I need a method that would add a few objects (2-10) to my array, skipping these that are nils:
NSMutableArray *arr = [[NSMutableArray alloc] init];
[arr addObjectsSkipNils:obj1, obj2, obj3];
How I can write this method in an elegant way?
This category method would work:
#interface NSMutableArray (MyAdditions)
- (void)addObjectsSkipNilsWithCount:(NSUInteger)count objects:(id)obj, ...;
#end
#implementation NSMutableArray (MyAdditions)
- (void)addObjectsSkipNilsWithCount:(NSUInteger)count objects:(id)obj, ...
{
va_list ap;
va_start(ap, obj);
// First object:
if (obj != nil)
[self addObject:obj];
// Remaining objects:
for (NSUInteger i = 1; i < count; i++) {
id myobj = va_arg(ap, id);
if (myobj != nil)
[self addObject:myobj];
}
va_end(ap);
}
#end
Example:
NSMutableArray *a = [NSMutableArray array];
[a addObjectsSkipNilsWithCount:3 objects:#"foo", nil, #"bar"];
NSLog(#"%#", a);
// Output: ( foo, bar )
You have to specify the number of objects explicitly, because nil cannot be used as terminator for the variable argument list. (And bad things can happen if the count is greater than the actual number of objects supplied !)
You can use:
[yourMainArray removeObjectIdenticalTo:[NSNull null]];
Now if you want to copy this to arr you can do quite easily.
This code works fine, but each time i look at it, i die a little bit inside. :(
Can you help me streamline it a bit?
Is there a more elegant way to .append(SomeString)? (To give you some perspective, code prints elements of the Linked List)
- (NSString *) description {
Node* tempNode = [self firstNode];
if (tempNode == nil) {
return #"List contains no elements";
}
NSString *desc= [[NSString alloc] initWithString:#"(null) -- "];
desc = [desc stringByAppendingString:[firstNode nodeCharacter]];
desc = [desc stringByAppendingString:#" -- "];
while ([tempNode nextNode] != nil) {
desc = [desc stringByAppendingString:[[tempNode nextNode]nodeCharacter]];
desc = [desc stringByAppendingString:#" -- "];
tempNode = [tempNode nextNode];
}
return [desc stringByAppendingString:#" (null)"];
}
First of all, if you want to build a string step by step or modify it, stop using stringByAppendingString:, and use NSMutableString instead of NSString !!!
Then, for your matter, you can even use stringWithFormat: to build part of your string.
Finally, you forgot to manage your memory: you alloc/init your string but never release it (and as your reassign the desc variable the line after, you loose track of the allocated memory and have a leak.
So here is the revised code:
- (NSString *) description {
Node* tempNode = [self firstNode];
if (tempNode == nil) {
return #"List contains no elements";
}
NSMutableString *desc = [NSMutableString stringWithFormat:#"(null) -- %# -- ",[firstNode nodeCharacter]];
while ((tempNode = [tempNode nextNode]) != nil) {
[desc appendFormat:#"%# -- ",[tempNode nodeCharacter]];
}
[desc appendingString:#" (null)"];
return desc;
}
(Note that you may also build an NSArray of the nodes of your list and use componentsJoinedByString then at the end… so you have multiple possibilities here anyway)
Yes use an NSMutableString which will involve much less memory allocations.
- (NSString *) description {
Node* tempNode = [self firstNode];
if (tempNode == nil) {
return #"List contains no elements";
}
//Autorelease string to prevent memory leak
NSMutableString *desc= [NSMutableString stringWithString:#"(null) -- "];
[desc appendString:[firstNode nodeCharacter]];
[desc appendString:#" -- "];
while ([tempNode nextNode] != nil) {
[desc appendString:[[tempNode nextNode]nodeCharacter]];
[desc appendString:#" -- "];
tempNode = [tempNode nextNode];
}
[desc appendString:#" (null)"];
return desc;
}
Rather than using the while loop, take a look at Cocoa's Fast Enumeration. It is supported by NSArrays already, and allow you to rapidly enumerate through array elements:
for (id object in myArray)
// Do something with object
You can adopt fast enumeration (or use NSEnumerator) in your node elements, then loop through them:
// Use mutable strings
NSMutableString *desc = [[NSMutableString alloc] initWithString:#"(null) -- "];
for (Node *node in nodeList) {
[desc appendString:[node nodeCharacter];
[desc appendString:#" -- "];
}
return [desc stringByAppendingString:#" (null)"];
Have you looked at NSMutableString? It has -appendString: methods.
