Im trying to manual sort on the below array.
The issue here is, the result varies while reading the item from the "for-loop enumuration" (noted as //(2)) verses reading it as a subscript (noted as //(1)). It could be a minor issue hiding behind my eye. Appreciate your time.
var mySortArray : Array<Int> = []
mySortArray = [1,5,3,3,21,11,2]
for (itemX,X) in mySortArray.enumerated() {
for (itemY,Y) in mySortArray.enumerated() {
// if mySortArray[itemX] < mySortArray[itemY] // (1)
if X < Y // (2)
{
//Swap the position of item in the array
mySortArray.swapAt(itemX, itemY)
}
}
}
print(mySortArray)
// Prints [1, 2, 3, 3, 5, 11, 21] ( for condition // (1))
// Prints [2, 1, 3, 5, 11, 3, 21] ( for condition // (2))
mySortArray = [1,5,3,3,21,11,2]
print("Actual Sort Order : \(mySortArray.sorted())")
// Prints Actual Sort Order : [1, 2, 3, 3, 5, 11, 21]
The problem here is that the function .enumerated() returns a new sequence and iterates that. Think of it as a new array.
So, you are working with 3 different arrays here.
You have an unsorted array that you want to fix. Lets call this the w ("working array") and then you have you array x and array y.
So, w is [1,5,3,3,21,11,2], x and y are effectively the same as w at the beginning.
Now you get your first two values that need to swap...
valueX is at index 1 of x (5). valueY is at index 2 of y (3).
And you swap them... in w.
So now w is [1,3,5,3,21,11,2] but x and y are unchanged.
So now you indexes are being thrown off. You are comparing items in x with items in y and then swapping them in we which is completely different.
You need to work with one array the whole time.
Of course... there is also the issue that your function is currently very slow. O(n^2) and there are much more efficient ways of sorting.
If you are doing this as an exercise in learning how to write sort algorithms then keep going. If not you should really be using the .sort() function.
Really what you want to be doing is not using .enumerated() at all. Just use ints to get (and swap) values in w.
i.e. something like
for indexX in 0..<w.count {
for indexY in indexX..<w.count {
// do some comparison stuff.
// do some swapping stuff.
}
}
Related
Say that I have a batch of arrays, and I would like to alter them based on conditions of particular values located by indices.
For example, say that I would like to increase and decrease particular values if the difference between those values are less than two.
For a single 1D array it can be done like this
import numpy as np
single2 = np.array([8, 8, 9, 10])
if abs(single2[1]-single2[2])<2:
single2[1] = single2[1] - 1
single2[2] = single2[2] + 1
single2
array([ 8, 7, 10, 10])
But I do not know how to do it for batch of arrays. This is my initial attempt
import numpy as np
single1 = np.array([6, 0, 3, 7])
single2 = np.array([8, 8, 9, 10])
single3 = np.array([2, 15, 15, 20])
batch = np.array([
np.copy(single1),
np.copy(single2),
np.copy(single3),
])
if abs(batch[:,1]-batch[:,2])<2:
batch[:,1] = batch[:,1] - 1
batch[:,2] = batch[:,2] + 1
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
Looking at np.any and np.all, they are used to create an array of booleans values, and I am not sure how they could be used in the code snippet above.
My second attempt uses np.where, using the method described here for comparing particular values of a batch of arrays by creating new versions of the arrays with values added to the front/back of the arrays.
https://stackoverflow.com/a/71297663/3259896
In the case of the example, I am comparing values that are right next to each other, so I created copies that shift the arrays forwards and backwards by 1. I also use only the particular slice of the array that I am comparing, since the other numbers would also be used in the comparison in np.where.
batch_ap = np.concatenate(
(batch[:, 1:2+1], np.repeat(-999, 3).reshape(3,1)),
axis=1
)
batch_pr = np.concatenate(
(np.repeat(-999, 3).reshape(3,1), batch[:, 1:2+1]),
axis=1
)
Finally, I do the comparisons, and adjust the values
batch[:, 1:2+1] = np.where(
abs(batch_ap[:,1:]-batch_ap[:,:-1])<2,
batch[:, 1:2+1]-1,
batch[:, 1:2+1]
)
batch[:, 1:2+1] = np.where(
abs(batch_pr[:,1:]-batch_pr[:,:-1])<2,
batch[:, 1:2+1]+1,
batch[:, 1:2+1]
)
print(batch)
[[ 6 0 3 7]
[ 8 7 10 10]
[ 2 14 16 20]]
Though I am not sure if this is the most computationally efficient nor programmatically elegant method for this task. Seems like a lot of operations and code for the task, but I do not have a strong enough mastery of numpy to be certain about this.
