How to plot specific points in array with matplotlib?

Question

I generated two arrays with 10 different values. How do I plot 3 specific values within each array using matplotlib? Here is my code so far:

import numpy as np
import matplotlib as plt
x = np.array(1,2,3,4,5,6,7,8,9,10)
y = np.array(1,2,3,4,5,6,7,8,9,10)

I only want to plot the points 3,4,5 of the x-array and it's corresponding y values. I have tried this:

plt.plot(x[2,3,4], y[2,3,4])
plt.show()

But I get the error "too many indices for array." However, if I write

plt.plot(x[2], y[2])
plt.show()

the second element in the arrays will plot.

Try: x = np.array(1,2,3,4,5,6,7,8,9,10) --> x = np.array([1,2,3,4,5,6,7,8,9,10]) the same for y. And in plot: x[2:5], y[2:5] — Tony Tannous
– Tony Tannous, Commented Mar 7, 2017 at 21:34
I think you also want to import the pyplot submodule: import matplotlib.pyplot as plt. — Stephen Shank
– Stephen Shank, Commented Mar 7, 2017 at 22:02
Thanks a lot! How do I do this if I wanted to only plot 3 different random elements in each array? For my actual data, the elements are not are in order from least to greatest — astrogirl79
– astrogirl79, Commented Mar 7, 2017 at 22:39
Then instead of plotting x[2,3,4], you can type x[np.array([2,3,4])] ! If x is a numpy array you can select elements with a another numpy array :) — Alexandre Kempf
– Alexandre Kempf, Commented Mar 8, 2017 at 15:39

Community · Accepted Answer · 2017-05-23 12:25:19Z

5

The problem is the syntax of x[3, 4, 5]. It is wrong what you want to do is x[3], x[4], x[5], which are the respective elements of the array.

print(x[3], x[4], x[5]) # print 4, 5, 6

A more comfortable way to do this is:

plt.plot(x[2:5], y[2:5])
plt.show()

Where x[2:5] returns from the third to the fifth element.

As Tony Tannous says, the creation of the array is also wrong. np.array needs a list!

Then you also have to change the creation of x and y:

x = np.array([1,2,3,4,5,6,7,8,9,10])
y = np.array([1,2,3,4,5,6,7,8,9,10])

Adding [ and ] to make it a list.

Surely you should see the documentation of Indexing

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answered Mar 7, 2017 at 21:39

Lucas

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1 Comment

You still have to wrap the array elements by [] in np.array. docs.scipy.org/doc/numpy/reference/generated/numpy.array.html