1.4. Integer Arithmetic

1.4.1. Addition and Subtraction

We start with the integers and integer arithmetic, not because arithmetic is exciting, but because the symbolism should be mostly familiar. Of course arithmetic is important in many cases, but Python is probably more often used to manipulate text and other sorts of data, as in the sample program in Running A Sample Program.

Python understands numbers and standard arithmetic. For the whole section on integer arithmetic, where you see a set-off line in typewriter font, type individual lines at the >>> prompt in the Python Shell. Press Enter after each line to get Python to respond:

2 + 3
5 - 7

Python should evaluate and print back the value of each expression. Of course the first one does not require any calculation. It appears that the shell just echoes back what you printed.

The Python Shell is an interactive interpreter. As you can see, after you press Return (or Enter), it is evaluating the expression you typed in, and then printing the result automatically. This is a very handy environment to check out simple Python syntax and get instant feedback. For more elaborate programs that you want to save, we will switch to an Editor Window later.

1.4.2. Multiplication, Parentheses, and Precedence

Try in the Shell:

2 x 3

You should get your first syntax error. The x should have become highlighted, indicating the location where the Python interpreter discovered that it cannot understand you: Python does not use x for multiplication as you may have done in grade school. The x can be confused with the use of x as a variable (more on that later).

Instead the symbol for multiplication is an asterisk *. Enter each of the following. You may include spaces or not. The Python interpreter can figure out what you mean either way. (The space can make understanding quicker for a human reader.) Try in the Shell:

2 + 3 * 4

If you expected the last answer to be 20, think again: Python uses the normal precedence of arithmetic operations: Multiplications and divisions are done before addition and subtraction, unless there are parentheses. Try

2 * (4 - 1)

Now try the following in the Shell, exactly as written, followed by Enter, with no closing parenthesis:

5 * (2 + 3

Look carefully. There is no answer given at the left margin of the next line and no prompt >>> to start a new expression. If you are using Idle, the cursor has gone to the next line and has only indented slightly. Python is waiting for you to finish your expression. It is smart enough to know that opening parentheses are always followed by the same number of closing parentheses. The cursor is on a continuation line. Type just the matching close-parenthesis and Enter,


and you should finally see the expression evaluated. (In some versions of the Python interpreter, the interpreter puts ‘...’ at the beginning of a continuation line, rather than just indenting.)

Negation also works. Try in the Shell:

-(2 + 3)

1.4.3. Division and Remainders

If you think about it, you learned several ways to do division. Eventually you learned how to do division resulting in a decimal. Try in the Shell:


As you saw in the previous section, numbers with decimal points in them are of type float in Python. They are discussed more in Floats, Division, Mixed Types.

In early grade school you would likely say “14 divided by 4 is 3 with a remainder of 2”. The problem here is that the answer is in two parts, the integer quotient 3 and the remainder 2, and neither of these results is the same as the decimal result. Python has separate operations to generate each part. Python uses the doubled division symbol // for the operation that produces just the integer quotient, and introduces the symbol % for the operation of finding the remainder. Try each in the Shell:


Now predict and then try each of


Finding remainders will prove more useful than you might think in the future!

OPTIONAL from here to the end of the section, considering negative numbers in integer quotients and remainders:

In grade school you were probably always doing such remainder calculations with positive integers, as we have above. This is all we are liklely to need for this course. However, the remainder operation is defined for all integers, even a negative divisor. We will probably not need it in this course, but try

17 % -5
-17 % 5
-17 % -5

You can stop here and just beware mixing in negative numbers in your integer arithmetic, or go on to a fuller explanation:

Go back to positive numbers first.

17 divided by 5: 17//5 is 3; 17 % 5 is 2. The basic relationship is 17 = 5*3 + 2:

dividend = quotient * divisor + remainder

We want that to always be true.

The issue is really with integer division when the dividend and divisor are not both positive: the value of this determines the remainder, solving above:

remainder = dividend - quotient * divisor

With positive numbers the integer quotient is always no bigger than the real quotient. In its definition, Python requires that this is true, no matter what the signs of the operands :

17/(-5) is -3.2, so 17//(-5) must be -4 (not greater than -3.25),

By the general relationship between quotients and remainders, that forces

17 % (-5) = 17 - (-4)*(-5) = -3

Swapping the negative signs,

-17 / 5 is still -3.2 so

-17 // 5 is still -4,

but now look at the remainder formula:

-17%5 = -17 - (-4)*5 = 3

Last sign switch: both negative

-17 / (-5) is 3.2 so

-17 // (-5) is 3

Remainder formula:

-17% (-5) = -17 - (3)*(-5) = -2

Note the sign of nonzero remainders alway matches the sign of the divisor IN PYTHON.

Side note: Many other languages (Java, C++, ...) use a different definition for integer division, saying its MAGNITUDE is never larger than the real division. This forces the remainders for mixed sign divisions to be different than in Python. Example where this is significant: In Python you can identify an arbitrary odd number because it has remainder 1 when dividing by 2. This is true in Python whether the odd number is positive or negative, but it is NOT true in the other languages!