Edit: You could also use a recursive function on your node to traverse the list and build up the string. I would make some simple public method like - (NSString *)description to call the first method and then use a private method internally to do your dirty work, like so:
- (NSString *)recursiveDescriptionWithSubnode:(Node *)node {
if(!node) {
return [self nodeCharacter];
}
else {
return [[self nodeCharacter] stringByAppendingString:[self recursiveDescriptionWithSubnode:[self nextNode]];
}
}
Note that this isn't tail recursive and so for a long list this would build up a sizable autorelease pool and call stack. Making it tail recursive is left as an exercise for the reader (but you could use NSMutableString to do it).
Instead of building up a string, you could create an array of strings, that you ultimately return as a single, joined string separating each value with your " -- " string.
If you know how many elements you might have, you could create the array with that capacity, which might be slightly more efficient under the hood for NSMutableArray.
I've created a custom sorting by creating a new category for the NSString class. Below is my code.
#implementation NSString (Support)
- (NSComparisonResult)sortByPoint:(NSString *)otherString {
int first = [self calculateWordValue:self];
int second = [self calculateWordValue:otherString];
if (first > second) {
return NSOrderedAscending;
}
else if (first < second) {
return NSOrderedDescending;
}
return NSOrderedSame;
}
- (int)calculateWordValue:(NSString *)word {
int totalValue = 0;
NSString *pointPath = [[NSBundle mainBundle] pathForResource:#"pointvalues"ofType:#"plist"];
NSDictionary *pointDictionary = [[NSDictionary alloc] initWithContentsOfFile:pointPath];
for (int index = 0; index < [word length]; index++) {
char currentChar = [word characterAtIndex:index];
NSString *individual = [[NSString alloc] initWithFormat:#"%c",currentChar];
individual = [individual uppercaseString];
NSArray *numbersForKey = [pointDictionary objectForKey:individual];
NSNumber *num = [numbersForKey objectAtIndex:0];
totalValue += [num intValue];
// cleanup
individual = nil;
numbersForKey = nil;
num = nil;
}
return totalValue;
}
#end
My question is whether I create a point dictionary to determine the point value associated with each character in the alphabet based on a plist. Then in my view controller, I call
NSArray *sorted = [words sortedArrayUsingSelector:#selector(sortByPoint:)];
to sort my table of words by their point values. However, creating a new dictionary each time the -sortByPoint: method is called is extremely inefficient. Is there a way to create the pointDictionary beforehand and use it for each subsequent call in the -calculateWordValue:?
This is a job for the static keyword. If you do this:
static NSDictionary *pointDictionary = nil
if (pointDictionary==nil) {
NSString *pointPath = [[NSBundle mainBundle] pathForResource:#"pointvalues" ofType:#"plist"];
pointDictionary = [[NSDictionary alloc] initWithContentsOfFile:pointPath];
}
pointDictionary will be persistent for the lifetime of your app.
One other optimization is to build a cache of scores by using this against each of your words:
[dict setObject:[NSNumber numberWithInt:[word calculateWordValue:word]] forKey:word];
Then use the keysSortedByValueUsingSelector: method to extract your list of words (note the selector chould be compare:, since the objects being compared are the NSNumbers).
Finally, the word argument on your method is redundant. Use self instead:
-(int)calculateWordValue {
...
for (int index = 0; index < [self length]; index++)
{
char currentChar = [self characterAtIndex:index];
...
}
...
}
Change your sortByPoint:(NSString *) otherString method to take the dictionary as a parameter, and pass it your pre-created dictionary.
sortByPoint:(NSString *)otherString withDictionary:(NSDictionary *)pointDictionary
EDIT: Won't work because of usage in sortedArrayWithSelector. Apologies. Instead, you may be better off creating a wrapper class for your point dictionary as a singleton which you then obtain a reference to each time your sort function runs.
In calculateWordValue:
NSDictionary *pointDictionary = [[DictWrapper sharedInstance] dictionary];
DictWrapper has an NSDictionary as a property, and a class method sharedInstance (to return the singleton. You have to set that dictionary and pre-initialize it before you do you first sorting.
Is there any way to create a new
NSString from a format string like #"xxx=%#, yyy=%#" and a NSArray of objects?
In the NSSTring class there are many methods like:
- (id)initWithFormat:(NSString *)format arguments:(va_list)argList
- (id)initWithFormat:(NSString *)format locale:(id)locale arguments:(va_list)argList
+ (id)stringWithFormat:(NSString *)format, ...
but non of them takes a NSArray as an argument, and I cannot find a way to create a va_list from a NSArray...
