<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"

           "http://www.w3.org/TR/REC-html40/loose.dtd">

<META http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <META name="GENERATOR" content="hevea 1.10"> <LINK rel="stylesheet" type="text/css" href="book.css"> <TITLE>Variables, expressions and statements</TITLE> </HEAD> <BODY > <A HREF="book002.html"><IMG SRC="previous_motif.gif" ALT="Previous"></A> <A HREF="index.html"><IMG SRC="contents_motif.gif" ALT="Up"></A> <A HREF="book004.html"><IMG SRC="next_motif.gif" ALT="Next"></A>

<A NAME="htoc15">Chapter 2</A> Variables, expressions and statements

<A NAME="toc12"></A><A NAME="htoc16">2.1</A> Values and types

A value is one of the basic things a program works with,

like a letter or a number. The values we have seen so far are 1, 2, and

'Hello, World!'.

These values belong to different types:

2 is an integer, and 'Hello, World!' is a string, so-called because it contains a “string” of letters. You (and the interpreter) can identify

strings because they are enclosed in quotation marks.

The print statement also works for integers.

<FONT COLOR=blue><FONT SIZE=4>>>> print 4
4
</FONT></FONT>

If you are not sure what type a value has, the interpreter can tell you.

<FONT COLOR=blue><FONT SIZE=4>>>> type('Hello, World!')
<type 'str'>
>>> type(17)
<type 'int'>

</FONT></FONT>

Not surprisingly, strings belong to the type str and

integers belong to the type int. Less obviously, numbers with a decimal point belong to a type called float, because these numbers are represented in a

format called floating-point.

<FONT COLOR=blue><FONT SIZE=4>>>> type(3.2)
<type 'float'>
</FONT></FONT>

What about values like '17' and '3.2'?

They look like numbers, but they are in quotation marks like

strings.

<FONT COLOR=blue><FONT SIZE=4>>>> type('17')
<type 'str'>
>>> type('3.2')
<type 'str'>
</FONT></FONT>

They’re strings.

When you type a large integer, you might be tempted to use commas

between groups of three digits, as in 1,000,000. This is not a

legal integer in Python, but it is legal:

<FONT COLOR=blue><FONT SIZE=4>>>> print 1,000,000
1 0 0
</FONT></FONT>

Well, that’s not what we expected at all! Python interprets 1,000,000 as a comma-separated sequence of integers, which it prints with spaces between.

This is the first example we have seen of a semantic error: the code

runs without producing an error message, but it doesn’t do the

“right” thing.

<A NAME="toc13"></A><A NAME="htoc17">2.2</A> Variables

One of the most powerful features of a programming language is the

ability to manipulate variables. A variable is a name that

refers to a value.

An assignment statement creates new variables and gives them values:

<FONT COLOR=blue><FONT SIZE=4>>>> message = 'And now for something completely different'
>>> n = 17
>>> pi = 3.1415926535897931
</FONT></FONT>

This example makes three assignments. The first assigns a string

to a new variable named message; the second gives the integer 17 to n; the third

assigns the (approximate) value of π to pi.

A common way to represent variables on paper is to write the name with

an arrow pointing to the variable’s value. This kind of figure is called a state diagram because it shows what state each of the variables is in (think of it as the variable’s state of mind).

This diagram shows the result of the previous example:

To display the value of a variable, you can use a print statement:

<FONT COLOR=blue><FONT SIZE=4>>>> print n
17
>>> print pi
3.14159265359
</FONT></FONT>

The type of a variable is the type of the value it refers to.

<FONT COLOR=blue><FONT SIZE=4>>>> type(message)
<type 'str'>
>>> type(n)
<type 'int'>
>>> type(pi)
<type 'float'>

</FONT></FONT>

Exercise 1
If you type an integer with a leading zero, you might get
a confusing error:

<EM><FONT COLOR=blue><FONT SIZE=4>>>> zipcode = 02492
                  ^
SyntaxError: invalid token
</FONT></FONT></EM>

Other number seem to work, but the results are bizarre:

<EM><EM><FONT COLOR=blue><FONT SIZE=4>>>> zipcode = 02132
>>> print zipcode
1114

</FONT></FONT></EM></EM>

Can you figure out what is going on? Hint: print the values 01, 010, 0100 and 01000.

