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Objective C — What is the fastest and most efficient way to enumerate an array?
(3 answers)
Closed 8 years ago.
NSEnumerator* friendsEnumerator = [friends objectEnumerator];
id aFriend;
while ((aFriend = [friendsEnumerator nextObject])) {
printf("%s\n", [aFriend UTF8String]);
}
int friendsCount = [friends count];
for(int i = 0; i < friendsCount; i++) {
printf("%s\n", [[friends objectAtIndex: i] UTF8String]);
}
for(NSString* aFriend in friends) {
printf("%s\n", [aFriend UTF8String]);
}
Case 3 is the fastest and generally better approach:
Fast enumeration is the preferred method of enumerating the contents of a collection because it provides the following benefits:
The enumeration is more efficient than using NSEnumerator directly.
List item
The syntax is concise.
The enumerator raises an exception if you modify the collection while enumerating.
You can perform multiple enumerations concurrently.
You can read more about it here
You can enumerate an array using the below method also.The stop parameter is important for performance, because it allows the enumeration to stop early based on some condition determined within the block.
[friends enumerateObjectsUsingBlock:^(id obj, NSUInteger index, BOOL *stop){
if ('some condition') {
NSLog(#"Object Found: %# at index: %i",obj, index);
*stop = YES;
}
} ];
First things first: option 1 and 3 are the same in terms of operations, both use the NSFastEnumeration protocol to provide a fast access to objects in collections.
As the name "NSFastEnumeration" implies, enumerations are faster then for-loops, as they don't need array bounds checking for each single object.
So it comes down to a matter of taste between 1 and 3. Personally I prefer forin-loops as they seem to be more elegant.
Related
I'm just starting out with objective c (coming from java) and I'm working on a calculator program just to practice with the syntax and some basic stuff. The way I'm going about it is having the user input a string and looking through for operators (taking order of operations into account) and then finding the term surrounding that operator, calculating it, replacing the term with the answer, and repeating for all the terms; however, I'm having an issue with the method I'm using to calculate the term. I pass in the index of the operator and have it loop backwards until it hits another operator to find the number immediately before it, and do the same forwards for the number after. My issue is that the loop does not stop when it hits the operators, and instead just continues until the end of the string in both directions. It's probably something really simple that I've overlooked but I've been trying to figure this out for a while and can' seem to get it. I've included an SSCCE of just the first half of the method, with a predetermined string and operator index. (also, a secondary question: is there any better way to post code blocks on this site rather than manually putting in 4 spaces before every line?)
#import <Foundation/Foundation.h>
int firstNumInTerm(int index);
NSString *calculation;
int main(int argc, const char * argv[])
{
#autoreleasepool {
calculation = #"51-43+378*32";
int firstNumber = firstNumInTerm(9);
NSLog(#"The number before the term is: %i", firstNumber);
}
return 0;
}
int firstNumInTerm(int index){
int firstNumIndex = index - 1;
int firstNumLength = 1;
NSRange prevChar = NSMakeRange(firstNumIndex - 1, 1);
while ([calculation substringWithRange:prevChar] != #"*" &&
[calculation substringWithRange:prevChar] != #"/" &&
[calculation substringWithRange:prevChar] != #"+" &&
[calculation substringWithRange:prevChar] != #"-" &&
firstNumIndex > 0) {
NSLog(#"prevChar: %#", [calculation substringWithRange:prevChar]);//TEST
firstNumIndex--; firstNumLength++;
prevChar = NSMakeRange(firstNumIndex - 1, 1);
}
NSRange firstRange = NSMakeRange(firstNumIndex, firstNumLength);
int firstNum = [[calculation substringWithRange:firstRange] intValue];
NSLog(#"firstNum String: %#", [calculation substringWithRange:firstRange]);//TEST
NSLog(#"firstNum int: %i", firstNum);//TEST
return firstNum;
}
The problem with this line:
[calculation substringWithRange:prevChar] != #"*" is that you are comparing the value of two pointers. [calculation substringWithRange:prevChar] returns a pointer to an NSString object, as does the NSString literal statement #"*". The simplest way to compare two strings is by using the isEqualToString: method of NSString. For example:
NSString *myName = #"Stephen";
NSString *yourName = #"Matt";
if([myName isEqualToString:yourName]){
printf("We have the same name!");
}
else{
printf("We do not have the same name");
}
If you are going to be doing a lot of string comparisons, it might be wise to write a macro, such as:
#define STREQ(x,y) [x isEqualToString:y]
Regarding copy/pasting code into StackOverflow:
Since I use XCode 99% of the time, I find it handy to select the text I am going to copy and then hit Cmd-]. This shifts the text to the right one tab-width. I then Cmd-c to copy and then Cmd-[ to undo the right-shift.
