Python-为什么我们必须在while循环中初始化标识符而不在for循环中初始化标识符？

Question

I learned Visual Basic before and now I'm learning Python. I realized that in Python, we don't have to initialize a value for the identifier in for loop , but it's the other way round in while loop . It looks unfamiliar to me as in Visual Basic, we have to declare a variable as the identifier in both loops.

For example, from Python wiki,

x = 1
while True:
    print "To infinity and beyond! We're getting close, on %d now!" % (x)
    x += 1

runs perfectly. However, after I commented x = 1 , it generates an error. Why can't it run like for loop where x starts from 0?

In addition, even if we assign a value for the identifier in for loop , for example,

num = 10
for num in range(5):
    print num
print num

the loop will overwrite the value of the assignment. Why is it so?

I know there are plenty of for loop and while loop questions here, but mostly they are case studies of OP's code. What I want is a more general explanation about the Python structure itself and some underlying operations of the language when for loop or while loop is executed. Also, in this case, I'm referring to Python 2.7.11. I will be very grateful if you can point out the difference in terms of the structure for 3.5.1. Thanks a lot.

Answer 1

x = 1
while True:
    print "To infinity and beyond! We're getting close, on %d now!" % (x)
    x += 1

If you comment x = 1 , this line raises an error:

x += 1

because it means, x = x + 1 , as x is not defined, it raises an error(python doesn't know for what value add 1)

num = 10
for num in range(5):
    print num
print num

you're using the same name for both variables, inside loop and outside of this, that's why you have another value of num inside a loop.

Good practice is to have another name for variables inside a loop variable, because you really don't need to have the same for loop variable, as you have outside a loop

so, you can write something like this:

num = 10
for i in range(5):
    # play with num and i variables
    num += i * 2
 print num

Answer 2

because the 'range' says you what is the succession of values and define num as a local value. You see few words but there is a more complex process.

An example may be Javascript language. In the for loop, you see the initial value of the value iterated through the loop, you see the maximal value and the incrementation. for(var num = 0; var <5; var+=1) in python you see for num in range(5). THe while loop is more explicit.

The fact that num is a name used for a global variable and a local variable may cause confusion. In reality the num that you see in the loop is an other variable.

Answer 3

Because Python's for loop is more like C#'s foreach or JS's for of ,
whereas VB's for is more like Pascal's for .

I'll use Pascal instead of VB, because I know it better.

Pascal's for :

var i: byte;
begin
    for i := 0 to 5 do
    begin
        writeln(i);
    end;
end.

Each Pascal's for loop can be easily replicated in Python using range :

for i in range(5):
    print(i)

But, Python's for is something more:

for c in "HelloWorld!":
    print(c)

You cannot replicate it in Pascal without directly dealing with indexes.

Pascal's and VB's for works with indexes – they iterate from some index to another, whereas Python's for loop works with iterables (array, strings, files, generators and much more).