Add element in first an last position - Python numpy array

Question

I want to add in my ndarray an element in the first and last position. For this example, I want to add 0 in the first position, and 1441 in the last position. But how?

<type 'numpy.ndarray'>
Out[185]:
array([  10,   20,   30,   40,   50,   60,   70,   80,   90,  100,  110,
        120,  130,  140,  150,  160,  170,  180,  190,  200,  210,  220,
        230,  240,  250,  260,  270,  280,  290,  300,  310,  320,  330,
        340,  350,  360,  370,  380,  390,  400,  410,  420,  430,  440,
        450,  460,  470,  480,  490,  500,  510,  520,  530,  540,  550,
        560,  570,  580,  590,  600,  610,  620,  630,  640,  650,  660,
        670,  680,  690,  700,  710,  720,  730,  740,  750,  760,  770,
        780,  790,  800,  810,  820,  830,  840,  850,  860,  870,  880,
        890,  900,  910,  920,  930,  940,  950,  960,  970,  980,  990,
       1000, 1010, 1020, 1030, 1040, 1050, 1060, 1070, 1080, 1090, 1100,
       1110, 1120, 1130, 1140, 1150, 1160, 1170, 1180, 1190, 1200, 1210,
       1220, 1230, 1240, 1250, 1260, 1270, 1280, 1290, 1300, 1310, 1320,
       1330, 1340, 1350, 1360, 1370, 1380, 1390, 1400, 1410, 1420, 1430,
       1440], dtype=int64)

Instead of existing elements or insert one before current "first" and one after current "last"? — Axalix
– Axalix, Commented Dec 14, 2017 at 0:43

akuiper · Accepted Answer · 2017-12-14 00:43:45Z

9

Use numpy.concatenate:

import numpy as np
a = np.array([1,2,3])
np.concatenate(([0], a, [4]))
# array([0, 1, 2, 3, 4])

Or numpy.r_:

np.r_[0, a, 4]
# array([0, 1, 2, 3, 4])

answered Dec 14, 2017 at 0:43

akuiper

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T.Liu · Accepted Answer · 2017-12-14 00:55:11Z

4

Assume that:

a = numpy.array([10, ... , 1440]);

Insert 0 at the first position:

np.insert(a, 0, 0);

Reference at numpy.insert.

Append 1441 at the last position:

numpy.append(a, 1441);

Reference at numpy.append.

answered Dec 14, 2017 at 0:55

T.Liu

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sfjac · Accepted Answer · 2017-12-14 01:26:08Z

1

Just to add to the existing answers, in case this isn't already clear: You can't expand an existing numpy array in place. So if you have

a = np.array([1,2,3])

There is no operation that you can do to a that would cause all existing references to a to now be [0, 1, 2, 3, 4], for example. If you need that sort of semantic, then you'll need to wrap a in a container object of some sort.

edited Dec 14, 2017 at 1:26

answered Dec 14, 2017 at 1:08

sfjac

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