It is actually not hard to create a va_list from an NSArray. See Matt Gallagher's excellent article on the subject.
Here is an NSString category to do what you want:
#interface NSString (NSArrayFormatExtension)
+ (id)stringWithFormat:(NSString *)format array:(NSArray*) arguments;
#end
#implementation NSString (NSArrayFormatExtension)
+ (id)stringWithFormat:(NSString *)format array:(NSArray*) arguments
{
char *argList = (char *)malloc(sizeof(NSString *) * arguments.count);
[arguments getObjects:(id *)argList];
NSString* result = [[[NSString alloc] initWithFormat:format arguments:argList] autorelease];
free(argList);
return result;
}
#end
Then:
NSString* s = [NSString stringWithFormat:#"xxx=%#, yyy=%#" array:#[#"XXX", #"YYY"]];
NSLog( #"%#", s );
Unfortunately, for 64-bit, the va_list format has changed, so the above code no longer works. And probably should not be used anyway given it depends on the format that is clearly subject to change. Given there is no really robust way to create a va_list, a better solution is to simply limit the number of arguments to a reasonable maximum (say 10) and then call stringWithFormat with the first 10 arguments, something like this:
+ (id)stringWithFormat:(NSString *)format array:(NSArray*) arguments
{
if ( arguments.count > 10 ) {
#throw [NSException exceptionWithName:NSRangeException reason:#"Maximum of 10 arguments allowed" userInfo:#{#"collection": arguments}];
}
NSArray* a = [arguments arrayByAddingObjectsFromArray:#[#"X",#"X",#"X",#"X",#"X",#"X",#"X",#"X",#"X",#"X"]];
return [NSString stringWithFormat:format, a[0], a[1], a[2], a[3], a[4], a[5], a[6], a[7], a[8], a[9] ];
}
Based on this answer using Automatic Reference Counting (ARC): https://stackoverflow.com/a/8217755/881197
Add a category to NSString with the following method:
+ (id)stringWithFormat:(NSString *)format array:(NSArray *)arguments
{
NSRange range = NSMakeRange(0, [arguments count]);
NSMutableData *data = [NSMutableData dataWithLength:sizeof(id) * [arguments count]];
[arguments getObjects:(__unsafe_unretained id *)data.mutableBytes range:range];
NSString *result = [[NSString alloc] initWithFormat:format arguments:data.mutableBytes];
return result;
}
One solution that came to my mind is that I could create a method that works with a fixed large number of arguments like:
+ (NSString *) stringWithFormat: (NSString *) format arguments: (NSArray *) arguments {
return [NSString stringWithFormat: format ,
(arguments.count>0) ? [arguments objectAtIndex: 0]: nil,
(arguments.count>1) ? [arguments objectAtIndex: 1]: nil,
(arguments.count>2) ? [arguments objectAtIndex: 2]: nil,
...
(arguments.count>20) ? [arguments objectAtIndex: 20]: nil];
}
I could also add a check to see if the format string has more than 21 '%' characters and throw an exception in that case.
#Chuck is correct about the fact that you can't convert an NSArray into varargs. However, I don't recommend searching for the pattern %# in the string and replacing it each time. (Replacing characters in the middle of a string is generally quite inefficient, and not a good idea if you can accomplish the same thing in a different way.) Here is a more efficient way to create a string with the format you're describing:
NSArray *array = ...
NSAutoreleasePool *pool = [NSAutoreleasePool new];
NSMutableArray *newArray = [NSMutableArray arrayWithCapacity:[array count]];
for (id object in array) {
[newArray addObject:[NSString stringWithFormat:#"x=%#", [object description]]];
}
NSString *composedString = [[newArray componentsJoinedByString:#", "] retain];
[pool drain];
I included the autorelease pool for good housekeeping, since an autoreleased string will be created for each array entry, and the mutable array is autoreleased as well. You could easily make this into a method/function and return composedString without retaining it, and handle the autorelease elsewhere in the code if desired.
This answer is buggy. As noted, there is no solution to this problem that is guaranteed to work when new platforms are introduced other than using the "10 element array" method.
