I am generating a random equation say like 2*3+4..... and using DDMathparser to evaluate it. Here I have a class method which is supposed to return a random equation(stored inside a mutable array) only if it evaluates to a integer.
however it keeps returning Null and i can't figure out why. Please help me out.!
#import "Equation.h"
#import "DDMathParser.h"
#implementation Equation
-(NSMutableArray*)randEquation{
NSMutableArray* usableEquation=[[NSMutableArray alloc]init];
while(1){
NSArray *nums = #[#"1", #"2", #"3", #"4", #"5",#"6",#"7",#"8",#"9"];
unsigned index1=arc4random()%9;
NSString* num = [NSString stringWithFormat:#"%#", [nums objectAtIndex:index1]];
NSArray *symbols = #[#"+", #"-", #"*", #"/"];
unsigned index=arc4random()%4;
NSString* symb = [NSString stringWithFormat:#"%#", [symbols objectAtIndex:index]];
NSMutableArray *arrayOfSymbolsAndNumbers = [[NSMutableArray alloc] init];
for( int i=0;i<=10;i++){
if (i%2==0) {
[arrayOfSymbolsAndNumbers addObject:num];
}
else{
[arrayOfSymbolsAndNumbers addObject:symb];
}
}
NSMutableString *stringOfSymbolsAndNumbers=[[NSMutableString alloc]init];
for (NSObject * obj in arrayOfSymbolsAndNumbers)
{
[stringOfSymbolsAndNumbers appendString:[obj description]];
}
usableEquation=arrayOfSymbolsAndNumbers;
NSNumber *result=[stringOfSymbolsAndNumbers numberByEvaluatingString];
float resultFloat = [result floatValue];
float checker=resultFloat;
if (floor(checker)==checker) {
break;
}
else{
continue;
}
}
return usableEquation;
}
#end
NSLog(#"The content of array is%#",[equation randEquation]);
Based on your code, for this log to output The content of array is(null) means that equation is nil. Your randEquation (while not efficient) looks ok, the problem is that you haven't created the equation instance when you run the log statement.
I have an NSArray, and I've calculated a list of integers that represent (in binary) the elements I need to pull out of the array into a new array.
For example, I have 7, 11, and 13, whose bit patterns are 000111, 001011, and 001101. I want to grab three arrays, made of elements 0,1,2, then elements 0,1,3, and then 0,2,3 out of the main array.
Construct an NSIndexSet from the bit patterns you have:
#implementation NSIndexSet (NonContiguous)
+ (instancetype)indexSetFromMask:(NSUInteger)mask
{
NSMutableIndexSet * set = [NSMutableIndexSet indexSet];
for( NSUInteger i = 0; i < (sizeof(NSUInteger) * 8); i++ ){
if( mask & (1l << i) ){
[set addIndex:i];
}
}
return set;
}
#end
Then use objectsAtIndexes:
[origArray objectsAtIndexes:[NSIndexSet indexSetFromMask:7]];
// etc.
Assuming you want the output to be in the form [[a,b,c],[a,b,d],[a,c,d]] for the example, you could do something like this:
NSArray *sourceArray = [[NSArray alloc] initWithObjects:#"a",#"b",#"c",#"d",...,nil];
NSArray *grabArray = [[NSArray alloc] initWithObjects:[NSNumber numberWithInt:7],[NSNumber numberWithInt:11],[NSNumber numberWithInt:13],...,nil];
NSMutableArray *resultArray = [[NSMutableArray alloc] init];
Then for each element of grabArray, add the correct elements to resultArray:
for (NSNumber num in grabArray) {
int n = [num intValue];
int bit = floor(log2(n)); //get highest bit in the current number
NSMutableArray *curr = [[NSMutableArray alloc] init];
while (n>0) {
if (n>pow(bit,2)) { //if this bit is a 1
[curr addObject:[sourceArray objectAtIndex:bit]];
}
n-=pow(bit,2);
bit-=1;
}
[resultArray addObject:curr];
}
Then resultArray should be the array you are looking for. It will add the objects in reverse order, so if order matters you would want to use [curr insertObject: [sourceArray objectAtIndex:bit] atIndex:0]; instead.
How can I create array of floating point numbers in Objective-C?
Is it possible?
