Looping using array name?

Question

Im trying to loop over arrays with similar name, as such:

a0=[]
a1=[]
a2=[]
a3=[]
a4=[]

for k in range(len(mlac)):     # mlac.shape (17280, 5)
    for I in range(len(mlac[0])):
        'a%i'%I.append(mlac[k][I])
        # or : 'a{}'.format(I).append(mlac[k][I])

But both are considered strings on the loop and cannot append.

So for each iteration of 'I', we take the corresponding column of 'mlac' and add to the array 'a' of corresponding iteration.

_Technically,_ you could get away with `globals()[f'a{I}'].append()`, but definitely use a list of lists as everyone suggests. — B Remmelzwaal, Apr 02 '23 at 20:57
Does this answer your question? [How can I select a variable by (string) name?](https://stackoverflow.com/questions/47496415/how-can-i-select-a-variable-by-string-name) — user3840170, Apr 02 '23 at 21:05
Dont even know whats going on there.. im not a programmer, just know the basics to do some data analysis. — Lyu, Apr 02 '23 at 21:07

score 1 · Accepted Answer · answered Apr 02 '23 at 20:55

Trying to take a string and use it directly as a variable name is not a great idea.

If you're allowed to change those arrays, you should change them to a list of lists or a dictionary. If not, you can load those existing arrays into another data structure, where you can easily iterate over them.

a0=[]
a1=[]
a2=[]
a3=[]
a4=[]

list_of_lists = [a0, a1, a2, a3, a4]

for k in range(len(mlac)):     # mlac.shape (17280, 5)
    for I in range(len(mlac[0])):
        list_of_lists[I].append(mlac[k][I])

The arrays are being passed by reference, so your original arrays will have their contents modified as you want.

score 1 · Answer 2 · answered Apr 02 '23 at 20:55

1

Easy fix by Sembei:

A = [[],[],[],[],[]]
for k in range(len(mlac)):
    for i in range(len(mlac[0])):
        A[i].append(mlac[k][i])

answered Apr 02 '23 at 20:55

Lyu

133
5

score 0 · Answer 3 · answered Apr 02 '23 at 21:08

Would using the eval() function be applicable to your use case?

Here is a simple code snippet demonstrating how it can be used:

a0=[]
a1=[]
a2=[]
a3=[]
a4=[]
a5=[]

mlac = [[y for y in range(5)] for x in range(5)] # Mock version of your mlac multidimentional array

for k in range(len(mlac)):     
    for I in range(len(mlac[0])):
        eval(f"a{I}.append(mlac[k][I])")

print(a0)
print(a1)
print(a2)
print(a3)
print(a4)

'''
Output:
[0, 0, 0, 0, 0]
[1, 1, 1, 1, 1]
[2, 2, 2, 2, 2]
[3, 3, 3, 3, 3]
[4, 4, 4, 4, 4]
'''

More examples on how to use the eval() function can be found here: Python eval(): Evaluate Expressions Dynamically

Looping using array name?

3 Answers3