This works
mask = abs(batch[:,1]-batch[:,2])<2
batch[mask,1] -= 1
batch[mask,2] += 1
So, something is bugging me with the syntax in Swift for performing operations on Arrays of Ints.
What I wanna do is this : I have an array of Ints which is outputted from a function, its size (count) varies between say 2 and 6 for now, depending on buttons I press in my app.
For each array that is outputted and that contain n ints, I want to create n arrays on which to perform an other action later on.
These "sub" arrays are supposed to be calculated this way :
newArray1's values should be array's values - the value of the first index of newArray1
newArray2's values should be array's values - the value of the second index of newArray2
etc... (I'll automate the number of newArrays according to the array.count)
An other condition applying for those new arrays is that if at a given index the value is negative, I add 12 (so it'll occur for newArray2 at index 1, for newArray3 at indexes 1 & 2, etc... as long as those newArrays are created).
Here's how I wanted to perform that (I created this with dummy arbitrary array in the playground for the sake of testing before inserting the correct stuff in my app code) :
var array : [Int] = [2,4,6,8,9]
var newArray2 = [Int]()
var increment2 = Int()
increment2 = array[1]
newArray2 = array.map {$0 - increment2}
for i in 0..<newArray2.count {
if array[i] < 0 {
newArray2[i] = array[i] + 12
} else {
newArray2[i] = array[i]
}
}
print(array)
print(newArray2)
So of course it doesn't work because I can't seem to figure how to correctly perform operations on Arrays...
Intuitively it seems in my first if statement I'm comparing not the element at index i but i itself, not sure how to reformat that though...
Any help is most welcome, thanks in advance ! :)
[EDIT: I just edited the names of newArray1 to newArray2, same for increments, so that I have negative values and it matches the index value of 1 which is the second element of my main array]
You seem to mean this:
let arr = [2,4,6,8,9]
var results = [[Int]]()
for i in arr.indices {
results.append(arr.map {
var diff = $0-arr[i]
if diff < 0 { diff += 12 }
return diff
})
}
// results is now:
// [[0, 2, 4, 6, 7],
// [10, 0, 2, 4, 5],
// [8, 10, 0, 2, 3],
// [6, 8, 10, 0, 1],
// [5, 7, 9, 11, 0]]
There are two arrays, an array of images and an array of the corresponding labels. (e.g pictures of figures and it's values)
The occurrences in the labels are unevenly distributed.
What I want is to cut both arrays in such a way, that the labels are evenly distributed. E.g. every label occurs 2 times.
To test I've just created two 1D arrays and it was working:
labels = np.array([1, 2, 3, 3, 1, 2, 1, 3, 1, 3, 1,])
images = np.array(['A','B','C','C','A','B','A','C','A','C','A',])
x, y = zip(*sorted(zip(images, labels)))
label = list(set(y))
new_images = []
new_labels = []
amount = 2
for i in label:
start = y.index(i)
stop = start + amount
new_images = np.append(new_images, x[start: stop])
new_labels = np.append(new_labels, y[start: stop])
What I get/want is this:
new_labels: [ 1. 1. 2. 2. 3. 3.]
new_images: ['A' 'A' 'B' 'B' 'C' 'C']
(It is not necessary, that the arrays are sorted)
But when I tried it with the right data (images.shape = (35000, 32, 32, 3), labels.shape = (35000)) I've got an error:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
This does not help me a lot:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
I think that my solution is quite dirty anyhow. Is there a way to do it right?
Thank you very much in advance!
When your labels are equal, the sort function tries to sort on the second value of the tuples it has as input, since this is an array in the case of your real data, (instead of the 1D data), it cannot compare them and raises this error.
Let me explain it a bit more detailed:
x, y = zip(*sorted(zip(images, labels)))
First, you zip your images and labels. What this means, is that you create tuples with the corresponding elements of images and lables. The first element from images by the first element of labels, etc.
In case of your real data, each label is paired with an array with shape (32, 32, 3).
Second you sort all those tuples. This function tries first to sort on the first element of the tuple. However, when they are equal, it will try to sort on the second element of the tuples. Since they are arrays it cannot compare them en throws an error.
You can solve this by explicitly telling the sorted function to only sort on the first tuple element.
x, y = zip(*sorted(zip(images, labels), key=lambda x: x[0]))
If performance is required, using itemgetter will be faster.
from operator import itemgetter
x, y = zip(*sorted(zip(images, labels), key=itemgetter(0)))
I need to know, how to save integers from stdin into array, given by first integer in line... Ehm... hope you understand. I will give you an example.
