Basic Python Loop

How does Python know what 'i' is when it is not defined, shouldn't there be an error? Probably a simple explanation, but I am new to learning Python. def doubles (sum): return

Solution 1:

Haha :-) People are marking down your question, but I know that is one question must have came in every person's mind. Specially those who learned Python through Online courses and not through a teacher in person.

Well let me explain that in layman's term,

The method that you used is specially used for 1) lists and 2) lists within lists. For eg,

example1= ['a','b','c'] # This is a simple list
example2 = [['a','b','c'],['a','b','c'],['a','b','c']] # This is list within lists.

Now, 'a','b' & 'c' are items in list.

So by saying,

for i in example1:
    print i

we are actually saying,

for item in the list(example1):
    print item

-------------------------

People use 'i', probably taken as abbreviation to item, or something else. I don't know the history.

But, the fact is that, we can use anything instead or 'i' and Python will still consider it as an item in list.

Let me give you examples again. example1= ['a','b','c'] # This is a simple list example2 = [['a','b','c'],['a','b','c'],['a','b','c']] # This is list within lists.

for i in example1:
    print i

[out]: a
       b
       c

now in example2, items are lists within lists. --- also, now i will use the word 'item' instead of 'i' --- the results regardless would be the same for both.

for item in example2:
    print item

[out]: ['a','b','c']
       ['a','b','c']
       ['a','b','c']

people also use singulars and plurals to remember things, so lets we have a list of alphabet.

letters=['a','b','c','d']
for letter in letters:
    print letter

[out]: a
       b
       c
       d

Hope that helps. There is much more to explain. Keep researching and keep learning.

Regards, Md. Mohsin

Solution 2:

Using a variable as a loop control variable does assign to it each time through the loop.

As to "what it is"... Python is dynamically typed. The only thing it "is" is a name, just like any other variable.

Solution 3:

i is assigned the value in the loop itself, it has no value (it is not defined) before the Python interpreter reaches the for line.

Its similar to how other for loops define variables. In C++ for example:

for(int i=0; i<5; i++){
    cout << i << endl;
}

Here the variable i is only exists once the for loop is called.

Solution 4:

i is assigned a value when the for loop runs, so the Python interpreter will not raise an error when the loop is run

Solution 5:

long story short it creates a new variable without having to be defined and its value is whatever number your loop is on, for example if you had written:

num = 0
for i in range(3):
     print(num)
     num = num + 1

so for the first time this loop ran 'i' would equal 0 (because python lists/loops etc always start on 0 not 1), the second time it would equal 1, etc. and the 'num' you can ignore it's just an example of code you could have in a loop which would print out numbers in ascending order. Levin