In vb.net how do you delete a row in a two dimensional array?
If you need to remove items from an array you probably shouldn't use an array but should be using a list of some kind (List(Of List(Of String)) or something.
If you do want to stick with the array, there's two different solutions described on this page, one slow shift everything by hand and one faster that copies the memory. The samples are for one dimensional arrays but should be fairly easy to adapt.
If it is the last row you want to remove and you are using the second dimension to represent rows, you can use ReDim with the preserve option like so:
Dim myArray(2,1)
ReDim Preserve myArray(2, 2)
Warning: I suggestion you check out this article before using the above example: The Redim Preserve Performance Trap
If you need to remove a row in the middle, you are going to have to shift everything down a row first, then truncate the last dimension of the array.
This coupled with the need to pivot your concept of rows to the second dimension probably makes it more trouble than it is worth. Chances are, you are using the wrong type in the first place if you need to arbitrarily delete items like that. Traditional arrays (especially multi-dimensional ones) are really best used for fixed-size data sets.
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How would I go about selecting the nth values of an array and adding them to another array.
For example, if i have an NSArray which has 100 objects and I want to add every 5th object? I understand how to select the 5th object and how to add to a new array etc, but just looking for the best way to do this. This is for image manipulation, so will be dealing with arrays of up to 2m pixel values.
Is the best way to just use for loops?
You can using striding:
.stride(to: 100, by: 5)
So to create a new array:
Array(0.stride(to: 10, by: 2).map( { myArray[$0] }))
UPDATE: As Leo Dabus points out, the above will start at element 0 (and take every 2nd). If you want to start at the 5th and take every 5th, you would use:
Array(4.stride(to: 100, by: 5).map( { myArray[$0] }))
Using loops is pretty good: they are easy to read, and they are about as efficient as anything else that you may want to use for this purpose. The only optimization to the for loop approach is to reserve a specific number of elements upfront, because you know how many elements you are going to write.
If you are going to make the same selection from multiple arrays (e.g. processing an array of arrays), consider creating NSIndexSet, and applying it with objectsAtIndexes to perform the selection. This may give your code slightly better readability, because the for loop for creating indexes would be separate from the process of selection.
Finally, if you need to optimize for speed, and your arrays store wrapped primitives, consider using plain arrays instead of NSArray to avoid wrapping and unwrapping. This has a potential of giving you the most improvement, because by eliminating additional memory accesses for unwrapping it would also significantly improve locality of reference, which has crucial importance for cache use optimization.
I have an understanding about how multiple dimensional arrays work and how to use them except for one thing, In what situation would we need to use them and why?
Basically multi dimension arrays are used if you want to put arrays inside an array.
Say you got 10 students and each writes 3 tests. You can create an array like: arr_name[10][3]
So, calling arr_name[0][0] gives you the result of student 1 on lesson 1.
Calling arr_name[5][2] gives you the result of student 6 on test 3.
You can do this with a 30 position array, but the multi dimension is:
1) easier to understand
2) easier to debug.
Here are a couple examples of arrays in familiar situations.
You might imagine a 2 dimensional array is as a grid. So naturally it is useful when you're dealing with graphics. You might get a pixel from the screen by saying
pixel = screen[20][5] // get the pixel at the 20th row, 5th column
That could also be done with a 3 dimensional array to represent 3d space.
An array could act like a spreadsheet. Here the rows are customers, and the columns are name, email, and date of birth.
name = customers[0][0]
email = customers[0][1]
dateofbirth = customers[0][2]
Really there is a more fundamental pattern underlying this. Things have things have things... and so on. And in a sense you're right to wonder whether you need multidimensional arrays, because there are other ways to represent that same pattern. It's just there for convenience. You could alternatively
Have a single dimensional array and do some math to make it act multidimensional. If you indexed pixels one by one left to right top to bottom you would end up with a million or so elements. Divide by the width of the screen to get the row. The remainder is the column.
Use objects. Instead of using a multidimensional array in example 2 you could have a single dimensional array of Customer objects. Each Customer object would have the attributes name, email and dob.
So there's rarely one way to do something. Just choose the most clear way. With arrays you're accessing by number, with objects you're accessing by name.
Such solution comes as intuitive when you are faced with accessing a data element identified by a multidimensional vector. So if "which element" is defined by more than two "dimensions".
Good uses for 2D or Two D arrays might be:
Matrix Math i.e. rotation things in space on a plane and more.
Maps like game maps, top or side views for either actual graphics or descriptive data.
Spread Sheet like storage.
Multi Columns of display table data.
Kinds of Graphics work.
I know there could be much more, so maybe someone else can add to this list in their answers.
As an aid to learning objective c/oop, I'm designing an iOS app to store and display periodic bodyweight measurements. I've got a singleton which returns a mutablearray of the shared store of measurement object. Each measurement will have at least a date and a body weight, and I want to be able to add historic measurements.
I'd like to display the measurements in date order. What's the best way to do this? As far as I can see the options are as follows: 1) when adding a measurement - I override addobject to sort the shared store every time after a measurement is added, 2) when retrieving the mutablearray I sort it, or 3) I retrieve the mutablearray in whatever order it happens to be in the shared store, then sort it when displaying the table/chart.
It's likely that the data will be retrieved more frequently than a new datum is added, so option 1 will reduce redundant sorting of the shared store - so this is the best way, yes?
You can use a modified version of (1). Instead of sorting the complete array each time a new object is inserted, you use the method described here: https://stackoverflow.com/a/8180369/1187415 to insert the new object into the array at the correct place.
Then for each insert you have only a binary search to find the correct index for the new object, and the array is always in correct order.
Since you said that the data is more frequently retrieved than new data is added, this seems to be more efficient.
If I forget your special case, this question is not so easy to answer. There are two basic solutions:
Keep array unsorted and when you try to access the element and array is not sorted, then sort it. Let's call it "lazy sorting".
Keep array sorted when inserting elements. Note this is not about appending new element at the end and then sort the whole array. This is about finding where the element should be (binary search) and place it there. Let's call it "sorted insert".
Both techniques are correct and useful and deciding which one is better depends on your use cases.
Example:
You want to insert hundreds of elements into the array, then access the elements, then again insert hundreds of elements, then access. In summary, you will be inserting values in big chunks. In this case, lazy sorting will be better.
You will often insert individual elements and you will access the elements often. Then sorted insert will have better performance.
Something in the middle (between inserting 1 and inserting tens of elements). You probably don't care which one of the methods will be used.
(Note that you can use also specialized structures to keep an array sorted, not based on NSArray, e.g. structures based on a balanced tree, while keeping number of elements in the subtree).
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.