Python For Loop

Question

I understand everything about how a for loop works except in this example...

state_names = ['Alabama', 'California', 'Oklahoma', 'Florida']
vowels = list('aeiou')
output = []

for state in state_names:
  state_list = list(state.lower())

In the for statement, I don't understand where state is coming from... I see the obvious that state_names is present, but what exactly is 'state'? It seems like python knows that state is tied to the strings within state_names.

Answer 1 · 2014-10-04T17:11:20Z

October 4, 2014 5:11pm

Just think of it as a convenient shortcut to prevent you having to manually assign the current item in the loop to a variable yourself. In this example, 'state' could be called anything, it doesn't matter.

So at any time in the loop, you can refer to state in order to access the current item of the array being looped over.

Hope that clears it up, these kind of for loops tend to confuse beginners but they are very intuitive and super handy once you get used to them.

Answer 2 · 2014-10-04T18:59:03Z

October 4, 2014 6:59pm

Good description, Ethan Lowry . +1

In the hopes to simplify further, in case anyone is still having trouble understanding, let's go over the key points.

The for loop receives an object which has items in it. Then the loop does something to each item consecutively (one at a time). In many cases,the loop considers all items in the object and starts at the beginning (the first item) of the object and goes to the end (the last item).

To illustrate, let's say:

There is a car wash; it washes each car one after another
- Therefore, the car wash consists of a loop
The Object is the line of cars or set of customers, which is to say all the cars in the line waiting to enter the car wash
The car that enters the car wash is the current priority of the car wash

When a car enters the car wash some things happen. This is what happens inside the for loop. When a car is inside the car wash that car (the current item or index) gets washed. When that is done, the car exits and the next car in the line up enters. This process repeats until each item has been dealt with.

So in programming terms we can say,

In the context of a car wash:
1. car_wash consider the line of customers waiting to get their car washed
2. when a car enters the car wash then wash the car
  - we will call this car the current_car. This is arbitrary, we can can name it whatever we like but it makes sense to keep within the context of the object it is inside of
3. when the car is washed it exits and the next one enters
Repeat steps 2 and 3 until there are no more cars in the line-up

In Python we can say:

1 line_of_cars_to_be_washed = ['car1', 'car2', 'car3', 'car4', 'car5']
2 # within the context of car_wash
3 # wash each car in the line
4 for current_car in line_of_cars_to_be_washed:
5    # the current car enters the car wash
6    car_wash.wash_car(current_car)
7    # car is clean and leaves
8    # if there are still customers waiting to get car washed
9        #  then go back to line 4 until all cars are washed

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Christopher Johnson

Christopher Johnson

Python For Loop

2 Answers

Ethan Lowry

Ethan Lowry

Christopher Johnson

Christopher Johnson

Sean T. Unwin

Sean T. Unwin

Christopher Johnson

Christopher Johnson