Astrid Astrid - 1 year ago 155
Python Question

From scatter plot to 2D array

My mind has gone completely blank on this one.

I want to do what I think is very simple.

Suppose I have some test data:

import pandas as pd
import numpy as np
df = pd.DataFrame(np.array([range(k),
[x + 1 for x in range(k)],
[x + 4 for x in range(k)],
[x + 9 for x in range(k)]]).T,columns=list('abcd'))

where rows correspond to time and columns to angles, and it looks like this:

a b c d
0 0 1 4 9
1 1 2 5 10
2 2 3 6 11
3 3 4 7 12
4 4 5 8 13
5 5 6 9 14
6 6 7 10 15
7 7 8 11 16
8 8 9 12 17
9 9 10 13 18

Then for reasons I convert it to and ordered dictionary:

def highDimDF2Array(df):
from collections import OrderedDict # Need to preserve order

vels = [1.42,1.11,0.81,0.50]

# Get dataframe shapes
cols = df.columns

trajectories = OrderedDict()
for i,j in enumerate(cols):
x = df[j].values
x = x[~np.isnan(x)]

maxTimeSteps = len(x)
tmpTraj = np.empty((maxTimeSteps,3))
# This should be fast
tmpTraj[:,0] = range(maxTimeSteps)
# Remove construction nans
tmpTraj[:,1] = x

trajectories[j] = tmpTraj

return trajectories

Then I plot it all

import matplotlib.pyplot as plt
m = highDimDF2Array(df)
M = np.vstack(m.values())
plt.title('Angle $[^\circ]$ vs. Time $[s]$')

enter image description here

Now all I want to do is to put all of that into a 2D numpy array with the properties:

  • Time is mapped to the x-axis (or y doesn't matter)

  • Angle is mapped to the y-axis

  • The entries in the matrix correspond to the values of the coloured dots in the scatter plot

  • All other entries are treated as
    (i.e. those that are undefined by a point in the scatter plot)

In 3D the colour would correspond to the height.

I was thinking of using something like this: 3d Numpy array to 2d but am not quite sure how.

Answer Source

You can convert the values in M[:,1] and M[:,2] to integers and use them as indices to a 2D numpy array. Here's an example using the value for M you defined.

out = np.empty((20,10))
out[:] = np.NAN
N = M[:,[0,1]].astype(int)
out[N[:,1], N[:,0]] = M[:,2]
plt.title('Angle $[^\circ]$ vs. Time $[s]$')
plt.imshow(out, interpolation='none', origin = 'lower')

enter image description here

Here you can convert M to integers directly but you might have to come up with a function to map the columns of M to integers depending on the resolution of the array you are creating.

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