You can create a dynamic array (size decided at runtime, not compile time) in different ways, depending on the language you wish to use:
Objective-C
NSArray *array = [[NSArray alloc] initWithObjects:
[NSNumber numberWithFloat:1.0f],
[NSNumber numberWithFloat:2.0f],
[NSNumber numberWithFloat:3.0f],
nil];
...
[array release]; // If you aren't using ARC
or, if you want to change it after creating it, use an NSMutableArray:
NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:0];
[array addObject:[NSNumber numberWithFloat:1.0f]];
[array addObject:[NSNumber numberWithFloat:2.0f]];
[array addObject:[NSNumber numberWithFloat:3.0f]];
...
[array replaceObjectAtIndex:1 withObject:[NSNumber numberWithFloat:99.9f]];
...
[array release]; // If you aren't using ARC
Or using the new-ish Objective-C literals syntax:
NSArray *array = #[ #1.0f, #2.0f, #3.0f ];
...
[array release]; // If you aren't using ARC
C
float *array = (float *)malloc(sizeof(float) * 3);
array[0] = 1.0f;
array[1] = 2.0f;
array[2] = 3.0f;
...
free(array);
C++ / Objective-C++
std::vector<float> array;
array[0] = 1.0f;
array[1] = 2.0f;
array[2] = 3.0f;
For an dynamic approach you can use NSNumber object and add it to NSMutableArray, or if you need only static array then use suggestions from comments, or use standard C.
like:
NSMutableArray *yourArray = [NSMutableArray array];
float yourFloat = 5.55;
NSNumber *yourFloatNumber = [NSNumer numberWithFloat:yourFloat];
[yourArray addObject:yourFloatNumber];
and then to retrive:
NSNumber *yourFloatNumber = [yourArray objectAtIndex:0]
float yourFloat = [yourFloatNumber floatValue];
If you are using Xcode 4.4+, you can try this:
NSArray *a = #[ #1.1f, #2.2f, #3.3f];
Here is all new literals of LLVM Compiler 4.0.
How about something like this?
#interface DoubleArray : NSObject
#property(readonly, nonatomic) NSUInteger count;
#property(readonly, nonatomic) double *buffer;
- (instancetype)init NS_UNAVAILABLE;
- (instancetype)initWithCount:(NSUInteger)count NS_DESIGNATED_INITIALIZER;
- (double)valueAtIndex:(NSUInteger)idx;
- (void)setValue:(double)value atIndex:(NSUInteger)idx;
#end
#implementation DoubleArray
- (void)dealloc
{
if (_buffer != 0) {
free(_buffer);
}
}
- (instancetype)initWithCount:(NSUInteger)count
{
self = [super init];
if (self) {
_count = count;
_buffer = calloc(rows * columns, sizeof(double));
}
return self;
}
- (double)valueAtIndex:(NSUInteger)idx
{
return *(_buffer + idx);
}
- (void)setValue:(double)value atIndex:(NSUInteger)idx
{
*(_buffer + idx) = value;
}
#end
It's a basic array. You can extend this with more complex features like appending, indexed removal etc.
I'm new to Objective-C and I'm trying to create a simple dictionary style app for personal use. Right now I'm attempting to make a loop that prints randomly selected NSArrays that have been added to an NSDictionary. I'd like to print each array only once. Here is the code I'm working with:
NSArray *catList = [NSArray arrayWithObjects:#"Lion", #"Snow Leopard", #"Cheetah", nil];
NSArray *dogList = [NSArray arrayWithObjects:#"Dachshund", #"Pitt Bull", #"Pug", nil];
...
NSMutableDictionary *wordDictionary = [[NSMutableDictionary alloc] init];
[wordDictionary setObject: catList forKey:#"Cats"];
[wordDictionary setObject: dogList forKey:#"Dogs"];
...
NSInteger keyCount = [[wordDictionary allKeys] count];
NSInteger randomKeyIndex = arc4random() % keyCount;
int i = keyCount;
for (i=i; i>0; i--) {
NSString *randomKey = [[wordDictionary allKeys] objectAtIndex:randomKeyIndex];
NSMutableArray *randomlySelectedArray = [wordDictionary objectForKey:randomKey];
NSLog(#"%#", randomlySelectedArray);
}
This code prints the same array "i" times. Any pointers on how to exclude previously printed arrays from being printed again?
