings:
NSString *formattedString = [NSString stringWithFormat:#"C%d", length];
[someArray insertObject:formattedString];
It's often easier to create your formatted string on a line of its own in Objective-C, since as you can see the call can be fairly verbose!
I have an NSArray of NSStrings and would like to know how to compare each item in the array with every other item in the array to see if there is any strings different from the rest.
I have seen a c++ example
for (int i = 0; i < list.size(); i++) {
for (int j = i+1; j < list.size(); j++) {
// compare list.get(i) and list.get(j)
}
}
but was woundering if there is a better easier way in objective C? also the other thing I need to do is make sure the item doesn't compare itself while it loops through.
Any help or examples would be greatly appreciated.
UPDATE ** BOLD is the updated part of the question **
If I read your question correctly, you want the strings that only appear once in the list, correct?
NSCountedSet *counted = [NSCountedSet setWithArray:list];
for (NSString *string in counted) {
NSUInteger count = [counted countForObject:string];
if (count == 1) {
// process "string", it appears in the list just once
}
}
If you just want to know if there is more than one different value in the list then do this:
NSSet *set = [NSSet setWithArray:list];
if (set.count == 1) {
// There is only one distinct value in the list
} else {
// There is more than one distinct value in the list
}
I'd use an NSMutableDictionary. This is very similar to "merging two lists into unique values", the apple docs actually explain the complicated way somewhere. I forgot where I found it, but the easy way is here: Merge two arrays while preserving the original array order
So what you'd do is loop through everything, see if there's a key (set to the string), if not, add one via the setObject: forKey: method, then enumerate through the dictionary or just grab the allKeys value after.
Use two sets. If the string goes into the first set without conflict, add it to the second set. If the string encounters a conflict in the first set, remove it from the second set. When you've processed all the strings the second set contains the unique ones.
Is there a quick method that gets the largest word from an array of words?
NSMutableArray wordlist
Something like the following should do the trick:
NSString *longest = nil;
for(NSString *str in wordlist) {
if (longest == nil || [str length] > [longest length]) {
longest = str;
}
}
I'm not aware of any simpler method.
You could use something like this example to sort an Array (but instead of the 'quality' sort in the example use a length sort on your strings) and then the longest string will be either at the top or at the end (depending on your sorting).
I don't know any objective C myself, but my solution would be to keep an integer 'longest' and a string 'longestWord' and initialise it to 0 and "". Then loop over the list and check if the current word is longer then the 'longest' value. If it is, store the new length and the current word. At the end of the loop, you have the longest word stored in the 'longestWord' variable.
Hope this helps