<A NAME="toc14"></A><A NAME="htoc18">2.3</A> Variable names and keywords

Programmers generally choose names for their variables that are meaningful—they document what the variable is used for.

Variable names can be arbitrarily long. They can contain

both letters and numbers, but they have to begin with a letter. It is legal to use uppercase letters, but it is a good idea to begin variable names with a lowercase letter (you’ll

see why later).

The underscore character (_) can appear in a name.

It is often used in names with multiple words, such as

my_name or airspeed_of_unladen_swallow.

If you give a variable an illegal name, you get a syntax error:

<FONT COLOR=blue><FONT SIZE=4>>>> 76trombones = 'big parade'
SyntaxError: invalid syntax
>>> more@ = 1000000
SyntaxError: invalid syntax
>>> class = 'Advanced Theoretical Zymurgy'
SyntaxError: invalid syntax
</FONT></FONT>

76trombones is illegal because it does not begin with a letter. more@ is illegal because it contains an illegal character, @. But what’s wrong with class?

It turns out that class is one of Python’s keywords. The

interpreter uses keywords to recognize the structure of the program,

and they cannot be used as variable names.

Python has 31 keywords:

<FONT COLOR=blue><FONT SIZE=4>and       del       from      not       while    
as        elif      global    or        with     
assert    else      if        pass      yield    
break     except    import    print              
class     exec      in        raise              
continue  finally   is        return             
def       for       lambda    try
</FONT></FONT>

You might want to keep this list handy. If the interpreter complains

about one of your variable names and you don’t know why, see if it

is on this list.

<A NAME="toc15"></A><A NAME="htoc19">2.4</A> Statements

A statement is a unit of code that the Python interpreter can

execute. We have seen two kinds of statements: print

and assignment.

When you type a statement in interactive mode, the interpreter executes it and displays the result, if there is one.

A script usually contains a sequence of statements. If there

is more than one statement, the results appear one at a time

as the statements execute.

For example, the script

<FONT COLOR=blue><FONT SIZE=4>print 1
x = 2
print x
</FONT></FONT>

produces the output

<FONT COLOR=blue><FONT SIZE=4>1
2

</FONT></FONT>

The assignment statement produces no output.

<A NAME="toc16"></A><A NAME="htoc20">2.5</A> Operators and operands

Operators are special symbols that represent computations like

addition and multiplication. The values the operator is applied to

are called operands.

The operators +, -, *, / and **

perform addition, subtraction, multiplication, division and

exponentiation, as in the following examples:

<FONT COLOR=blue><FONT SIZE=4>20+32   hour-1   hour*60+minute   minute/60   5**2   (5+9)*(15-7)
</FONT></FONT>

In some other languages, ^ is used for exponentiation, but

in Python it is a bitwise operator called XOR. I won’t cover bitwise operators in this book, but you can read about

them at wiki.python.org/moin/BitwiseOperators.

The division operator might not do what you expect:

<FONT COLOR=blue><FONT SIZE=4>>>> minute = 59
>>> minute/60
0
</FONT></FONT>

The value of minute is 59, and in conventional arithmetic 59

divided by 60 is 0.98333, not 0. The reason for the discrepancy is

that Python is performing floor division^{<A NAME="text4" HREF="#note4">1</A>}.

When both of the operands are integers, the result is also an

integer; floor division chops off the fraction

part, so in this example it rounds down to zero.

If either of the operands is a floating-point number, Python performs floating-point division, and the result is a float:

<FONT COLOR=blue><FONT SIZE=4>>>> minute/60.0
0.98333333333333328
</FONT></FONT>

<A NAME="toc17"></A><A NAME="htoc21">2.6</A> Expressions

An expression is a combination of values, variables, and operators.

A value all by itself is considered an expression, and so is a variable, so the following are all legal expressions

(assuming that the variable x has been assigned a value):

<FONT COLOR=blue><FONT SIZE=4>17
x
x + 17
</FONT></FONT>

If you type an expression in interactive mode, the interpreter evaluates it and displays the result:

<FONT COLOR=blue><FONT SIZE=4>>>> 1 + 1
2
</FONT></FONT>

But in a script, an expression all by itself doesn’t

do anything! This is a common

source of confusion for beginners.

