Question

Let’s say i have a multidimensional list l:

l = [['a', 1],['b', 2],['c', 3],['a', 4]]

and I want to return another list consisting only of the rows that has ‘a’ in their first list element:

m = [['a', 1],['a', 4]]

What’s a good and efficient way of doing this?

Answer 1

Definitely a case for a list comprehension:

m = [row for row in l if 'a' in row[0]]

Here I’m taking your “having ‘a’ in the first element” literally, whence the use of the in operator. If you want to restrict this to “having ‘a’ as the first element” (a very different thing from what you actually wrote!-), then

m = [row for row in l if 'a' == row[0]]

is more like it;-).