The answer by solidsun was working well, until I went to compile with 64-bit architecture. This caused an error:
EXC_BAD_ADDRESS type EXC_I386_GPFLT
The solution was to use a slightly different approach for passing the argument list to the method:
+ (id)stringWithFormat:(NSString *)format array:(NSArray*) arguments;
{
__unsafe_unretained id * argList = (__unsafe_unretained id *) calloc(1UL, sizeof(id) * arguments.count);
for (NSInteger i = 0; i < arguments.count; i++) {
argList[i] = arguments[i];
}
NSString* result = [[NSString alloc] initWithFormat:format, *argList] ;// arguments:(void *) argList];
free (argList);
return result;
}
This only works for arrays with a single element
There is no general way to pass an array to a function or method that uses varargs. In this particular case, however, you could fake it by using something like:
for (NSString *currentReplacement in array)
[string stringByReplacingCharactersInRange:[string rangeOfString:#"%#"]
withString:currentReplacement];
EDIT: The accepted answer claims there is a way to do this, but regardless of how fragile this answer might seem, that approach is far more fragile. It relies on implementation-defined behavior (specifically, the structure of a va_list) that is not guaranteed to remain the same. I maintain that my answer is correct and my proposed solution is less fragile since it only relies on defined features of the language and frameworks.
For those who need a Swift solution, here is an extension to do this in Swift
extension String {
static func stringWithFormat(format: String, argumentsArray: Array<AnyObject>) -> String {
let arguments = argumentsArray.map { $0 as! CVarArgType }
let result = String(format:format, arguments:arguments)
return result
}
}
Yes, it is possible. In GCC targeting Mac OS X, at least, va_list is simply a C array, so you'll make one of ids, then tell the NSArray to fill it:
NSArray *argsArray = [[NSProcessInfo processInfo] arguments];
va_list args = malloc(sizeof(id) * [argsArray count]);
NSAssert1(args != nil, #"Couldn't allocate array for %u arguments", [argsArray count]);
[argsArray getObjects:(id *)args];
//Example: NSLogv is the version of NSLog that takes a va_list instead of separate arguments.
NSString *formatSpecifier = #"\n%#";
NSString *format = [#"Arguments:" stringByAppendingString:[formatSpecifier stringByPaddingToLength:[argsArray count] * 3U withString:formatSpecifier startingAtIndex:0U]];
NSLogv(format, args);
free(args);
You shouldn't rely on this nature in code that should be portable. iPhone developers, this is one thing you should definitely test on the device.
- (NSString *)stringWithFormat:(NSString *)format andArguments:(NSArray *)arguments {
NSMutableString *result = [NSMutableString new];
NSArray *components = format ? [format componentsSeparatedByString:#"%#"] : #[#""];
NSUInteger argumentsCount = [arguments count];
NSUInteger componentsCount = [components count] - 1;
NSUInteger iterationCount = argumentsCount < componentsCount ? argumentsCount : componentsCount;
for (NSUInteger i = 0; i < iterationCount; i++) {
[result appendFormat:#"%#%#", components[i], arguments[i]];
}
[result appendString:[components lastObject]];
return iterationCount == 0 ? [result stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] : result;
}
Tested with format and arguments:
NSString *format = #"xxx=%#, yyy=%# last component";
NSArray *arguments = #[#"XXX", #"YYY", #"ZZZ"];
Result: xxx=XXX, yyy=YYY last component
NSString *format = #"xxx=%#, yyy=%# last component";
NSArray *arguments = #[#"XXX", #"YYY"];
Result: xxx=XXX, yyy=YYY last component
NSString *format = #"xxx=%#, yyy=%# last component";
NSArray *arguments = #[#"XXX"];
Result: xxx=XXX last component
NSString *format = #"xxx=%#, yyy=%# last component";
NSArray *arguments = #[];
Result: last component
NSString *format = #"some text";
NSArray *arguments = #[#"XXX", #"YYY", #"ZZZ"];
Result: some text
I found some code on the web that claims that this is possible however I haven't managed to do it myself, however if you don't know the number of arguments in advance you also need to build the format string dynamically so I just don't see the point.
You better off just building the string by iterating the array.
You might find the stringByAppendingString: or stringByAppendingFormat: instance method handy .
One can create a category for NSString and make a function which receives a format, an array and returns the string with replaced objects.
