# function changing input value

The following code is supposed to implement a basic percolation process on a matrix of 0's and 1's. For some reason, it's changing the value of the matrix I input, and I can't figure out why. I'm sure it's a basic error on my part, but if someone can set me straight, I'd appreciate it!

Here's the function I defined:

```
n = 4;
def percolate(B):
update = B;
for i in [0..n-1]:
for j in [0..n-1]:
nbr_sum = 0;
if (i==0):
nbr_sum += B[i+1,j];
elif (i==n-1):
nbr_sum += B[i-1,j];
else:
nbr_sum += B[i-1,j]+B[i+1,j];
if (j==0):
nbr_sum += B[i,j+1];
elif (j==n-1):
nbr_sum += B[i,j-1];
else:
nbr_sum += B[i,j-1]+B[i,j+1];
if (nbr_sum > 1):
update[i,j] = 1;
return update;
```

When I execute

```
Bid = matrix.identity(n);
percolate(Bid)
```

I get the expected output

```
[1 1 0 0]
[1 1 1 0]
[0 1 1 1]
[0 0 1 1]
```

but the value of `Bid`

also changes to the same value (as opposed to continuing to be the n x n identity matrix, as desired).

Use

`update = B.copy()`

, otherwise`update`

is the same as`B`

. By the way, no need for all these`;`