I'm wondering if removeObjectForKey: could be of any use.
You just need to re-calculate the random key index every time you go through the loop, and then, as you suggest, use removeObjectForKey:.
Something like this:
NSArray *catList = [NSArray arrayWithObjects:#"Lion", #"Snow Leopard", #"Cheetah", nil];
NSArray *dogList = [NSArray arrayWithObjects:#"Dachshund", #"Pitt Bull", #"Pug", nil];
//...
NSMutableDictionary *wordDictionary = [[NSMutableDictionary alloc] init];
[wordDictionary setObject: catList forKey:#"Cats"];
[wordDictionary setObject: dogList forKey:#"Dogs"];
//...
while ([wordDictionary count] > 0) {
NSInteger keyCount = [wordDictionary count];
NSInteger randomKeyIndex = arc4random() % keyCount;
NSString *randomKey = [[wordDictionary allKeys] objectAtIndex:randomKeyIndex];
NSMutableArray *randomlySelectedArray = [wordDictionary objectForKey:randomKey];
NSLog(#"%#", randomlySelectedArray);
[wordDictionary removeObjectForKey: randomKey];
}
In your code, you generate a random randomKeyIndex, then use it without changing its value i times in the loop. So you get i times the same array.
NSInteger randomKeyIndex = arc4random() % keyCount;
// ...
for (i=i; i>0; i--) {
NSString *randomKey = [[wordDictionary allKeys] objectAtIndex:randomKeyIndex];
// ...
}
As you say removeObjectForKey is an option for you, you can change your code into something like this:
NSInteger keyCount = [[wordDictionary allKeys] count];
for (i=keyCount; i>0; i--) {
NSInteger randomKeyIndex = arc4random() % keyCount;
NSString *randomKey = [[wordDictionary allKeys] objectAtIndex:randomKeyIndex];
NSMutableArray *randomlySelectedArray = [wordDictionary objectForKey:randomKey];
[wordDictionary removeObjectForKey:randomKey];
keyCount--;
NSLog(#"%#", randomlySelectedArray);
}
I need to reverse my NSArray.
As an example:
[1,2,3,4,5] must become: [5,4,3,2,1]
What is the best way to achieve this?
There is a much easier solution, if you take advantage of the built-in reverseObjectEnumerator method on NSArray, and the allObjects method of NSEnumerator:
NSArray* reversedArray = [[startArray reverseObjectEnumerator] allObjects];
allObjects is documented as returning an array with the objects that have not yet been traversed with nextObject, in order:
This array contains all the remaining objects of the enumerator in enumerated order.
For obtaining a reversed copy of an array, look at danielpunkass' solution using reverseObjectEnumerator.
For reversing a mutable array, you can add the following category to your code:
#implementation NSMutableArray (Reverse)
- (void)reverse {
if ([self count] <= 1)
return;
NSUInteger i = 0;
NSUInteger j = [self count] - 1;
while (i < j) {
[self exchangeObjectAtIndex:i
withObjectAtIndex:j];
i++;
j--;
}
}
#end
Some benchmarks
1. reverseObjectEnumerator allObjects
This is the fastest method:
NSArray *anArray = #[#"aa", #"ab", #"ac", #"ad", #"ae", #"af", #"ag",
#"ah", #"ai", #"aj", #"ak", #"al", #"am", #"an", #"ao", #"ap", #"aq", #"ar", #"as", #"at",
#"au", #"av", #"aw", #"ax", #"ay", #"az", #"ba", #"bb", #"bc", #"bd", #"bf", #"bg", #"bh",
#"bi", #"bj", #"bk", #"bl", #"bm", #"bn", #"bo", #"bp", #"bq", #"br", #"bs", #"bt", #"bu",
#"bv", #"bw", #"bx", #"by", #"bz", #"ca", #"cb", #"cc", #"cd", #"ce", #"cf", #"cg", #"ch",
#"ci", #"cj", #"ck", #"cl", #"cm", #"cn", #"co", #"cp", #"cq", #"cr", #"cs", #"ct", #"cu",
#"cv", #"cw", #"cx", #"cy", #"cz"];
NSDate *methodStart = [NSDate date];
NSArray *reversed = [[anArray reverseObjectEnumerator] allObjects];
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(#"executionTime = %f", executionTime);
Result: executionTime = 0.000026
2. Iterating over an reverseObjectEnumerator
This is between 1.5x and 2.5x slower:
NSDate *methodStart = [NSDate date];
NSMutableArray *array = [NSMutableArray arrayWithCapacity:[anArray count]];
NSEnumerator *enumerator = [anArray reverseObjectEnumerator];
for (id element in enumerator) {
[array addObject:element];
}
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(#"executionTime = %f", executionTime);
Result: executionTime = 0.000071
3. sortedArrayUsingComparator
This is between 30x and 40x slower (no surprises here):
NSDate *methodStart = [NSDate date];
NSArray *reversed = [anArray sortedArrayUsingComparator: ^(id obj1, id obj2) {
return [anArray indexOfObject:obj1] < [anArray indexOfObject:obj2] ? NSOrderedDescending : NSOrderedAscending;
}];
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(#"executionTime = %f", executionTime);
Result: executionTime = 0.001100
So [[anArray reverseObjectEnumerator] allObjects] is the clear winner when it comes to speed and ease.
