In Python, I am trying to apply an operator to a two layer nested array. For example,
a = [[\'2.3\',\'.2\'],[\'-6.3\',\'0.9\']]
for j in range(2)
for i in r
One-liner with mix of map
and listcomp:
a = [map(float, suba) for suba in a] # Only works on Py2
Or variants:
# Both of the below work on Py2 and Py3
a = [list(map(float, suba)) for suba in a]
a = [[float(x) for x in suba] for suba in a]
Choose based on your personal preference and target Python version. For large nested lists on CPython 2, the first variant is probably the fastest (if the inner lists are huge, it avoids lookup overhead to get the float
constructor and byte code execution for the inner list
s), and the list
wrapped equivalent might eventually win on CPython 3; for small nested lists on all versions, the nested list comprehensions is usually going to be the fastest.
The first argument requires a function, and as such, you can utilize the lambda operator to map float to the sub-lists. This is an example:
a = map(lambda b : map(float, b), a)
Under the hood, map
is a loop too. You should just use a list comprehension, however there is a nice way to do it with Python2 - you just need a partial
function
>>> a = [['2.3','.2'],['-6.3','0.9']]
>>> from functools import partial
>>> map(partial(map, float), a)
[[2.3, 0.2], [-6.3, 0.9]]
I don't think there is a nice way to convert this to Python3 though
>>> list(map(list, map(partial(map, float), a))) #YUKKK!
[[2.3, 0.2], [-6.3, 0.9]]
With list comprehension:
a = [[float(j) for j in i] for i in a]