,Chapter 1. Programming as a Way of
Thinking
The first goal of this book is to teach you how to program in Python. But learning to program
means learning a new way to think, so the second goal of this book is to help you think like a
computer scientist. This way of thinking combines some of the best features of mathematics,
engineering, and natural science. Like mathematicians, computer scientists use formal languages
to denote ideas—specifically computations. Like engineers, they design things, assembling
components into systems and evaluating trade-offs among alternatives. Like scientists, they
observe the behavior of complex systems, form hypotheses, and test predictions.
We will start with the most basic elements of programming and work our way up. In this chapter,
we’ll see how Python represents numbers, letters, and words. And you’ll learn to perform
arithmetic operations.
You will also start to learn the vocabulary of programming, including terms like operator,
expression, value, and type. This vocabulary is important—you will need it to understand the rest
of the book, to communicate with other programmers, and to use and understand virtual assistants.
Arithmetic Operators
An arithmetic operator is a symbol that represents an arithmetic computation. For example, the
plus sign, +, performs addition:
30 + 12
42
The minus sign, –, is the operator that performs subtraction:
43 - 1
42
The asterisk, *, performs multiplication:
6 * 7
,42
And the forward slash, /, performs division:
42.0
Notice that the result of the division is 42.0 rather than 42. That’s because there are two types of
numbers in Python:
integers, which represent whole numbers, and
floating-point numbers, which represent numbers with a decimal point.
If you add, subtract, or multiply two integers, the result is an integer. But if you divide two integers,
the result is a floating-point number. Python provides another operator, //, that performs integer
division. The result of integer division is always an integer:
84 // 2
42
Integer division is also called “floor division” because it always rounds down (toward the “floor”):
85 // 2
42
Finally, the operator ** performs exponentiation; that is, it raises a number to a power:
7 ** 2
49
In some other languages, the caret, ^, is used for exponentiation, but in Python it is a bitwise
operator called XOR. If you are not familiar with bitwise operators, the result might be unexpected:
, 7 ^ 2
5
I won’t cover bitwise operators in this book, but you can read about them
at http://wiki.python.org/moin/BitwiseOperators.
Expressions
A collection of operators and numbers is called an expression. An expression can contain any
number of operators and numbers. For example, here’s an expression that contains two operators:
6 + 6 ** 2
42
Notice that exponentiation happens before addition. Python follows the order of operations you
might have learned in a math class: exponentiation happens before multiplication and division,
which happen before addition and subtraction.
In the following example, multiplication happens before addition:
12 + 5 * 6
42
If you want the addition to happen first, you can use parentheses:
(12 + 5) * 6
102
Every expression has a value. For example, the expression 6 * 7 has the value 42.
Arithmetic Functions
Thinking
The first goal of this book is to teach you how to program in Python. But learning to program
means learning a new way to think, so the second goal of this book is to help you think like a
computer scientist. This way of thinking combines some of the best features of mathematics,
engineering, and natural science. Like mathematicians, computer scientists use formal languages
to denote ideas—specifically computations. Like engineers, they design things, assembling
components into systems and evaluating trade-offs among alternatives. Like scientists, they
observe the behavior of complex systems, form hypotheses, and test predictions.
We will start with the most basic elements of programming and work our way up. In this chapter,
we’ll see how Python represents numbers, letters, and words. And you’ll learn to perform
arithmetic operations.
You will also start to learn the vocabulary of programming, including terms like operator,
expression, value, and type. This vocabulary is important—you will need it to understand the rest
of the book, to communicate with other programmers, and to use and understand virtual assistants.
Arithmetic Operators
An arithmetic operator is a symbol that represents an arithmetic computation. For example, the
plus sign, +, performs addition:
30 + 12
42
The minus sign, –, is the operator that performs subtraction:
43 - 1
42
The asterisk, *, performs multiplication:
6 * 7
,42
And the forward slash, /, performs division:
42.0
Notice that the result of the division is 42.0 rather than 42. That’s because there are two types of
numbers in Python:
integers, which represent whole numbers, and
floating-point numbers, which represent numbers with a decimal point.
If you add, subtract, or multiply two integers, the result is an integer. But if you divide two integers,
the result is a floating-point number. Python provides another operator, //, that performs integer
division. The result of integer division is always an integer:
84 // 2
42
Integer division is also called “floor division” because it always rounds down (toward the “floor”):
85 // 2
42
Finally, the operator ** performs exponentiation; that is, it raises a number to a power:
7 ** 2
49
In some other languages, the caret, ^, is used for exponentiation, but in Python it is a bitwise
operator called XOR. If you are not familiar with bitwise operators, the result might be unexpected:
, 7 ^ 2
5
I won’t cover bitwise operators in this book, but you can read about them
at http://wiki.python.org/moin/BitwiseOperators.
Expressions
A collection of operators and numbers is called an expression. An expression can contain any
number of operators and numbers. For example, here’s an expression that contains two operators:
6 + 6 ** 2
42
Notice that exponentiation happens before addition. Python follows the order of operations you
might have learned in a math class: exponentiation happens before multiplication and division,
which happen before addition and subtraction.
In the following example, multiplication happens before addition:
12 + 5 * 6
42
If you want the addition to happen first, you can use parentheses:
(12 + 5) * 6
102
Every expression has a value. For example, the expression 6 * 7 has the value 42.
Arithmetic Functions
