basics.md

Basics: variables, assignment, lists

There are two ways to run Python programs. You can either type them out line by line in the python interpreter (by running just the python command) and see the result of each line, or you can save your program to a file like test.py and run the file using python test.py.

Because the second method only prints values if you call the print function, all examples here will use print so that they will work on both methods.

Variables and assignments

Try the following program:

x = 2
print(x)
x = x + 1
print(x)

Programs are executed line by line, so the order of lines is important. Does this program

x = 2
x = x + 1
x = 4
print(x)

have the same output as this one?

x = 2
x = 4
x = x + 1
print(x)

When you assign the value of one variable to another variable, does changing one affect the other?

x = 8
y = x
x = x + 1
print(y)

What about here?

x = 8
x = x + 1
y = x
print(y)

Lists

If you want to store more than one value in a variable, you can use a list.

l = [2, 4, 7]
print(l)
print(l[0])
print(l[2])
l[1] = 5
print(l)

Does assignment copy lists?

l = [1, 2, 3]
m = l
l[2] = 4
print(m)

You can slice a list to get a section of it:

l = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
print(l[0])
print(l[2:5])
print(l[4:])
print(l[:4])
print(l[-1])
print(l[2:-2])
print(l[2:15])

What does the len function return?

print(len(l))
print(len([1, 4]))
print(len([]))