为什么我的python for循环中没有定义我的变量？

Question

Here is the code:

import math
with open("test.stl") as file:
    vertices = [map(float, line.split()[1:4])
                for line in file
                if line.lstrip().startswith('vertex')]

    normals = [map(float, line.split()[2:5])
                for line in file
                if line.lstrip().startswith('facet')]

V=len(vertices)
ordering=[]
N=len(normals)

for i in range(0,N):
    p1=vertices[3*i]
    p2=vertices[3*i+1]
    p3=verticies[3*i+2]
    print p1

    x1=p1[0]
    y1=p1[1]
    z1=p1[2]

    x2=p2[0]
    y2=p2[1]
    z2=p2[2]

    x3=p3[0]
    y3=p3[1]
    z3=p3[2]

    a=[x2-x1,y2-y1,z2-z1]
    b=[x3-x1,y3-y1,z3-z1]

    a1=x2-x1
    a2=y2-y1
    a3=z2-z1
    b1=x3-x1
    b2=y3-y1
    b3=z3-z1

    normal=normals[i]

    cross_vector=[a2*b3-a3*b2,a3*b1-a1*b3,a1*b2-a2*b1]

    if cross_vector==normal:
        ordering.append([i,i+1,i+2])
    else:
        ordering.append([i,i+2,i+1])
print ordering
print cross_vector

If I try to add print p1 (or any of the other variables such as cross_vector) inside of the for loop, there aren't any errors but no output and if I try to print them outside of the for loop it says NameError: name '(variable name)' is not defined. So if none of these variables are being defined, obviously my ordering array prints as [] (blank). How can I change this. Do variables have to be declared before they are defined?

Edit: Here is the error output when the code above is run:

Traceback (most recent call last):
  File "convert.py", line 52, in <module>
    print cross_vector
NameError: name 'cross_vector' is not defined

As explained above this happens with any variable defined in the for loop, I am just using cross_vector as an example.

Answer 1

This line:

vertices = [map(float, line.split()[1:4])
            for line in file
            if line.lstrip().startswith('vertex')]

reads through all the lines in the file. After that, you're at the end of the file, and there's nothing left to read. So

normals = [map(float, line.split()[2:5])
            for line in file
            if line.lstrip().startswith('facet')]

is empty ( normals == [] ). Thus

N=len(normals)

sets N to 0, meaning that this loop:

for i in range(0,N):

is never executed. That's why printing from inside it does nothing -- the loop isn't being run.

Answer 2

To solve the problem diagnosed by DSM, use:

import math
import itertools
with open("test.stl") as file:
    i1, i2 = itertools.tee(file)
    vertices = [map(float, line.split()[1:4])
                for line in i1
                if line.lstrip().startswith('vertex')]

    normals = [map(float, line.split()[2:5])
                for line in i2
                if line.lstrip().startswith('facet')]

You might also want to try and drop the list comprehension, and work with iterators throughout, to save on memory for large files.

---

Edit:

At present, you load the entire file into memory, and then create two more full size lists in memory. Instead, you can write it in a way that only reads from the file in memory as required. As an example, we can replace the list comprehensions with generator comprehensions:

import math
import itertools
with open("test.stl") as file:
    i1, i2 = itertools.tee(file)
    vertexIter = (map(float, line.split()[1:4])
                  for line in i1
                  if line.lstrip().startswith('vertex'))

    normalIter = (map(float, line.split()[2:5])
                  for line in i2
                  if line.lstrip().startswith('facet'))

Here, we've avoided using any memory at all.

For this to be useful, you need to be able to replace your loop, from:

for i in range(0,N):
    p1=vertices[3*i]
    p2=vertices[3*i+1]
    p3=verticies[3*i+2]
    normal = normals[i]

    # processing

To a single iterator:

for normal, p1, p2, p3 in myMagicIterator:
    # processing

One way I can think of doing this is:

myMagicIterator = itertools.izip(
    normalIter,
    itertools.islice(vertexIter, 0, 3),
    itertools.islice(vertexIter, 1, 3),
    itertools.islice(vertexIter, 2, 3)
)

Which is the iterator equivalent of:

myNormalList = zip(normals, vertices[0::3], vertices[1::3], vertices[2::3])

Answer 3

Declare them outside of it (before the for loop) and see what happens. Even if it would be ok to declare them in the for loop, you would probably like to have a "default" value of them when the loop doesn't run.

And please try to post a lot smaller example if necessary.