You can't do that in Objective-C: [calculation substringWithRange:prevChar] != #"*"
Instead, you need to do :
[[calculation substringWithRange:prevChar] compare:#"*"] != NSOrderedSame
(I know, it's longer, but arithmetic operators aren't overloaded for string like they are in Java).
I see others have answered this to correct the issue with your string comparison operations, but a better way to split this string up would be using NSString's native parsing methods. For example:
NSArray *numbers = [ calculation componentsSeparatedByCharactersInSet:
[ NSCharacterSet characterSetWithCharactersInString: #"*/+-" ] ];
Will give you an array containing each of the numbers (in order) in your string. You could come up with custom parsing routines, but using NSString's is going to likely be more straightforward and a lot less buggy. It will also be easier for someone else to read and understand.
while((![[calculation substringWithRange:prevChar] isEqualToString:#"*"]) && …){
}
or
NSArray *operators = #[#"+", #"-", #"*", #"/"];
while(![operators contains:[calculation substringWithRange:prevChar]])
I'm using the value of two sliders as the first search criteria.
Now I want a second criteria. I have a
NSMutableArray *suitsArray;
which is containing up to 10 words, or it could be empty (0 words). Example of array:
2012-08-12 03:13:14.825 App[4595:f803] The content of array is(
mushroom,
grill,
pizza
)
What I want is this:
If suitsArray is like above, containing words mushroom, grill and pizza
and the "Suits" value in a dictionary is: "This wine suits mushroom dishes, grilled food, pizza and chicken."
This dictionary is added to searchResultsArray, same with all other dictionaries where "Suits" value containing words mushroom, grill and pizza.
But if suitsArray is empty of objects, skip this critera.
But I'm not sure how to write the code for it. Can you help me on it?
-(IBAction)searchButtonPressed:(id)sender{
resultObjectsArray = [NSMutableArray array];
for(NSDictionary *object in allObjectsArray)
{
NSString *objectPrice = [object objectForKey:#"75 cl price"];
NSString *suitsString = // the words in suitsArray
NSString *objectSuits = [object objectForKey:#"Suits"];
BOOL priceConditionGood = YES;
if (minPrisSlider.value <= maxPrisSlider.value && (winePrice.floatValue < minPrisSlider.value || winePrice.floatValue > maxPrisSlider.value))
priceConditionGood = NO;
if (priceConditionGood)
[resultObjectsArray addObject:object];
}
ResultsTableViewController *nextController = [[self storyboard] instantiateViewControllerWithIdentifier:#"ResultsController"];
nextController.objectsArray = [[NSMutableArray alloc]initWithArray:resultObjectsArray];
[self.navigationController pushViewController:nextController animated:YES];
}
It would be better to see if suitsArray contains the string objectSuits, rather than using rangeOfString.
if ([suitsArray containsObject:objectSuits]);
[resultObjectsArray addObject:object];
After edit: I think I understand now. This method looks at each word in suitsArray, and if it finds any of them in objectSuits, then object is added to the resultObjectArray.
for (NSString *aWord in suitsArray) {
if ([objectSuits rangeOfString:aWord].length == aWord.length) {
[resultObjectArray addObject:objectSuits];
break;
}
}
Second Edit: After further thought, the above code has a flaw in the logic that may or may not be important -- because it uses rangeOfString it will find words inside other words. So, if objectSuits was "This wine goes well with cheesecake" and suitsArray contained the word cheese, it would find a match (I can't imagine a wine that would be good with cheese would be good with cheesecake). So, here is a better solution, I think, that breaks objectSuit up into individual words and puts them into a set. suitsArray is also converted to a set, so that we can use intersectsSet: to find if there are any common words.
NSCharacterSet *sepSet = [NSCharacterSet characterSetWithCharactersInString:#" ,.;"];
NSArray *words = [objectSuits componentsSeparatedByCharactersInSet:sepSet];
NSSet *objectSuitsWords = [NSSet setWithArray:words];
NSSet *suitsSet = [NSSet setWithArray:suitsArray];
BOOL ans = [suitsSet intersectsSet:objectSuitsWords];
So, if ans is 1, then that object should be added to the results array. Notice that sepSet starts with a space and includes a comma, period, and semicolon. There might be other things you might want to include, but I think this should work in most cases.
Is it possible to pass a block of code, for example:
int i = 0;
while (i < [array count]) {
//Code to pass in here
i++;
}
Reason being that i need to perform various actions on every item in an array at different times, it'd be nice to have 1 method and pass it a block of code to run.