DasBoot has the right approach, but there are a few mistakes in his code. Here's a completely generic code snippet that will reverse any NSMutableArray in place:
/* Algorithm: swap the object N elements from the top with the object N
* elements from the bottom. Integer division will wrap down, leaving
* the middle element untouched if count is odd.
*/
for(int i = 0; i < [array count] / 2; i++) {
int j = [array count] - i - 1;
[array exchangeObjectAtIndex:i withObjectAtIndex:j];
}
You can wrap that in a C function, or for bonus points, use categories to add it to NSMutableArray. (In that case, 'array' would become 'self'.) You can also optimize it by assigning [array count] to a variable before the loop and using that variable, if you desire.
If you only have a regular NSArray, there's no way to reverse it in place, because NSArrays cannot be modified. But you can make a reversed copy:
NSMutableArray * copy = [NSMutableArray arrayWithCapacity:[array count]];
for(int i = 0; i < [array count]; i++) {
[copy addObject:[array objectAtIndex:[array count] - i - 1]];
}
Or use this little trick to do it in one line:
NSArray * copy = [[array reverseObjectEnumerator] allObjects];
If you just want to loop over an array backwards, you can use a for/in loop with [array reverseObjectEnumerator], but it's likely a bit more efficient to use -enumerateObjectsWithOptions:usingBlock::
[array enumerateObjectsWithOptions:NSEnumerationReverse
usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
// This is your loop body. Use the object in obj here.
// If you need the index, it's in idx.
// (This is the best feature of this method, IMHO.)
// Instead of using 'continue', use 'return'.
// Instead of using 'break', set '*stop = YES' and then 'return'.
// Making the surrounding method/block return is tricky and probably
// requires a '__block' variable.
// (This is the worst feature of this method, IMHO.)
}];
(Note: Substantially updated in 2014 with five more years of Foundation experience, a new Objective-C feature or two, and a couple tips from the comments.)
After reviewing the other's answers above and finding Matt Gallagher's discussion here
I propose this:
NSMutableArray * reverseArray = [NSMutableArray arrayWithCapacity:[myArray count]];
for (id element in [myArray reverseObjectEnumerator]) {
[reverseArray addObject:element];
}
As Matt observes:
In the above case, you may wonder if -[NSArray reverseObjectEnumerator] would be run on every iteration of the loop — potentially slowing down the code. <...>
Shortly thereafter, he answers thus:
<...> The "collection" expression is only evaluated once, when the for loop begins. This is the best case, since you can safely put an expensive function in the "collection" expression without impacting upon the per-iteration performance of the loop.
Georg Schölly's categories are very nice. However, for NSMutableArray, using NSUIntegers for the indices results in a crash when the array is empty. The correct code is:
#implementation NSMutableArray (Reverse)
- (void)reverse {
NSInteger i = 0;
NSInteger j = [self count] - 1;
while (i < j) {
[self exchangeObjectAtIndex:i
withObjectAtIndex:j];
i++;
j--;
}
}
#end
The most efficient way to enumerate an array in reverse:
Use enumerateObjectsWithOptions:NSEnumerationReverse usingBlock. Using #JohannesFahrenkrug's benchmark above, this completed 8x quicker than [[array reverseObjectEnumerator] allObjects];:
NSDate *methodStart = [NSDate date];
[anArray enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
//
}];
NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(#"executionTime = %f", executionTime);
NSMutableArray *objMyObject = [NSMutableArray arrayWithArray:[self reverseArray:objArrayToBeReversed]];
// Function reverseArray
-(NSArray *) reverseArray : (NSArray *) myArray {
return [[myArray reverseObjectEnumerator] allObjects];
}
Reverse array and looping through it:
[[[startArray reverseObjectEnumerator] allObjects] enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
...
