How to Split a List Based on a Condition

How to split a list based on a condition?

good = [x for x in mylist if x in goodvals]
bad  = [x for x in mylist if x not in goodvals]

is there a more elegant way to do this?

That code is perfectly readable, and extremely clear!

# files looks like: [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi'), ... ]
IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
images = [f for f in files if f[2].lower() in IMAGE_TYPES]
anims  = [f for f in files if f[2].lower() not in IMAGE_TYPES]

Again, this is fine!

There might be slight performance improvements using sets, but it's a trivial difference, and I find the list comprehension far easier to read, and you don't have to worry about the order being messed up, duplicates being removed as so on.

In fact, I may go another step "backward", and just use a simple for loop:

images, anims = [], []

for f in files:
    if f.lower() in IMAGE_TYPES:
        images.append(f)
    else:
        anims.append(f)

The a list-comprehension or using set() is fine until you need to add some other check or another bit of logic - say you want to remove all 0-byte jpeg's, you just add something like..

if f[1] == 0:
    continue

Split list based on condition in Python

Using itertools.groupby

Ex:

from itertools import groupby

mylist = [2, 3, 4, 0, 0, 0, 6, 7, 8, 0, 4, 9, 0, 2, 0, 2, 0, 20, 0]
result = [list(v) for k, v in groupby(mylist, lambda x: x!=0) if k]
print(result)

Output:

[[2, 3, 4], [6, 7, 8], [4, 9], [2], [2], [20]]

Slicing a list into sublists based on condition

I've quickly written one way to do this, I'm sure there are more efficient ways, but this works at least:

num_list =[97, 122, 99, 98, 111, 112, 113, 100, 102]

arrays = [[num_list[0]]] # array of sub-arrays (starts with first value)
for i in range(1, len(num_list)): # go through each element after the first
    if num_list[i - 1] < num_list[i]: # If it's larger than the previous
        arrays[len(arrays) - 1].append(num_list[i]) # Add it to the last sub-array
    else: # otherwise
        arrays.append([num_list[i]]) # Make a new sub-array 
print(arrays)

Hopefully this helps you a bit :)

Split data in list based on condition

Here you go:

import numpy as np
data = np.array(['A1', 'C3', 'B2', 'A2', 'D3', 'C2', 'A3', 'D2', 'C1', 'B1', 'D1', 'B3'])

# define some condition
condition = ['B', 'D']
boolean_selection = [np.any([ c in d for c in condition]) for d in data]

split1 = data[boolean_selection]
split2 = data[np.logical_not(boolean_selection)]

Python3: How can I split a list based on condition?

Use itertools.groupby with a custom key function that changes every time we see a new header. In this function, we increment ctr.

from itertools import groupby

lis = ['>a', 'b', 'c', '>d', 'e', '>f', '>g']

def group_by_header(lis: list):
    def header_counter(x: str):
        if x.startswith('>'):
            header_counter.ctr += 1
        return header_counter.ctr
    header_counter.ctr = 0

    return groupby(lis, key=header_counter)

print([list(l) for k, l in group_by_header(lis)])
# [['>a', 'b', 'c'], ['>d', 'e'], ['>f'], ['>g']]

Split list of tuples into seprate list based on a condition

I here post the solution of @Mustafa Aydin so that other users can find the solution soon.

inds, = np.nonzero(np.array(a)[:, 0] == 1)
[sub.tolist() for sub in np.split(a, inds) if sub.size]

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