Selecting all first element of a nested list / array

Question

I have the following nested list.

 peaks = [[313, 0.7608371709999999], [695, 0.6970320329999999], [996, 0.652950446], [1367, 0.570015382], [1518, 0.516312078], [1726, 0.5599991639999999], [1871, 0.5025837670000001], [2084, 0.563103466], [2240, 0.5229157010000001], [2478, 0.611383482], [2622, 0.59167585], [2987, 0.68685803], [3225, 0.524618916], [3573, 0.738796993], [3938, 0.8191861140000001], [4302, 0.82872904], [4666, 0.814913208], [5038, 0.7919655170000001], [5397, 0.773237498], [5762, 0.78905603]]

I want to extract the first element to a separate list/array and the second to another list/array.

I have done this using a for loop,

max_peaksIdx = []
max_peaksVal = []
for i in max_peaks:
    max_peaksIdx.append(i[0])
    max_peaksVal.append(i[1])

#out : max_peaksIsdx : [313, 695, 996, 1367, 1518, 1726, 1871, 2084, 2240, 2478, 2622, 2987, 3225, 3573, 3938, 4302, 4666, 5038, 5397, 5762]
#out: maxpeaksVal :[0.7608371709999999, 0.6970320329999999, 0.652950446, 0.570015382, 0.516312078, 0.5599991639999999, 0.5025837670000001, 0.563103466, 0.5229157010000001, 0.611383482, 0.59167585, 0.68685803, 0.524618916, 0.738796993, 0.8191861140000001, 0.82872904, 0.814913208, 0.7919655170000001, 0.773237498, 0.78905603]

I am curious to know if the same result can be obtained using numpy slicing? (without using a for loop)

something like,

max_peaksIdx = peaks[:][0]  #this doesn't work, just picks the first element. but get a similar result shown above, but with one line code.

Anuja Negi · Accepted Answer · 2020-02-03 06:59:49Z

4

Yes, it can be done!

import numpy as np

peaks = np.array(peaks)

max_peaksIsdx = peaks[:,0]
max_peaksVal = peaks[:,1]

answered Feb 3, 2020 at 6:59

Anuja Negi

412 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Chris · Accepted Answer · 2020-02-03 06:38:29Z

2

You can use zip with unpacking:

max_peaksIdx, max_peaksVal = zip(*peaks)

Output:

max_peaksIdx
# (313, 695, 996, ...)
max_peaksVal
# (0.7608371709999999, 0.6970320329999999, 0.652950446,...)

If you have to construct numpy array:

arr = np.array(peaks)
max_peaksIdx, max_peaksVal = arr[:, 0], arr[:, 1]

But zip is about 9x faster:

%timeit max_peaksIdx, max_peaksVal = zip(*peaks)
# 1.19 µs ± 91.5 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%%timeit
arr = np.array(peaks)
max_peaksIdx, max_peaksVal = arr[:, 0], arr[:, 1]
# 10 µs ± 120 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

answered Feb 3, 2020 at 6:38

Chris

29.8k3 gold badges34 silver badges56 bronze badges

1 Comment

imantha Over a year ago

Amazin. Thanks definitely what I was looking for :)

Swati Srivastava · Accepted Answer · 2020-02-03 08:02:09Z

There are a number of ways to do so. Firstly, you can try zip and unpacking as

lis = []
for i in range(len(peaks[0])):
    lis.append(list(list(zip(*peaks))[i]) )

This will output lis as

[[313,
  695,
  996,
  1367,
  1518,
  1726,
  1871,
  2084,
  2240,
  2478,
  2622,
  2987,
  3225,
  3573,
  3938,
  4302,
  4666,
  5038,
  5397,
  5762],
 [0.7608371709999999,
  0.6970320329999999,
  0.652950446,
  0.570015382,
  0.516312078,
  0.5599991639999999,
  0.5025837670000001,
  0.563103466,
  0.5229157010000001,
  0.611383482,
  0.59167585,
  0.68685803,
  0.524618916,
  0.738796993,
  0.8191861140000001,
  0.82872904,
  0.814913208,
  0.7919655170000001,
  0.773237498,
  0.78905603]]

Only zip can also be used.

list( map(itemgetter(i), peaks))

Data_Is_Everything · Accepted Answer · 2020-02-03 06:42:12Z

-2

   max_peaksIdx = peaks[:][0]

can also be done as

   max_peaksIdx = peaks[0] #stores the first element in peaks
   max_peaksVal = peaks[1] #stores the second element in peaks

without the ":", all without the for loop.

answered Feb 3, 2020 at 6:42

Data_Is_Everything

2,0191 gold badge18 silver badges19 bronze badges

2 Comments

imantha Over a year ago

Sorry this wouldnt work. It just picks the first element. i.e [313, 0.7608371709999999]

Data_Is_Everything Over a year ago

it picks the first element to the first list created and the 2nd element to the 2nd list created, just going off your question. this should work without the for loop and is not a slicing approach :)

Collectives™ on Stack Overflow

Selecting all first element of a nested list / array

4 Answers 4

Comments

1 Comment

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

1 Comment

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related