# All Questions

51502 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...
2656 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...
50977 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...
42038 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: ...
12982 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. ...
1280065 views

### How to define a 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: T...
84523 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',...
251771 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 ...