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21
votes
2answers
49461 views

Convert matrix to 3-column table ('reverse pivot', 'unpivot', 'flatten', 'normalize')

I need to convert the Excel matrix FIRST in the table LATER: FIRST: P1 P2 P3 P4 F1 X F2 X X F3 X X F4 X X LATER: F P VALUE...
11
votes
1answers
2391 views

How do I compose a rotation matrix with human readable angles from scratch?

The one thing that has always hindered me from doing 3D programming is failing to understand how math works. I can go along with math fine in programming flow using methods and functions, then its all...
278
votes
9answers
49681 views

How can I index a MATLAB array returned by a function without first assigning it to a local variable?

For example, if I want to read the middle value from magic(5), I can do so like this: M = magic(5); value = M(3,3); to get value == 13. I'd like to be able to do something like one of these: value...
88
votes
2answers
39743 views

Reshape three column data frame to matrix ("long" to "wide" format)

I have a data.frame that looks like this. x a 1 x b 2 x c 3 y a 3 y b 3 y c 2 I want this in matrix form so I can feed it to heatmap to make a plot. The result should look something like: ...
42
votes
4answers
12513 views

Generate a matrix containing all combinations of elements taken from n vectors

This question pops up quite often in one form or another (see for example here or here). So I thought I'd present it in a general form, and provide an answer which might serve for future reference. ...
428
votes
19answers
1193759 views

How to define two-dimensional array in python

I want to define a two-dimensional array without an initialized length like this : Matrix = [][] but it does not work... I've tried the code below, but it is wrong too: Matrix = [5][5] Error: ...
317
votes
6answers
79219 views

Transpose/Unzip Function (inverse of zip)?

I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item. For example: original = [('a',...
248
votes
56answers
244430 views

How do you rotate a two dimensional array?

Inspired by Raymond Chen's post, say you have a 4x4 two dimensional array, write a function that rotates it 90 degrees. Raymond links to a solution in pseudo code, but I'd like to see some real world ...
27
votes
1answers
1236 views

Linear indexing, logical indexing, and all that

We are used to different forms of indexing in Matlab: standard (using integers along each dimension), logical (using logical values), linear (using a single index to traverse an array with more than...
77
votes
8answers
181880 views

Memory management / cannot allocate vector of size n Mb

I am running into issues trying to use large objects in R. For example: > memory.limit(4000) > a = matrix(NA, 1500000, 60) > a = matrix(NA, 2500000, 60) > a = matrix(NA, 3500000, 60) Erro...

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