在Python中，如何将列表中的第一个元素乘以一个数字（列表中的列表）？

Question

I have a list of lists like this:

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

I would like to multiply the first element of each of the inner lists by 10. So the expected output would be this:

[[10,2,3],[40,5,6]]

How could I do this?

Answer 1

Try this:

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

for i in my_list:
    i[0] *= 10  # multiply the first element of each of the inner lists by 10

print(my_list)

Output:

[[10, 2, 3], [40, 5, 6]]

Answer 2

If you're working with large arrays, then numpy works better than python alone, for many other reasons as well.

lst = [[1,2,3],[4,5,6]] # if you have your list in python

lst = np.array(lst) # simple one-liner to convert to an array

lst
Out[32]: 
array([[1, 2, 3],
       [4, 5, 6]])

lst[:,0] *= 10 # multiply first column only by 10. This removes the need
               # for python's "for" loops, which improves performance 
               # on larger arrays

lst
Out[34]: 
array([[10,  2,  3],
       [40,  5,  6]])