Merging list comprehensions in Python

Question

Is it possible to combine the following python list comprehensions into a single line? I know it's not necessary, I'm just curious.

rows = [row.split() for row in data]
flattened = [float(val) for sublist in rows for val in sublist]

Below is a snippet of the data file,

['  -.2098335E-03  -.2108988E-03  -.2119629E-03  -.2130240E-03  -.2140826E-03', '  -.2151421E-03  -.2161973E-03  -.2172531E-03  -.2183025E-03  -.2193489E-03', '  -.2203825E-03  -.2214097E-03  -.2224521E-03  -.2235475E-03  -.2246843E-03'....]

I'm trying to extract each of the strings and make a single list of numbers. The code above currently does that, I was wanting to see if it could get even more condensed.

Thanks!

does this help? [float(val) for sublist in [row.split() for row in data] for val in sublist] — DirtyBit
– DirtyBit, Commented Jul 14, 2020 at 17:33
edit your question include your data and how you wanna get your output, and what you hav done till — Adhun Thalekkara
– Adhun Thalekkara, Commented Jul 14, 2020 at 17:35

OysterShucker · Accepted Answer · 2020-07-14 20:01:19Z

2

Since sublist is your row.split(), simply replace it.

data = ['0.3 0.6 0.9', '0.1 0.4 0.3']

flattened = [float(val) for row in data for val in row.split()]

print(flattened) #[0.3, 0.6, 0.9, 0.1, 0.4, 0.3]

If we expand it, it becomes more clear.

data = ['0.3 0.6 0.9', '0.1 0.4 0.3']

#the generator is identically replacing this part
flattened = list()
for row in data:
    for val in row.split():
        flattened.append(float(val))

All of the answers that suggest the solution below, are trying to wrap the source loop with the content loop.

#               v--contents--v      wrapping       v--source       
[float(val) for sublist in (row.split() for row in data) for val in sublist]

It should be the other way around.

[float(val) for row in data for val in row.split()]

edited Jul 14, 2020 at 20:01

answered Jul 14, 2020 at 18:02

OysterShucker

5,5613 gold badges12 silver badges27 bronze badges

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

Comments

Anshika Singh · Accepted Answer · 2020-07-14 18:44:24Z

Nested List Comprehensions

Nested List Comprehensions are nothing but a list comprehension within another list comprehension which is quite similar to nested for loops.

When we create a new list (of squares) from the elements of the existing list, using list comprehensions, we write:

l1=[1,2,3]
l2=[i**2 for i in l1]
print(l2)

This gives the output:

[1, 4, 9]

We can also nest two or even more for loops in this comprehension.

For example:

l1=[1,2,3]
l2=[4,5,6]
l3=[[i**2,j**2] for i in l1 for j in l2]
print(l3)

The output:

[[1, 16], [1, 25], [1, 36], [4, 16], [4, 25], [4, 36], [9, 16], [9, 25], [9, 36]]

Similarly:

l1=[1,2]
l2=[3,4]
l3=[5,6]
l4=[[i,j,k] for i in l1 for j in l2 for k in l3]
print(l4)

Output:

[[1, 3, 5], [1, 3, 6], [1, 4, 5], [1, 4, 6], [2, 3, 5], [2, 3, 6], [2, 4, 5], [2, 4, 6]]

Similarly your code can be re-written using the help of list comprehensions as:

flattened = [float(val) for sublist in (row.split() for row in data) for val in sublist]

Hope this helped you!

Luke Storry · Accepted Answer · 2020-07-14 17:36:21Z

1

You can just use [float(val) for sublist in (row.split() for row in data) for val in sublist] but it is much more readable (and easier to debug in the future) to keep them out on seperate lines

answered Jul 14, 2020 at 17:36

Luke Storry

6,7821 gold badge11 silver badges25 bronze badges

Comments

Gustav Rasmussen · Accepted Answer · 2020-07-14 17:41:07Z

0

One liner:

data = ["1 2",
        "3 4",
        "5 6",
        "7 8",
        "9 10"
        ]

rows = [float(val) for row in data for sublist in row.split() for val in sublist]
print(rows)

Returns:

[1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 1.0, 0.0]

answered Jul 14, 2020 at 17:41

Gustav Rasmussen

4,0394 gold badges32 silver badges56 bronze badges

Comments

DirtyBit · Accepted Answer · 2020-07-14 17:44:30Z

0

If the intention is only to merge into a one-liner, you can use a generator expression (thanks to @OneCricketeer) that would generate item only when in demand and making it memory efficient than the lists:

flattened = [float(val) for sublist in (row.split() for row in data) for val in sublist]

edited Jul 14, 2020 at 17:44

answered Jul 14, 2020 at 17:35

DirtyBit

16.8k5 gold badges37 silver badges56 bronze badges

5 Comments

OneCricketeer Over a year ago

I would write (row.split() for row in data) to not make a secondary list

DirtyBit Over a year ago

@OneCricketeer wouldn't that create a tuple instead?

chepner Over a year ago

@DirtyBit No. Commas create tuples, not parentheses. (The exception is the empty tuple, since () is not a valid parenthesized expression.) Parentheses simply prevent the comma from being interpreted as some other part of Python's grammar (for instance, in the argument list to a function.)

Luke Storry Over a year ago

@DirtyBit using curved brackets in comprehensions makes it a generator expression (basically a list that isn't generated in memory, but lazy evaluated). See docs.python.org/3/reference/…

DirtyBit Over a year ago

@OneCricketeer and hence memory efficient!

Collectives™ on Stack Overflow

Merging list comprehensions in Python

5 Answers 5

Comments

Nested List Comprehensions

Comments

Comments

Comments

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Nested List Comprehensions

Comments

Comments

Comments

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related