CODEWITHSUNDEEP
python-3.xnumpy
An student
An student

Reputation: 392

Array swapping in python

I want to swap elements between two array starting from a particular array index value keeping other values prior to the array index intact. 

import numpy as np
r = np.array([10, 20, 30, 40, 50, 60])
p = np.array([70, 80, 90, 100, 110, 120])

t = []
for i in range(len(r)):
    for j in range(len(p)):
        if i >= 3 and j >= 3:
            t.append(p[j])
            p[j] = r[i]
            for k in t:
                r[i] = k

The above code does the task but the values are in reverse order. The value that I want in array p after swapping is:

[70, 80, 90, 40, 50, 60]

and the value that i want in array r after swapping is:

[10, 20, 30, 100, 110, 120]

But in array p I am getting:

[70, 80, 90, 60, 50, 40]

and in array r I am getting:

[10, 20, 30, 120, 110, 100]

I don't know what is wrong with the code.

Upvotes: 1

Views: 7898

Answers (3)

hpaulj
hpaulj

Reputation: 231395

You can copy a slice of one array on to the other:

In [113]: r = np.array([10, 20, 30, 40, 50, 60])
     ...: p = np.array([70, 80, 90, 100, 110, 120])
     ...: 
In [114]: t = p.copy()
In [115]: t[3:]=r[3:]
In [116]: t
Out[116]: array([70, 80, 90, 40, 50, 60])

You could also join slices:

In [117]: np.concatenate((p[:3], r[3:]))
Out[117]: array([70, 80, 90, 40, 50, 60])

Those answers create a new array. I think that's clearer than doing an inplace swap. But here's how I'd do the swap

In [128]: temp = r[3:].copy()
In [129]: r[3:]=p[3:]
In [130]: p[3:]=temp
In [131]: r
Out[131]: array([ 10,  20,  30, 100, 110, 120])
In [132]: p
Out[132]: array([70, 80, 90, 40, 50, 60])

I use copy in temp because otherwise a slice produces a view, which will get modified in the next copy. That issue has come up recently when swapping rows of a 2d array.

With lists the swapping is easier - because r[3:] makes a copy.

In [139]: r=r.tolist()
In [140]: p=p.tolist()
In [141]: temp = r[3:]
In [142]: r[3:], p[3:] = p[3:], r[3:]
In [143]: r
Out[143]: [10, 20, 30, 100, 110, 120]
In [144]: p
Out[144]: [70, 80, 90, 40, 50, 60]

Upvotes: 1

Mike Müller
Mike Müller

Reputation: 85462

You can achieve the same without looping:

r = np.array([10, 20, 30, 40, 50, 60])
p = np.array([70, 80, 90, 100, 110, 120])

i = 3
temp = p[i:].copy()
p[i:] = r[i:]
r[i:] = temp

Now:

>>> p
array([70, 80, 90, 40, 50, 60])
>>> r
array([ 10,  20,  30, 100, 110, 120])

Upvotes: 2

Umang Gupta
Umang Gupta

Reputation: 16450

import numpy as np
r = np.array([10, 20, 30, 40, 50, 60])
p = np.array([70, 80, 90, 100, 110, 120])

for i in range(len(r)):
    if (i>=3):
        p[i],r[i] = r[i],p[i]

Above code will do the work for you. You don't need to run two for loop and t array if I understand your problem right. All you want is to swap at some indexes. You can just swap at those indexes as above no need of a temporary array t.

Upvotes: 3

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