python - Can I set a function to call a specific index from a list?

Question

I have n lists (really arrays, but I'm treating them as lists) of length p. I want to pull the first three values from the first, second, and third rows. This is what I have so far:

k0= K[0][0:3]
k1= K[1][0:3]
k2= K[2][0:3]

where

k0= [a,b,c]
k1= [d,e,f]
k2= [g,h,i]

such that

concatenated_ko = []

for j in range(0,3):
    k = "k"+str(j)
    concatenated_ko.append(k)

print(concatenated_ko)

which currently prints

['k0','k1','k2']

but should print

[a,b,c,d,e,f,h,i,j]

Edit: I think what it boils down to is that I initially use an object from <class 'numpy.ndarray'> which changes to <class 'str'>. Forcing a type change from a string to an array by np.ndarray(k) doesn't work either.

Any way to change it back?

question from:https://stackoverflow.com/questions/65928953/can-i-set-a-function-to-call-a-specific-index-from-a-list

Answer 1

You've said you "want to pull the first three values from the first, second, and third rows". So instead of creating the three lists k0, k1, and k2 separately, you could put that in your iteration and operate directly on K, by first slicing the first 3 columns.

A.

concatenated_ko = []
for row in K[:3]:
    concatenated_ko.extend(row[:3])

B. or, if you want to use indexes:

concatenated_ko = []
for idx in range(3):
    concatenated_ko.extend(K[idx][:3])

C. And suppose K is a 5x5 array:

>>> K
array([[  5,  12,  10,  70,   4],
       [102,   6, 120,  60,  22],
       [150, 100, 110,   2, 150],
       [ 15,  20,  22,  70,  30],
       [ 20,  55,   1,  70,   1]])

Then to get the first 3 columns of the first 3 rows as a single array, you can use ndarray.flatten():

>>> K[:3, :3].flatten()
array([  5,  12,  10, 102,   6, 120, 150, 100, 110])
>>> K[:3, :3].flatten().tolist()
[5, 12, 10, 102, 6, 120, 150, 100, 110]

---

Another option, not recommended: To make a minor modification to your original code, use eval to get the list from its variable name, and use extend instead of append:

# with k0, k1, k2 setup as per your question
for j in range(0,3):
    k = "k"+str(j)
    concatenated_ko.extend(eval(k))