Reading data in lists: Error

[Peter Otten]

> I ran this code only to get the following error:
> 
>  File "<string>", line 24
>     NYA=x[i]-x[i-1]
>       ^
> SyntaxError: invalid syntax
> 
> 
> I remember Mr.Skip Montanaro quoting in his email earlier that at the
> point I make the assignment neither x has no i'th elements. I am sure this
> is what is happening now.

How can you be sure when Python reports a SyntaxError?

> Can someone advise any possible work around to this problem ??

See Paul McGuire's post for a fix. As a general rule, you should break messy
expressions into small chunks that are easier to maintain. In your code you
could introduce a distance(x0, y0, x1, y1) function and test it separately
from the rest. Here's another approach that might be helpful if you are
familiar with complex numbers - yes, Python has them built in :-)

# assuming pres, x, y as in your post
FZ = []

# you should create the complex numbers directly in the for loop
# reading from the file - I'm just lazy
z = [complex(x0, y0) for x0, y0 in zip(x, y)]

for z0, z1, p in zip(z, z[1:], pres):
    d = z1-z0
    FZ.append(p*d/abs(d))

# assuming you accidentally swapped FX and FY
FX = [z.real for z in FZ]
FY = [-z.imag for z in FZ] # not sure about the sign

If you're confused by the zip trick in the for loop, here's how it works in
three simple steps:

>>> a = ["a", "b", "c"]
>>> b = ["x", "y", "z"]
>>> zip(a, b)
[('a', 'x'), ('b', 'y'), ('c', 'z')]

zip() will end, when the shortest list is exhausted:

>>> b[1:]
['y', 'z']
>>> zip(a, b[1:])
[('a', 'y'), ('b', 'z')]

>>> zip(a, a[1:])
[('a', 'b'), ('b', 'c')]

I. e, zip(alist, alist[1:]) will create a list of all adjacent pairs in
alist.

Peter