Adding value to one of the elements in array

Question

I have a list of n elements, and I'm trying to make a new nxn array in which all the rows are the elements of my previous list (for example, if I have a list u=[a,b], then the array is y=[u,u]=[[a,b],[a,b]]) and for all the values of index i=1,2,... I want all the elements y[i][i] to be added with additional values h (that is, if the list is p=[a,b], then the final product is q=[p,p]=[[a+h,b],[a,b+h]]).

If I made the program to be straightforward like this

u=[[1,2],[1,2]]
h=1e-3
for i in range(2):
  u[i][i]+=1
print(u)

it would print out what I want correctly

[[2, 2], [1, 3]]

but if I did it in a loop like this

x=[1,2]
u=[]
h=1e-3
for i in range(len(x)):
  u.append(x)
for i in range(2):
  u[i][i]+=1
print(u)

the program would assume that I'm trying to add 1 to each of its elements like this

[[2, 3], [2, 3]]

I did check "u" before adding 1 to its u[i][i] elements with the second code and it prints out u=[[1,2],[1,2]] as well. So what did I do wrong here?

Synthaze · Accepted Answer · 2021-01-03 02:57:50Z

1

x=[1,2]
u=[]
h=1e-3

for i in range(len(x)):
  u.append(x.copy())

for i in range(2):
  u[i][i] +=1

print(u)

You got almost there! In fact, when you append x to u twice, you make "linked twins". Which means: x stays in in u. And you have x twice. If you modify u[0] which is x, then you modify u[1] which is also x.

To avoid that, use x.copy().

answered Jan 3, 2021 at 2:57

Synthaze

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