Have you considered using blocks. It's just an idea and not working or compilable code
typedef int (^block_t)();
-(void) methodName:(block_t) code_block
{
int i = 0;
while (i < [array count]) {
code_block() //Code to pass in here
i++;
}
block_t youCode = ^{ NSLog("Just an example"); }
[self methodName:youCode];
You can definitely iterate an array and process a block of code on it. This feature has been a standard part of Objective C since 2.0, iOS since 4.0 and in addition was included in Snow Leopard. If you look up the NSArray class reference you will find the following functions:
enumerateObjectsAtIndexes:options:usingBlock:
Executes a given block using the
objects in the array at the specified
indexes.
enumerateObjectsUsingBlock: Executes a
given block using each object in the
array, starting with the first object
and continuing through the array to
the last object.
enumerateObjectsWithOptions:usingBlock:
Executes a given block using each
object in the array.
You can define the code block to be executed globally in your implementation file, or in place where its needed. You can find some good examples on block programming here: http://thirdcog.eu/pwcblocks/.
It sounds like you want "blocks", which are a feature of Objective-C similar to "lambdas", "anonymous functions" and "closures" in other languages.
See Apple's documentation: Blocks Programming Topics
You should put the code into a method and call the method like so:
-(void)foo{
int i = 0;
while (i < [array count]) {
[self myMethod];
i++;
}
}
-(void)myMethod{
//Code to pass in here
}
You could also store the method as a variable allowing you to change which method is called
SEL methodToCall = #selector(myMethod);
-(void)foo{
int i = 0;
while (i < [array count]){
[self performSelector:methodToCall];
i++;
}
}
-(void)myMethod{
//Code to pass in here
}
first of all you could give a more detailed example.
second you could take a look at Action or Func in case you need a return message (well something equivalent for obj-C if exists)
but again, not understanding what you need to do, it is hard to give you an answer
cause from you Q i could understand as #Trevor did that you need to run a method:
int i = 0;
while (i < [array count]) {
if (i % 2)
EvenMethod(array[i])
else
OddMethod(array[i])
i++;
}
If you can live with 4.0+ compatibility, I highly recommend the use of blocks.
- (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block
gives you all you need and you can define your block right where you need it. There are some other variants of this, too. Please refer to the 'NSArray' documentation.
Why not just define functions that do what you want, and call them at various times with the items in your array as arguments?
If your concern is that you don't want redundant code with multiple while loops you have here, you could write a function that takes an array and a function as an argument, then applies that function to every element in the array.
Adapted from here:
//------------------------------------------------------------------------------------
// 2.6 How to Pass a Function Pointer
// <pt2Func> is a pointer to a function which returns void and takes an NSObject*
void DoForAllItems( NSArray* array, void (*pt2Func)(NSObject*) )
{
int i = 0;
while (i < [array count]) {
(*pt2Func)([array objectAtIndex:i]);
i++;
}
}
// 'DoIt' takes an NSObject*
void DoIt (NSObject* object){ NSLog(#"DoIt"); }
// execute example code
void Pass_A_Function_Pointer()
{
NSArray* array= [NSArray arrayWithObjects:#"1", #"2", nil];
DoForAllItems(array, &DoIt);
}
http://thirdcog.eu/pwcblocks/#objcblocks
Blocks were added recently to Objective-C I hope they have found their way to the Iphone also.
and it was anwered here also before:
Using Objective-C Blocks
Regards
If you are using Objective C++, I would strongly recommend function objects (even over blocks). They have a nice syntax, are easy to use, and are supported everywhere on modern C++ compilers (MSVC++11, GNUC++11, and LLVMC++11):
void go( function<void (int arg)> func )
{
int i = 0;
while (i < [array count]) {
//Code to pass in here
func( i ) ; // runs func on i
i++;
}
}
calling it:
go( []( int arg ){ // the square brackets are the "capture"
printf( "arg was %d\n", arg ) ; // arg takes on whatever value was passed to it,
// just like a normal C function
} ) ;
Based on this SO question asked a few hours ago, I have decided to implement a swizzled method that will allow me to take a formatted NSString as the format arg into stringWithFormat, and have it not break when omitting one of the numbered arg references (%1$#, %2$#)
I have it working, but this is the first copy, and seeing as this method is going to be potentially called hundreds of thousands of times per app run, I need to bounce this off of some experts to see if this method has any red flags, major performance hits, or optimizations
#define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int))
#implementation NSString (UAFormatOmissions)
+ (id)uaStringWithFormat:(NSString *)format, ... {
if (format != nil) {
va_list args;
va_start(args, format);
// $# is an ordered variable (%1$#, %2$#...)
