Ndtyjky

I'm new to Python but having a hard time to understand the following While Loop code as it behaves very differently. I know this code work but I do not know HOW IT WORK. Top Python experts also have no idea.

x = 1

while x < 10:

    print x

    x = x + 1 ## How does this VAR manipulate Print as the VAR comes late in the code?

I do not know whether this related to control flow or global var. Please help me out to understand deeper.

I believe your question is about scope. In the above example, a new memory address for var x is created on the first line and any code on lines 2,3,4 that access x can read/write to that variable.

There are only two rules here that we need to follow:

1. 
   x must be declared before print so that print can access that piece of memory


2. 
   print must be in the same or inner scope to access 'x'

Example 1 - valid (because we follow rules 1 and 2)

x = 1

print x

Example 2 - valid (because we follow rules 1 and 2)

x = 1

while x < 10:

    print x

    x = x + 1

Example 3 - valid (because we follow rules 1 and 2)

x = 1

while x < 10:

    print x

    x = x + 1

print x

Example 4 - NOT valid (because we don't follow rule 1 and print is accessing a variable that has not yet been created in memory)

print x    

x = 1

Example 5 - NOT valid (because we don't follow rule 2 and print doesn't have access to our variable's scope, x no longer exists in memory)

y = 1

while y < 5:

    x = 1

    y = y + 1

print x

It would do you good to read this entire section on variable lifetime and scope in python all the way through Exercise 1 for good practice.

x = 1 #Declares the variable
while x < 10: #checks whether the value of x is less than 10 every time the loop runs
print x #prints value of x for the each iterations(1 for first iteration, 2 for second and so on)
x = x + 1 #this is the update part
Let me tell you one more thing, you do not have any case of global variable here.
If you have trouble in understanding whether its global or local declaration, I would recommend you to follow this link

Counter variable act like a tiny persistent storage. While loop checks whether the stored value in that tiny persistent storage Less than 11. If the value less than 10 then it starts the loop. Inside the loop, it instructs to Print value 1 because it's the current value in that tiny persistent storage after that the other "x" inside loop instruct that tiny persistent storage to update value by adding +1. This loop continues until it gets alert from persistent storage that value stored in persistent storage met 10 value. The loop will successfully exist.

This little software helped me to understand behind scenes http://pythontutor.com/visualize.html

Conclusion:

It seems For loops better for this kind things because it's clean and easy to maintain but understanding this concept is also important