Doing maths is a process: it happens inside the computer, between the input and the output. Python does arithmetic with operators, and several of them are written differently from the way you would write them on paper.

The operators

You have already met + joining two strings together. Between two numbers the same symbol adds them up instead, so what + does depends entirely on what sits on either side of it.

The two kinds of division

Python has two division operators, and they answer two different questions.

/ is ordinary division. It gives the full answer, including everything after the decimal point. A number with a decimal point in it is called a float:

print(10 / 4)

2.5

// is floor division, also called integer division. It throws away everything after the decimal point and keeps only the whole number part. A whole number is called an integer, or int for short:

print(10 // 4)

2

Notice that // does not round to the nearest whole number. 10 // 4 is 2, even though 2.5 is exactly halfway: the decimal part is simply dropped.

Remainders and powers

% is called modulo. It divides, like //, but gives you the part that is left over instead of the number of whole times. 4 goes into 10 twice, which uses up 8, so the remainder is 2:

print(10 % 4)

2

The % sign has nothing to do with percentages here: in Python it always means the remainder.

** is called exponentiation. It raises the first number to the power of the second, so 8 ** 2 is 8 squared, 8 times 8, and 2 ** 3 is 2 cubed, 2 times 2 times 2:

print(8 ** 2)
print(2 ** 3)

64
8

One star multiplies, two stars raise to a power.

Doing maths with variables

Numbers do not have to be typed straight into the calculation. If they are stored in variables, you can use the variable names instead, and Python looks inside each one before it calculates:

num1 = 5
num2 = 10

result = num1 + num2

print(result)

15

There are three steps there, and it is worth naming them:

1. Assign numbers to your variables.
2. Perform the calculation, and assign the result to a new variable.
3. Output the result.

Step 2 is the one that is easy to skip. A calculation on its own throws its answer away; it only survives if you assign it to something.

Changing a variable with maths

The variable you assign the result to may also be one of the variables used in the calculation. Python works out the right hand side first, and only then replaces the old value:

score = 0
score = score + 1

print(score)

1

Because adding to a variable’s own value is so common, Python has a shorthand for it. score += 1 means exactly the same thing as score = score + 1, and lives -= 1 means the same as lives = lives - 1:

score = 0
score += 1

print(score)

1

Try it yourself

Run the code below. Then swap the operator on the result line for each of the other six in turn, and check that every answer is the one you expected.

num1 = 20
num2 = 5

result = num1 * num2

print(result)