Exercise 2
Type the following statements in the Python interpreter to see
what they do:

<EM><FONT COLOR=blue><FONT SIZE=4>5
x = 5
x + 1
</FONT></FONT></EM>

Now put the same statements into a script and run it. What

is the output? Modify the script by transforming each expression into a print statement and then run it again.

<A NAME="toc18"></A><A NAME="htoc22">2.7</A> Order of operations

When more than one operator appears in an expression, the order of

evaluation depends on the rules of precedence. For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to

remember the rules:

Parentheses have the highest precedence and can be used
to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even
if it doesn’t change the result.
Exponentiation has the next highest precedence, so 2**1+1 is 3, not 4, and 3*1**3 is 3, not 27.
Multiplication and Division have the same precedence, which is higher than Addition and Subtraction, which also have the same precedence. So 2*3-1 is 5, not 4, and 6+4/2 is 8, not 5.
Operators with the same precedence are evaluated from left to right. So in the expression degrees / 2 * pi, the division happens first and the result is multiplied by pi. To divide by 2 π, you can reorder the operands or use parentheses.

<A NAME="toc19"></A><A NAME="htoc23">2.8</A> String operations

In general, you cannot perform mathematical operations on strings, even if the strings look like numbers, so the following are illegal:

<FONT COLOR=blue><FONT SIZE=4>'2'-'1'    'eggs'/'easy'    'third'*'a charm'
</FONT></FONT>

The + operator works with strings, but it

might not do what you expect: it performs concatenation, which means joining the strings by

linking them end-to-end. For example:

<FONT COLOR=blue><FONT SIZE=4>first = 'throat'
second = 'warbler'
print first + second
</FONT></FONT>

The output of this program is throatwarbler.

The * operator also works on strings; it performs repetition.

For example, ’Spam’*3 is 'SpamSpamSpam'. If one of the operands

is a string, the other has to be an integer.

This use of + and * makes sense by

analogy with addition and multiplication. Just as 4*3 is equivalent to 4+4+4, we expect 'Spam'*3 to be the same as 'Spam'+'Spam'+'Spam', and it is. On the other hand, there is a significant way in which string concatenation and repetition are different from integer addition and multiplication. Can you think of a property that addition has

that string concatenation does not?

<A NAME="toc20"></A><A NAME="htoc24">2.9</A> Comments

As programs get bigger and more complicated, they get more difficult

to read. Formal languages are dense, and it is often difficult to

look at a piece of code and figure out what it is doing, or why.

For this reason, it is a good idea to add notes to your programs to explain

in natural language what the program is doing. These notes are called

comments, and they start with the # symbol:

<FONT COLOR=blue><FONT SIZE=4># compute the percentage of the hour that has elapsed
percentage = (minute * 100) / 60
</FONT></FONT>

In this case, the comment appears on a line by itself. You can also put comments at the end of a line:

<FONT COLOR=blue><FONT SIZE=4>percentage = (minute * 100) / 60     # percentage of an hour
</FONT></FONT>

Everything from the # to the end of the line is ignored—it has no effect on the program.

Comments are most useful when they document non-obvious features of

the code. It is reasonable to assume that the reader can figure out

what the code does; it is much more useful to explain why.

This comment is redundant with the code and useless:

<FONT COLOR=blue><FONT SIZE=4>v = 5     # assign 5 to v
</FONT></FONT>

This comment contains useful information that is not in the code:

<FONT COLOR=blue><FONT SIZE=4>v = 5     # velocity in meters/second. 
</FONT></FONT>

Good variable names can reduce the need for comments, but

long names can make complex expressions hard to read, so there is

a tradeoff.

<A NAME="toc21"></A><A NAME="htoc25">2.10</A> Debugging

At this point the syntax error you are most likely to make is

an illegal variable name, like class and yield, which are keywords, or odd~job and US$, which contain

illegal characters.

If you put a space in a variable name, Python thinks it is two operands without an operator:

<FONT COLOR=blue><FONT SIZE=4>>>> bad name = 5
SyntaxError: invalid syntax
</FONT></FONT>

For syntax errors, the error messages don’t help much.

