I have two NSMutableArrays of strings that I wish to combine and shuffle together. However, I want it so that every time it shuffles the original two arrays, it always yields the same array in the same order. How can this be done?
Could I seed some constant value into the random function like in C++ srand(constant) but in Objective-C?
Edit: Or say I just have one array (not sure if this makes any difference) and I wanted to shuffle that one array in a predictable manner?
The way to do it is just to do srandom(constant) and use the random() function in Objective C. This way you'll always get the same shuffle and random values.
Related
I need to keep 90x90 array data for iphone app. how can i keep this data? making an multi-dimensional array is a solution for this big table. or is there an other solution.
If the matrix is always 90x90, then you should just use C arrays.
Unless you have a special need for passing the matrix around, searching using predicates, or need some other feature of NSArray, then keep it simple.
You can:
Use a single Obj-C array containing 8100 elements and map your rows and columns onto the single index yourself: index = (row * 90) + column;
Create an Obj-C array containing 90 Obj-C arrays of 90 elements each.
Hash the row and column together into a single key that you can use with a dictionary. This could be a good solution especially if the array is sparse.
Use a single- or multi-dimensional C array, especially if the elements of the array are plain old C types, like int. If you're storing objects, it's better to go with an Obj-C container.
Iphone's have a built in database SQL-Lite. I'd look into that to see if it meets you needs
I have two sorted NSMutableArrays (or I can use any other collection, not critical), I need to insert objects from the first array to the second and preserve sort order in the second array. What is the optimal (fastest) method to do that? I can implement all the known good algorithms, but my question is, if there is already some built-in method? If not, what is the best algorithm in my case?
The real answer would be: it depends, since you are asking: what is the fastest way of inserting objects from one array into another while preserving sort order.
There is no built in way of inserting in the right place of a sorted array. You can achieve the same effect by just adding the two arrays together but it won't be "the fastest way".
What is actually faster depends on many things like: how much data does the arrays contain, what is the ratio of data in array1 vs array2 (does one array contain much more data than the other)?, etc.
NOTE: You should probably begin with the simple solution and only optimize once you experience performance problems. Do measurements with a large data set though, to see that your solution works with whatever data your users may have.
Inserting items from one sorted array into another sorted array
If you want to merge the two arrays by inserting objects in the right place then normal algorithms apply. You should insert the smaller array into the bigger array and try to insert entire sorted sequences where possible instead of every item one by one.
For best performance you should try to make a batch insert using insertObjects:atIndexes: instead of inserting the object one by one.
You can use indexOfObject:inSortedRange:options:usingComparator: to find the index that each item should be inserted in the other array if you specify NSBinarySearchingInsertionIndex for the options. Also, the comparator you are using must be the same as the comparator that sorted the array, otherwise the result is "undefined".
With this in mind you would do something like this
Create mutable index
For every ITEM in SMALLER ARRAY
Find the index where to insert ITEM in LONGER ARRAY
Add (the insertion location + the location in the short array) as the index in the mutable set.
Next item.
Batch insert all items.
The documentation for insertObjects:atIndexes: tells you that "the corresponding location specified in indexes after earlier insertions have been made." Which in your case with two sorted array mean all items with a lower index will already have been added and thus you should add the index of the object in the short array to the value returned from indexOfObject:inSortedRange:options:usingComparator:.
Another (probably very premature optimization) you can do is decrease the sortedRange for every item in the loop so that you don't have to search through parts of the array that you know the item to be inserted is bigger than.
There are probably many other optimizations that can be made. You should still measure first! Hopefully this will get you started.
NSArray *newArray=[firstArray arrayByAddingObjectsFromArray:secondArray];
newArray = [newArray sortedArrayUsingSelector:#selector(localizedCaseInsensitiveCompare:)];
I would start by simply adding all of the objects of the first array to the second and then resorting the second. Time how long it takes. If it is acceptable, stop there.
If not, you could try a binary search to find the insertion point in the second array for each item in the first array. Since both arrays are sorted, you might be able to optimise the search by using the last insertion point as the lower bound each time round. Something like this:
NSInteger insertionPoint = -1;
for (id object in array1)
{
insertionPoint = [self binarySearch: array2 for: object lowerBound: insertionPoint + 1];
[array2 insertObject: object atIndex: insertionPoint];
}
The Cocoa class NSSortDescriptor together with sortedArrayUsingDescriptors: from NSArray should do what you are after.
