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<!DOCTYPE html>

<html>

<head>

<title>Conditional execution</title>

</head>

<body>

<hr />

<h1 class="chapter" id="sec33">Chapter&#XA0;3&#XA0;&#XA0;Conditional execution</h1>

<h2 class="section" id="sec34">3.1&#XA0;&#XA0;Boolean expressions</h2>

<a id="hevea_default122"></a>A boolean expression is an expression that is either true

or false. The following examples use the

operator ==, which compares two operands and produces

True if they are equal and False otherwise:<pre class="verbatim">>>> 5 == 5

True

>>> 5 == 6

False

</pre>True and False are special

values that belong to the type bool; they are not strings:<a id="hevea_default123"></a>

<a id="hevea_default128"></a><pre class="verbatim">>>> type(True)

>>> type(False)

</pre>The == operator is one of the comparison operators; the

others are:<pre class="verbatim"> x != y # x is not equal to y

x > y # x is greater than y

x < y # x is less than y

x >= y # x is greater than or equal to y

x <= y # x is less than or equal to y

x is y # x is the same as y

x is not y # x is not the same as y

</pre>Although these operations are probably familiar to you, the Python

symbols are different from the mathematical symbols. A common error

is to use a single equal sign (=) instead of a double equal sign

(==). Remember that = is an assignment operator and

== is a comparison operator. There is no such thing as

=< or =>.<a id="hevea_default129"></a>

<h2 class="section" id="sec35">3.2&#XA0;&#XA0;Logical operators</h2>

<a id="hevea_default132"></a>There are three logical operators: and, or, and not. The semantics (meaning) of these operators is

similar to their meaning in English. For example,x > 0 and x < 10 is true only if x is greater than 0

and less than 10.<a id="hevea_default133"></a>

<a id="hevea_default138"></a>n%2 == 0 or n%3 == 0 is true if either of the conditions

is true, that is, if the number is divisible by 2 or 3.Finally, the not operator negates a boolean

expression, so not (x > y) is true if x > y is false,

that is, if x is less than or equal to y.Strictly speaking, the operands of the logical operators should be

boolean expressions, but Python is not very strict.

Any nonzero number is interpreted as &#X201C;true.&#X201D;<pre class="verbatim">>>> 17 and True

True

</pre>This flexibility can be useful, but there are some subtleties to

it that might be confusing. You might want to avoid it (unless

you know what you are doing).

<h2 class="section" id="sec36">3.3&#XA0;&#XA0;Conditional execution</h2>

<a id="hevea_default143"></a>In order to write useful programs, we almost always need the ability

to check conditions and change the behavior of the program

accordingly. Conditional statements give us this ability. The

simplest form is the if statement:<pre class="verbatim">if x > 0 :

print 'x is positive'

</pre>The boolean expression after the if statement is

called the condition. We end the if

statement with a colon character (:) and the line(s)

after the if statement are indented. <div class="center"><img src="book005.png" /></div>If the logical condition is true, then the indented

statement gets executed. If the logical condition is

false, the indented statement is skipped.<a id="hevea_default144"></a>

<a id="hevea_default146"></a>if statements have the same structure as function definitions

or for loops. The statement consists of a header line

that ends with the colon character (:)

followed by an indented block. Statements like this are

called compound statements because they stretch

across more than one line.There is no limit on the number of statements that can appear in

the body, but there has to be at least one.

Occasionally, it is useful to have a body with no statements (usually

as a place keeper for code you haven&#X2019;t written yet). In that

case, you can use the pass statement, which does nothing.<a id="hevea_default147"></a>

<a id="hevea_default148"></a><pre class="verbatim">if x < 0 :

pass # need to handle negative values!

</pre>If you enter an if statement in the Python interpreter, the prompt will change

from three chevrons to three dots to indicate you are in the middle of a block of

statements as shown below:<pre class="verbatim">>>> x = 3

>>> if x < 10:

... print 'Small'

...

Small

>>>

</pre>

<h2 class="section" id="sec37">3.4&#XA0;&#XA0;Alternative execution</h2>

<a id="hevea_default151"></a>A second form of the if statement is alternative execution,

in which there are two possibilities and the condition determines

which one gets executed. The syntax looks like this:<pre class="verbatim">if x%2 == 0 :

print 'x is even'

else :

print 'x is odd'

</pre>If the remainder when x is divided by 2 is 0, then we

know that x is even, and the program displays a message to that

effect. If the condition is false, the second set of statements is

executed. <div class="center"><img src="book006.png" /></div>Since the condition must be true or false, exactly one of

the alternatives will be executed. The alternatives are called

branches, because they are branches in the flow of execution.<a id="hevea_default152"></a>

<h2 class="section" id="sec38">3.5&#XA0;&#XA0;Chained conditionals</h2>

<a id="hevea_default154"></a>Sometimes there are more than two possibilities and we need more than

two branches. One way to express a computation like that is a chained conditional:<pre class="verbatim">if x < y:

print 'x is less than y'

elif x > y:

print 'x is greater than y'

else:

print 'x and y are equal'

</pre>elif is an abbreviation of &#X201C;else if.&#X201D; Again, exactly one

branch will be executed. <div class="center"><img src="book007.png" /></div>There is no limit on the number of elif statements. If there is an else clause, it has to be

at the end, but there doesn&#X2019;t have to be one.<a id="hevea_default155"></a>

<a id="hevea_default156"></a><pre class="verbatim">if choice == 'a':

print 'Bad guess'

elif choice == 'b':

print 'Good guess'

elif choice == 'c':

print 'Close, but not correct'

</pre>Each condition is checked in order. If the first is false,

the next is checked, and so on. If one of them is

true, the corresponding branch executes, and the statement

ends. Even if more than one condition is true, only the

first true branch executes.