}];
To update this, in Swift it can be done easily with:
array.reverse()
As for me, have you considered how the array was populated in the first place? I was in the process of adding MANY objects to an array, and decided to insert each one at the beginning, pushing any existing objects up by one. Requires a mutable array, in this case.
NSMutableArray *myMutableArray = [[NSMutableArray alloc] initWithCapacity:1];
[myMutableArray insertObject:aNewObject atIndex:0];
Or the Scala-way:
-(NSArray *)reverse
{
if ( self.count < 2 )
return self;
else
return [[self.tail reverse] concat:[NSArray arrayWithObject:self.head]];
}
-(id)head
{
return self.firstObject;
}
-(NSArray *)tail
{
if ( self.count > 1 )
return [self subarrayWithRange:NSMakeRange(1, self.count - 1)];
else
return #[];
}
There is a easy way to do it.
NSArray *myArray = #[#"5",#"4",#"3",#"2",#"1"];
NSMutableArray *myNewArray = [[NSMutableArray alloc] init]; //this object is going to be your new array with inverse order.
for(int i=0; i<[myNewArray count]; i++){
[myNewArray insertObject:[myNewArray objectAtIndex:i] atIndex:0];
}
//other way to do it
for(NSString *eachValue in myArray){
[myNewArray insertObject:eachValue atIndex:0];
}
//in both cases your new array will look like this
NSLog(#"myNewArray: %#", myNewArray);
//[#"1",#"2",#"3",#"4",#"5"]
I hope this helps.
I don't know of any built in method.
But, coding by hand is not too difficult. Assuming the elements of the array you are dealing with are NSNumber objects of integer type, and 'arr' is the NSMutableArray that you want to reverse.
int n = [arr count];
for (int i=0; i<n/2; ++i) {
id c = [[arr objectAtIndex:i] retain];
[arr replaceObjectAtIndex:i withObject:[arr objectAtIndex:n-i-1]];
[arr replaceObjectAtIndex:n-i-1 withObject:c];
}
Since you start with a NSArray then you have to create the mutable array first with the contents of the original NSArray ('origArray').
NSMutableArray * arr = [[NSMutableArray alloc] init];
[arr setArray:origArray];
Edit: Fixed n -> n/2 in the loop count and changed NSNumber to the more generic id due to the suggestions in Brent's answer.
If all you want to do is iterate in reverse, try this:
// iterate backwards
nextIndex = (currentIndex == 0) ? [myArray count] - 1 : (currentIndex - 1) % [myArray count];
You can do the [myArrayCount] once and save it to a local variable (I think its expensive), but I’m also guessing that the compiler will pretty much do the same thing with the code as written above.
Swift 3 syntax :
let reversedArray = array.reversed()
Try this:
for (int i = 0; i < [arr count]; i++)
{
NSString *str1 = [arr objectAtIndex:[arr count]-1];
[arr insertObject:str1 atIndex:i];
[arr removeObjectAtIndex:[arr count]-1];
}
Here is a nice macro that will work for either NSMutableArray OR NSArray:
#define reverseArray(__theArray) {\
if ([__theArray isKindOfClass:[NSMutableArray class]]) {\
if ([(NSMutableArray *)__theArray count] > 1) {\
NSUInteger i = 0;\
NSUInteger j = [(NSMutableArray *)__theArray count]-1;\
while (i < j) {\
[(NSMutableArray *)__theArray exchangeObjectAtIndex:i\
withObjectAtIndex:j];\
i++;\
j--;\
}\
}\
} else if ([__theArray isKindOfClass:[NSArray class]]) {\
__theArray = [[NSArray alloc] initWithArray:[[(NSArray *)__theArray reverseObjectEnumerator] allObjects]];\
}\
}
To use just call: reverseArray(myArray);