#interface NSString (NSArrayFormat)
+ (NSString *)stringWithFormat:(NSString *)format arrayArguments:(NSArray *)arrayArguments;
#end
#implementation NSString (NSArrayFormat)
+ (NSString *)stringWithFormat:(NSString *)format arrayArguments:(NSArray *)arrayArguments {
static NSString *objectSpecifier = #"%#"; // static is redundant because compiler will optimize this string to have same address
NSMutableString *string = [[NSMutableString alloc] init]; // here we'll create the string
NSRange searchRange = NSMakeRange(0, [format length]);
NSRange rangeOfPlaceholder = NSMakeRange(NSNotFound, 0); // variables are declared here because they're needed for NSAsserts
NSUInteger index;
for (index = 0; index < [arrayArguments count]; ++index) {
rangeOfPlaceholder = [format rangeOfString:objectSpecifier options:0 range:searchRange]; // find next object specifier
if (rangeOfPlaceholder.location != NSNotFound) { // if we found one
NSRange substringRange = NSMakeRange(searchRange.location, rangeOfPlaceholder.location - searchRange.location);
NSString *formatSubstring = [format substringWithRange:substringRange];
[string appendString:formatSubstring]; // copy the format from previous specifier up to this one
NSObject *object = [arrayArguments objectAtIndex:index];
NSString *objectDescription = [object description]; // convert object into string
[string appendString:objectDescription];
searchRange.location = rangeOfPlaceholder.location + [objectSpecifier length]; // update the search range in order to minimize search
searchRange.length = [format length] - searchRange.location;
} else {
break;
}
}
if (rangeOfPlaceholder.location != NSNotFound) { // we need to check if format still specifiers
rangeOfPlaceholder = [format rangeOfString:#"%#" options:0 range:searchRange];
}
NSAssert(rangeOfPlaceholder.location == NSNotFound, #"arrayArguments doesn't have enough objects to fill specified format");
NSAssert(index == [arrayArguments count], #"Objects starting with index %lu from arrayArguments have been ignored because there aren't enough object specifiers!", index);
return string;
}
#end
Because NSArray is created at runtime we cannot provide compile-time warnings, but we can use NSAssert to tell us if number of specifiers is equal with number of objects within array.
Created a project on Github where this category can be found. Also added Chuck's version by using 'stringByReplacingCharactersInRange:' plus some tests.
Using one million objects into array, version with 'stringByReplacingCharactersInRange:' doesn't scale very well (waited about 2 minutes then closed the app). Using the version with NSMutableString, function made the string in about 4 seconds. The tests were made using simulator. Before usage, tests should be done on a real device (use a device with lowest specs).
Edit: On iPhone 5s the version with NSMutableString takes 10.471655s (one million objects); on iPhone 5 takes 21.304876s.
Here's the answer without explicitly creating an array:
NSString *formattedString = [NSString stringWithFormat:#"%# World, Nice %#", #"Hello", #"Day"];
First String is the target string to be formatted, the next string are the string to be inserted in the target.
No, you won't be able to. Variable argument calls are solved at compile time, and your NSArray has contents only at runtime.
Is __block keyword needed for writing for in loop in objective C when mutating an array or dictionary?
__block NSMutableArray *xxxx = [NSMutableArray new]; // is __block needed?
for (obj in objs) {
[xxx addObject];
}
No, it is not. The __block keyword is needed only when several conditions are met:
There is an Objective-C "block" — a segment of code captured as an object, and
There is a variable declared outside the block, and
The value of the variable is changed inside the block
This might be clearer by counterexample. Where don't you need the __block keyword?
You don't need it when your loop is a simple C loop, including plain for loops and loops that use NSFastEnumeration:
NSInteger sum = 0;
for (NSInteger i = 0; i < 10; i++) {
sum += i; // okay! this is a plain C for loop
}
NSArray *numbers = #[#1, #2, #3];
for (NSNumber *i in numbers) {
sum += [i integerValue]; // okay! this is an NSFastEnumeration object loop
}
When you have an actual Objective-C block, you don't need it if the body of the block doesn't really mutate the value of the variable. This is often the case if your block only sends Objective-C messages to an object:
NSMutableArray *evenNumbers = [NSMutableArray array];
NSArray *numbers = #[#1, #2, #3];
[numbers enumerateObjectsUsingBlock:^(NSNumber * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
if ([obj integerValue] % 2 == 0) {
[evenNumbers addObject:obj]; // okay! this is an ObjC message send, which does not mutate the pointer value of `evenNumbers`
}
}];
The only case you need it is when your block intends to mutate the value of a variable directly.
__block NSString *match = nil;
__block NSUInteger matchIndex = NSNotFound;
NSArray *candidates = #[#"foo", #"bar"];
[candidates enumerateObjectsUsingBlock:^(NSString * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
if ([obj isEqual:#"foo"]) {
// these must be __block, because we are mutating their values in this ObjC block
match = obj;
matchIndex = idx;
*stop = YES;
}
}];
No. __block is needed if value of variable is changed inside block, eg.
__block NSUInteger sum = 0;
[objs enumerateObjectsUsingBlock:
^(id _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
sum += obj.number; // just imaginary
}];
in your case value of variable is pointer and it is not changed.
No, in the code snippet you've posted you don't need __block modifier.