The most common messages are SyntaxError: invalid syntax and

SyntaxError: invalid token, neither of which is very informative.

The runtime error you are most likely to make is a “use before

def;” that is, trying to use a variable before you have assigned

a value. This can happen if you spell a variable name wrong:

<FONT COLOR=blue><FONT SIZE=4>>>> principal = 327.68
>>> interest = principle * rate
NameError: name 'principle' is not defined
</FONT></FONT>

Variables names are case sensitive, so LaTeX is not the same as latex.

At this point the most likely cause of a semantic error is

the order of operations. For example, to evaluate 1/2 π,

you might be tempted to write

<FONT COLOR=blue><FONT SIZE=4>>>> 1.0 / 2.0 * pi
</FONT></FONT>

But the division happens first, so you would get π / 2, which

is not the same thing! There is no way for Python to know what you meant to write, so in this case you don’t

get an error message; you just get the wrong answer.

<A NAME="toc22"></A><A NAME="htoc26">2.11</A> Glossary

value:: One of the basic units of data, like a number or string,
that a program manipulates.
<A NAME="@default165"></A>
type:: A category of values. The types we have seen so far are integers (type int), floating-point numbers (type float), and strings (type str). <A NAME="@default166"></A>
integer:: A type that represents whole numbers. <A NAME="@default167"></A>
floating-point:: A type that represents numbers with fractional parts. <A NAME="@default168"></A>
string:: A type that represents sequences of characters. <A NAME="@default169"></A>
variable:: A name that refers to a value. <A NAME="@default170"></A>
statement:: A section of code that represents a command or action. So far, the statements we have seen are assignments and print statements. <A NAME="@default171"></A>
assignment:: A statement that assigns a value to a variable. <A NAME="@default172"></A>
state diagram:: A graphical representation of a set of variables and the values they refer to. <A NAME="@default173"></A>
keyword:: A reserved word that is used by the compiler to parse a program; you cannot use keywords like if, def, and while as variable names. <A NAME="@default174"></A>
operator:: A special symbol that represents a simple computation like addition, multiplication, or string concatenation. <A NAME="@default175"></A>
operand:: One of the values on which an operator operates. <A NAME="@default176"></A>
floor division:: The operation that divides two numbers and chops off the fraction part. <A NAME="@default177"></A>
expression:: A combination of variables, operators, and values that represents a single result value. <A NAME="@default178"></A>
evaluate:: To simplify an expression by performing the operations in order to yield a single value.
rules of precedence:: The set of rules governing the order in which expressions involving multiple operators and operands are evaluated. <A NAME="@default179"></A> <A NAME="@default180"></A>
concatenate:: To join two operands end-to-end. <A NAME="@default181"></A>
comment:: Information in a program that is meant for other programmers (or anyone reading the source code) and has no effect on the execution of the program. <A NAME="@default182"></A>

<A NAME="toc23"></A><A NAME="htoc27">2.12</A> Exercises

Exercise 3 Assume that we execute the following assignment statements:

<FONT COLOR=black><FONT SIZE=3><EM>width = 17
height = 12.0
delimiter = '.'
</EM></FONT></FONT>

For each of the following expressions, write the value of the expression and the type (of the value of the expression).

width/2
width/2.0
height/3
1 + 2 * 5
delimiter * 5

Use the Python interpreter to check your answers.

Exercise 4
Practice using the Python interpreter as a calculator:
<A NAME="@default183"></A>

The volume of a sphere with radius r is 4/3 π r³. What is the volume of a sphere with radius 5? Hint: 392.6 is wrong!
Suppose the cover price of a book is $24.95, but bookstores get a 40% discount. Shipping costs $3 for the first copy and 75 cents for each additional copy. What is the total wholesale cost for 60 copies?
If I leave my house at 6:52 am and run 1 mile at an easy pace (8:15 per mile), then 3 miles at tempo (7:12 per mile) and 1 mile at easy pace again, what time do I get home for breakfast?
<A NAME="@default184"></A>

<A NAME="note4" HREF="#text4">1</A>: In Python 3.0, the result of this division is a float. The new operator // performs integer division.