if ([format rangeOfString:#"$#"].location == NSNotFound) {
//call apples method
NSString *s = [[[NSString alloc] initWithFormat:format arguments:args] autorelease];
va_end(args);
return s;
}
NSMutableArray *newArgs = [NSMutableArray arrayWithCapacity:NUMARGS(args)];
id arg = nil;
int i = 1;
while (arg = va_arg(args, id)) {
NSString *f = [NSString stringWithFormat:#"%%%d\$\#", i];
i++;
if ([format rangeOfString:f].location == NSNotFound) continue;
else [newArgs addObject:arg];
}
va_end(args);
char *newArgList = (char *)malloc(sizeof(id) * [newArgs count]);
[newArgs getObjects:(id *)newArgList];
NSString* result = [[[NSString alloc] initWithFormat:format arguments:newArgList] autorelease];
free(newArgList);
return result;
}
return nil;
}
The basic algorithm is:
search the format string for the %1$#, %2$# variables by searching for %#
if not found, call the normal stringWithFormat and return
else, loop over the args
if the format has a position variable (%i$#) for position i, add the arg to the new arg array
else, don't add the arg
take the new arg array, convert it back into a va_list, and call initWithFormat:arguments: to get the correct string.
The idea is that I would run all [NSString stringWithFormat:] calls through this method instead.
This might seem unnecessary to many, but click on to the referenced SO question (first line) to see examples of why I need to do this.
Ideas? Thoughts? Better implementations? Better Solutions?
Whoa there!
Instead of screwing with a core method that you very probably will introduce subtle bugs into, instead just turn on "Static Analyzer" in your project options, and it will run every build - if you get the arguments wrong it will issue a compiler warning for you.
I appreciate your desire to make the application more robust but I think it very likely that re-writing this method will more likely break your application than save it.
How about defining your own interim method instead of using format specifiers and stringWithFormat:? For example, you could define your own method replaceIndexPoints: to look for ($1) instead of %1$#. You would then format your string and insert translated replacements independently. This method could also take an array of strings, with NSNull or empty strings at the indexes that don't exist in the “untranslated” string.
Your method could look like this (if it were a category method for NSMutableString):
- (void) replaceIndexPointsWithStrings:(NSArray *) replacements
{
// 1. look for largest index in "self".
// 2. loop from the beginning to the largest index, replacing each
// index with corresponding string from replacements array.
}
Here's a few issues that I see with your current implementation (at a glance):
The __VA_ARGS__ thingy explained in the comments.
When you use while (arg = va_arg(args, id)), you are assuming that the arguments are nil terminated (such as for arrayWithObjects:), but with stringWithFormat: this is not a requirement.
I don't think you're required to escape the $ and # in your string format in your arg-loop.
I'm not sure this would work well if uaStringWithFormat: was passed something larger than a pointer (i.e. long long if pointers are 32-bit). This may only be an issue if your translations also require inserting unlocalised numbers of long long magnitude.
I am working on my first iPhone application and I've hit a wall. I'm trying to develop a 'statistics' page for a three entity relationship. My entities are the following:
Department - Name, Address, Building, etc.
People - Name, Gender (BOOL), Phone, etc
If I have fetched a specific department how do I filter those results and only return people that are Male (Gender == 0)?
If I do
NSLog(#"%d", [department.people count]);
I get the correct number of people in that department so I know I'm in the neighborhood. I know I could re-fetch and modify the predicate each time but with 20+ stats in my app that seems inefficient. Thanks for any advice!
You don't need to refetch:
NSPredicate* pred = [NSPredicate predicateWithFormat:#"gender == NO"];
NSUInteger count = [[department.people filteredArrayUsingPredicate:pred] count];
NSLog(#"%lu", (unsigned long)count);
Somehow gender==NO still looks strange though ;)
If copying is too expensive, you could use enumerators instead. E.g.:
NSUInteger CountIf(NSEnumerator* en, NSPredicate* pred) {
NSUInteger count = 0;
id obj;
while (obj = [en nextObject]) {
if([pred evaluateWithObject:obj])
++count;
}
return count;
}
NSUInteger count = CountIf([department.people objectEnumerator], predicate));
... though this would be ideally moved to a suitable category as say countOfObjectsMatchingPredicate:.
You could create NSPredicates representing your different filters and use NSSet's filteredSetWithPredicate: method. The count method will give you the number of entities matching the predicate. This isn't terribly efficient because you're creating a new set for each calculation, but it may be significantly faster than fetching each time.