python: replacing negative values in array read from file

Question

I'm populating an array, reading data from file:

Input:

Code:

f = [ i.strip() for i in open('file.txt', 'r') ]
a = []

for line in f:
   line = line.split()
   a.append(float(line[0]))

that produces the vector a=[2,4,-8,5,-2]. Now I want to replace all negative values with their values +10, in order to obtain a vector b=[2,4,2,5,8]. Without using a loop or a for cycle, how can I do that? I tried both using the np.where() function and the a[a<0], but they don't produce any result (they just work if I create a np.array[2,4,-8,5,-2] ex novo...). Thank you in advance.

After all there'll be a loop, no matter what function you choose to use. — Maroun
– Maroun, Commented Feb 2, 2016 at 8:38

Yen · Accepted Answer · 2016-02-02 08:50:36Z

4

one way not using numpy

a = list(map(lambda x:x+10 if x < 0 else x, a))

answered Feb 2, 2016 at 8:50

Yen

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1 Comment

machine yearning Over a year ago

Nice to see a functional answer in here. The more Pythonic way to write this notation would be a = [x+10 if x < 0 else x for x in a]

elzell · Accepted Answer · 2016-02-02 08:47:59Z

2

You could make a a numpy array:

a = np.array(a)

and then simply do:

a[a<0] += 10

answered Feb 2, 2016 at 8:47

elzell

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3 Comments

machine yearning Over a year ago

IN my opinion Numpy is wayyyy overkill unless you're reading a ton of values and doing performance-critical computations on them. There's a simple way of binary-filtering values in python, that's with an if-else expression.

elzell Over a year ago

@machineyearning But OP explicitely said "Without using a loop or a for cycle" ... One could argue though if list comprehension is actually a for loop.

machine yearning Over a year ago

In that case I'd also question whether numpy array iteration uses some kind of a loop. I had read OP as meaning he would like a way to do it with a single expression, rather than a syntax block. I digress though, OP did mention numpy, so it could actually be the case that this is a large and performance-critical set of numbers to crunch. In that case it's up to OP to choose the right answer that fits his/her needs.

okainov · Accepted Answer · 2016-02-02 09:00:04Z

2

It can be done with list comprehension without using NumPy. Assuming you have list of numbers a:

>>> a = [2, 4, -8, 5, -2]
>>> [number if number >= 0 else number + 10 for number in a]
[2, 4, 2, 5, 8]

answered Feb 2, 2016 at 9:00

okainov

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Comments

Simon · Accepted Answer · 2016-02-02 08:47:36Z

1

If you allow the use of numpy:

import numpy as np

f = [ i.strip() for i in open('C:/Users/simon/Desktop/file.txt', 'r') ]
a = []

for line in f:
   line = line.split()
   a.append(float(line[0]))

a = np.array(a)

print a

a[a<0] = a[a<0] + 10

print a

That will give you:

[ 2.  4. -8.  5. -2.]
[ 2.  4.  2.  5.  8.]

answered Feb 2, 2016 at 8:47

Simon

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