On stdin I have:
0 : [ 1, 2, 3 ]
5 : [ 10, 11, 12, 13]
6 : [ 2, 4, 9 ]
0 : [ 4, 9, 8 ]
5 : [ 9, 6, 7 ]
5 : [ 1 ]
And I need save these integers to the arrays like this:
0={1, 2, 3, 4, 9, 8}
5={10, 11, 12, 13, 9, 6, 7, 1}
6={2, 4, 9}
I absolutely don't how to do it. There is a problem, that the number of arrays(in this case - 0, 5, 6 - so 3 arrays ) can be very high and I need to work effectively with memory...So I guess i will need something like malloc and free to solve this problem, or am I wrong? The names of arrays (0, 5, 6) can be changed. Number of integers in brackets has no maximum limit.
Thank you for any help.
I go with the assumption, this is homework, and I go with the assumption, this isn't your first homework to do, so I won't present you a solution but instead some tips that would help you to solve it yourself.
Given the input line
5 : [ 10, 11, 12, 13]
I will call "5" the "array name" and 10, 11, 12 and 13 the values to add.
You should implement some system to map array names to indices. A trivial approach would be like this:
.
size_t num_arrays;
size_t * array_names;
Here, in your example input, num_arrays will end up being 3 with array_names[3] = { 0, 5, 6}. If you find a new array name, realloc and add the new array name. Also you need the actual arrays for the values:
int * * array;
you need to realloc array for each new array name (like you realloc array_names). array[0] will represent array array_names[0] here array 0, array[1] will represent array array_names[1] here array 5 and array[2] will represent array array_names[2] here array 6.
To access an array, find it's index like so:
size_t index;
for (size_t index = 0; index < num_arrays && array_names[index] != search; ++index) ;
The second step is easy. Once you figured out, you need to use array[index] to add elemens, realloc that one (array[index] = realloc(array[index], new size)) and add elements there array[index][i+old_size] = new_value[i].
Obviously, you need to keep track of the number of elements in your separate arrays as well ;)
Hint: If searching for the array names take too long, you will have to replace that trivial mapping part by some more sophisticated data structure, like a hash map or a binary search tree. The rest of the concept may stay more or less the same.
Should you have problems to parse the input lines, I suggest, you open a new question specific on this parsing part.
In algorithmic terms, you need map (associative array) from ints to arrays. This is solved long ago in most higher level languages.
If you have to implement it manually, you have a few options:
simple "master" array where you store your 0, 5, 6, 1000000 and then map them to indices 0, 1, 2, 3 by doing search in for each time you have to access it (it's too time consuming when ;
hash table: write simple hash function to map 0, 5, 6, 1000000 (they're called keys) to values less than 1000, allocate array of 1000 elements and then make "master" array structures for each hash function result;
some kind of tree (e.g. red-black tree), may be a bit complex to implement manually.
Last two structures are part of programming classic and are well described in various articles and books.
If I have the following array:
x = double([1, 1, 1, 10, 1, 1, 50, 1, 1, 1 ])
I want to do the following:
Group the array into groups of 5 which will each be evaluated separately.
Identify the MAX value each of the groups of the array
Remove that MAX value and put it into another array.
Finally, I want to print the updated array x without the MAX values, and the new array containing the MAX values.
How can I do this? I am new to IDL and have had no formal training in coding.
I understand that I can write the code to group and find the max values this way:
FOR i = 1, (n_elements(x)-4) do begin
print, "MAX of array", MAX( MAX(x[i-1:1+3])
ENDFOR
However, how do I implement all of what I specified above? I know I have to create an empty array that will append the values found by the for loop, but I don't know how to do that.
Thanks
I changed your x to have unique elements to make sure I wasn't fooling myself. It this, the number of elements of x must be divisible by group_size:
x = double([1, 2, 3, 10, 4, 5, 50, 6, 7, 8])
group_size = 5
maxes = max(reform(x, group_size, n_elements(x) / group_size), ind, dimension=1)
all = bytarr(n_elements(x))
all[ind] = 1
x_without_maxes = x[where(all eq 0)]
print, maxes
print, x_without_maxes
Lists are good for this, because they allow you to pop out values at specific indices, rather than rewriting the whole array again. You might try something like the following. I've used a while loop here, rather than a for loop, because it makes it a little easier in this case.
x = List(1, 1, 1, 10, 1, 1, 50, 1, 1, 1)
maxValues = List()
pos = 4
while (pos le x.length) do begin
maxValues.add, max(x[pos-4:pos].toArray(), iMax)
x.Remove, iMax+pos-4
pos += 5-1
endwhile
print, "Max Values : ", maxValues.toArray()
print, "Remaining Values : ", x.toArray()
This allows you to do what you want I think. At the end, you have a List object (which can easily be converted to an array) with the max values for each group of 5, and another containing the remaining values.
Also, please tag this as idl-programming-language rather than idl. They are two different tags.