Python noob here. I need to store an array of float arrays. I am doing this but its not working out:
distance = [] ##declare my array
distance.append ([]) ##add an empty array to the array
distance[len(distance)-1].append ([0,1,2,3.5,4.2]) ## store array in array[0]
print distance[0][1] ## this doesnt work, the array above got stored as 1 item
Use list.extend not list.append:
The difference between extend and append is that append appends the object passed to it as it is. While extend expects that item passed to it to be an iterable(list, tuple,string, etc) and appends it's items to the list.
Using append we can append any type of object; i.e iterable or non-iterable.
>>> lis = [1,2,3]
>>> lis.append(4) #non-iterable
>>> lis.append('foo') #iterable
>>> lis
[1, 2, 3, 4, 'foo']
But extend behaves differently and actually appends the individual items from the iterable to the list.
>>> lis = [1,2,3]
>>> lis.extend('foo') #string is an iterable in python
>>> lis
[1, 2, 3, 'f', 'o', 'o'] #extend appends individual characters to the list
>>> lis.extend([7,8,9]) #same thing happend here
>>> lis
[1, 2, 3, 'f', 'o', 'o', 7, 8, 9]
>>> lis.extend(4) #an integer is an not iterable so you'll get an error
TypeError: 'int' object is not iterable
Your Code
>>> distance = [[]]
>>> distance[-1].extend ([0,1,2,3.5,4.2])
>>> distance
[[0, 1, 2, 3.5, 4.2]]
This returns:
[[0, 1, 2, 3.5, 4.2]]
If you wanted to do this then there's no need to append the empty [] and then call list.extend, just use list.append directly:
>>> ditance = [] ##declare my array
>>> distance.append([0,1,2,3.5,4.2])
>>> distance
[[0, 1, 2, 3.5, 4.2]]
Use extend instead of append:
distance[-1].extend([0,1,2,3.5,4.2])
(Also, note that distance[len(distance)-1] can be written distance[-1].)
You can also do this (since you already initialized an empty list to distance[0]):
distance[len(distance)-1] += [0,1,2,3.5,4.2]
Here's what you've done:
There are several ways to accomplish your goal:
1.
distance[len(distance)-1].extend([0,1,2,3,4,5]) #this one has been addressed elseqhere
This next one should make the most sense to you. Loop through and append to you inner list
for item in [0,1,2,3,4]:
distance[ -1 ].append( item)
This last one is kind of cool and good to know, though really indirect here:
map( lambda item : distance[0].append( item ), [1,2,3,4,5] )
