Extracting a coordinate from a python list of tuples

Question

I have a python list

training_data=[(x_1, y_1), (x_2, y_2), ..., (x_n, y_n)]

and I wish to extract a list of x values

training_data_x=[x_1, x_2, ..., x_n]

I have tried

for j in range(0, len(training_data)):
    training_data_x=[i for i in training_data[j][0]]

and

training_data_x=[i for i in training_data[j][0] for j in range(0, len(training_data))]

but neither worked. How can I do this?

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Answer 1

A comprehension would do:

training_data_x = [x[0] for x in training_data]

Answered By: Netwave

Answer 2

I prefer a list comprehension with a self-documenting tuple unpacking:

[x for x,y in training_data]

Complete program:

training_data=[(1.5, 11), (2.5, 22), (7.5, 77)]
training_data_x = [x for x,y in training_data]
print(training_data_x)

Answered By: Robᵩ

Answer 3

Looks like you just have a list of tuples. I’m still a bit new to python, but I would try :

training_data_x =[]
for i in training_data:
training_data_x.append(i[0])

another way you could do is create a function that takes tuple and returns the x. Then map the function to your training_data_x variable.

Answer 4

You should unpack them rather than iterating over:

Here is the code:

training_data_x, training_data_y = list(list(zip(*training_data))[0]), list(list(zip(*training_data))[1])

Here is the walkthrough:

training_data gets unpacked and a zip object is created.
the "list(zip(..))" creates a list with x and y coordinates inside separate tuples something like [(x. coordinates), (y. Coordinates)]
Using indexing you extract x and y coordinates (as tuples)
use list to convert x and y tuples into separate lists.