# Erle Robotics Learning Python GitBook Free

## For loop with lists

If you want to do something with every item in the list, you can use a for loop. The command line is:

for variable in list_name:


A variable name follows the for keyword; it will be assigned the value of each list item in turn.

Then in list_namedesignates list_name as the list the loop will work on. The line ends with a colon (:) and the indented code that follows it will be executed once per item in the list.

###### Practice 1

What does this code?

>>> my_list = [1,9,3,8,5,7]
>>>
>>> for number in my_list:
...     print number*2
...
...


The for loop will automatically execute your code as many times as there are items in my_list.The result is:

...
2
18
6
16
10
14
>>>


If your list is a jumbled mess, you may need to sort() it.Note that .sort() modifies the list rather than returning a new list.

###### Practice 2

Write a forloop that iterates over start_list = [5, 3, 1, 2, 4] and .append()s each number squared (x ** 2) to square_list(initialized to empty list). Then sort square_list.

>>> start_list = [5, 3, 1, 2, 4]
>>> square_list = []
>>>
>>>
>>> for number in start_list:
...    n=star_list[number-1]
...    square_list.append(n**2)
...
>>> #Now we reordenate the list
...
>>> square_list.sort()
>>> print square_list
[1, 4, 9, 16, 25]
>>>
over start_list=[5, 3, 1, 2, 4] and .append()s each number squared (x ** 2) to square_list (which is initialized to empty list).
Then sort square_list.


###### Practice 3

Use a for loop to print out all of the elements in the list names.

names = ["Adam","Alex","Mariah","Martine","Columbus"]

>>> names = ["Adam","Alex","Mariah","Martine","Columbus"]
>>>
>>> for name in names:
...    print name
...
Alex
Mariah
Martine
Columbus
>>>


You can also use the forloop with strings:

###### Practice 4

We are going to print only the "i" letters for a string. Run this code and analyze what happend:


word = "Programming is fun!"

for letter in word:
# Only print out the letter i
if letter == "i":
print letter
`