Since you are using mutable arrays, you might want to use sortUsingDescriptors: which sorts the mutable array without creating a new one.
Look at the documentation here to see if any of the NSArray sort methods work for you. http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSArray_Class/NSArray.html. You can scroll down to the methods and there's 7 built-in ones for sorting. You could probably just combine the two arrays and run the sortedArrayUsingComparator: or one of the other methods.
Is there a way to perform a group conditional statement?
For example, I have a process which returns random numbers from an array (x) which I want to compare to the first 100 prime numbers.
Is there a function or method available by which I could do:
if (x in first_100_prime_numbers)
...
As Almo already said in a comment, many collection classes in Cocoa have a containsObject: method that you can use for this purpose. Note that performing containsObject: on an array will probably be much slower (especially when the array is large) than on a set or hash table as the method has to iterate over every item in the array until it finds a match.
I have an NSMutableArray of objects and a property per each object which contains a double. I want to get the Arithmetic Mode (most common value) from the NSMutableArray. What is the fastest way of doing this as it is running through a lot of data.
If you add them all at once then probably you will have to iterate over all the values, if you add them one at atime i would calculate the mode while adding.
Or partition the array into subarrays then calculate count of each value of each subarray in its own thread then calculate the mode using the counts. But probably taht will be not really faster than iterating over the array and a lot more complex.
I'm working with Objective-C, but probably it doesn't matter the programming language for this. So basically I have an array, with say, the integers 12, 5, and 17, and I want to be able to pull out the largest number, or the smallest, or second smallest, etc.
Basically I want to be able to sort them into ascending or decending order so I could pick out, for instance, the second smallest number by retrieving the objectAtIndex: 1 if it is sorted in ascending order. I feel like this is incredibly obvious but I can't think of how to do it at the moment, so I would love it if someone could enlighten me.
If you have an NSArray with NSNumber instances, then the sort you are looking for is as easy as this:
NSArray* sortedNumbers = [unorderedNumbers sortedArrayUsingSelector:#selector(intValue)];
It will sort ascending, so [sortedNumbers lastObject] will be the greatest value.
There are many more sorting methods on NSArray if you have more specific needs. NSArray sorting
Almost every high level language, including objective-c, have library to sort an array. But as you said that language does not matter, probably you are looking for the algorithm itself. There are a number a sorting algorithms with different computational complexity. You can find them in any standard algorithm book. Or these 2 pages might be helpful:
Sorting Algorithms in Wikipedia.
sorting-algorithms.com. Contains nice explanation with animation.
And if you are interested particularly in objective-c, check the Sorting section of NSArray reference. This contains an example to sort an array of integer.
Just sort the array in ascending order (I don't use objective C, but I am sure there is a function for it) and then get the element wherever you want...
To get the largest
array[array.length - 1]
Second largest
array[array.length -2]
Smallest
array[0]
Second smallest
array[1]
You should check to make sure that the array index is valid:
if (array.length - 2> 0) //Second largest element
return array[array.length - 2];
Or:
if (array.length > 1) //Second smallest element
return array[1];
See here for how to sort an array in objective C:
http://howtomakeiphoneapps.com/2009/03/how-to-sort-an-array-in-objective-c/
If you want to preserve the order of the original array, one method is to create a second array that just contains the numbers 0, 1, ... n, representing indexes into the first array. Then sort the second array, but instead of comparing its values, compare the corresponding values that it points to in the first array. (You could also just store pointers and sort based on the dereferenced pointers.)
Then to find the second-largest number, look up the index in the second-to-last position in the second array and see where it points to in the first array.
If you want to get fancy and avoid sorting, this lecture describes an algorithm for finding the k-largest element in linear time. I haven't actually used it, but it looks like it might be a good method if your data changes often, as you wouldn't have to maintain the extra array.
If your goal is to get the highest number, or lowest, or second-lowest, or what have you, and you only need one number from the result, then sorting is overkill. Instead you should just iterate over the entire array and keep track of the highest (or lowest, or 2 lowest (for the second-lowest)) number seen so far. If your language supports this, it'll be called a "fold". The only reason to actually sort the array is if you need to access multiple different ranked values from the array.