<h2 class="section" id="sec39">3.6&#XA0;&#XA0;Nested conditionals</h2>

<a id="hevea_default158"></a>One conditional can also be nested within another. We could have

written the trichotomy example like this:<pre class="verbatim">if x == y:

print 'x and y are equal'

else:

if x < y:

print 'x is less than y'

else:

print 'x is greater than y'

</pre>The outer conditional contains two branches. The

first branch contains a simple statement. The second branch

contains another if statement, which has two branches of its

own. Those two branches are both simple statements,

although they could have been conditional statements as well.<div class="center"><img src="book008.png" /></div>Although the indentation of the statements makes the structure

apparent, nested conditionals become difficult to read very

quickly. In general, it is a good idea to avoid them when you can.Logical operators often provide a way to simplify nested conditional

statements. For example, we can rewrite the following code using a

single conditional:<pre class="verbatim">if 0 < x:

if x < 10:

print 'x is a positive single-digit number.'

</pre>The print statement is executed only if we make it past both

conditionals, so we can get the same effect with the and operator:<pre class="verbatim">if 0 < x and x < 10:

print 'x is a positive single-digit number.'

</pre>

<h2 class="section" id="sec40">3.7&#XA0;&#XA0;Catching exceptions using try and except</h2>

<a id="catch1"></a>Earlier we saw a code segment where we used the <code>raw_input</code> and

int functions to read and parse an integer number entered by

the user. We also saw how treacherous doing this could be:<pre class="verbatim">>>> speed = raw_input(prompt)

What...is the airspeed velocity of an unladen swallow?

What do you mean, an African or a European swallow?

>>> int(speed)

ValueError: invalid literal for int()

>>>

</pre>When we are executing these statements in the Python interpreter,

we get a new prompt from the interpreter, think &#X201C;oops&#X201D; and move

on to our next statement. However if this code is placed in a

Python script and this error occurs, your script immediately

stops in its tracks with a traceback.

It does not execute the following statement.

<a id="hevea_default159"></a>Here is a sample program to convert a Fahrenheit temperature

to a Celsius temperature:

<a id="hevea_default162"></a><pre class="verbatim">inp = raw_input('Enter Fahrenheit Temperature:')

fahr = float(inp)

cel = (fahr - 32.0) * 5.0 / 9.0

print cel

</pre>If we execute this code and give it invalid input, it simply fails

with an unfriendly error message:<pre class="verbatim">python fahren.py

Enter Fahrenheit Temperature:72

22.2222222222

python fahren.py

Enter Fahrenheit Temperature:fred

Traceback (most recent call last):

File "fahren.py", line 2, in <module>

fahr = float(inp)

ValueError: invalid literal for float(): fred

</pre>There is a conditional execution structure built into

Python to handle these types of expected and unexpected

errors called &#X201C;try / except&#X201D;. The idea of try

and except is that you know that some sequence

of instruction(s) may have a problem and you want to

add some statements to be executed if an error occurs.

These extra statements (the except block) are ignored

if there is no error.You can think of the try and except feature

in Python as an &#X201C;insurance policy&#X201D; on a sequence of

statements.We can rewrite our temperature converter as follows:<pre class="verbatim">inp = raw_input('Enter Fahrenheit Temperature:')

try:

fahr = float(inp)

cel = (fahr - 32.0) * 5.0 / 9.0

print cel

except:

print 'Please enter a number'

</pre>Python starts by executing the

sequence of statements in the

try block. If all goes

well, it skips the except block and proceeds. If an

exception occurs in the try block,

Python jumps out of the try block and

executes the sequence of statements in the except block.<pre class="verbatim">python fahren2.py

Enter Fahrenheit Temperature:72

22.2222222222

python fahren2.py

Enter Fahrenheit Temperature:fred

Please enter a number

</pre>Handling an exception with a try statement is called catching an exception. In this example, the except clause

prints an error message. In general,

catching an exception gives you a chance to fix the problem, or try

again, or at least end the program gracefully.

<h2 class="section" id="sec41">3.8&#XA0;&#XA0;Short circuit evaluation of logical expressions</h2>

<a id="hevea_default163"></a>When Python is processing a logical expression such as

x >= 2 and (x/y) > 2, it evaluates the expression

from left-to-right. Because of the definition of and,

if x is less than 2, the expression x >= 2 is

False and so the whole expression is False regardless

of whether (x/y) > 2 evaluates to True or False.When Python detects that there is nothing to be gained by evaluating

the rest of a logical expression, it stops its evaluation and does

not do the computations in the rest of the logical expression.

When the evaluation of a logical expression stops because the overall

value is already known, it is called short-circuiting

the evaluation.<a id="hevea_default164"></a>

While this may seem like a fine point, the short circuit behavior

leads to a clever technique called the guardian pattern.

Consider the following code sequence in the Python interpreter:<pre class="verbatim">>>> x = 6

>>> y = 2

>>> x >= 2 and (x/y) > 2

True

>>> x = 1

>>> y = 0

>>> x >= 2 and (x/y) > 2

False

>>> x = 6

>>> y = 0

>>> x >= 2 and (x/y) > 2

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ZeroDivisionError: integer division or modulo by zero

>>>

</pre>The third calculation failed because Python was evaluating (x/y)

and y was zero which causes a runtime error. But the second example

did not fail because the first part of the expression x >= 2

evaluated to False so the (x/y) was not ever executed

due to the short circuit rule and there was no error.We can construct the logical expression to strategically place a guard

evaluation just before the evaluation that might cause an error as follows:<pre class="verbatim">>>> x = 1

>>> y = 0

>>> x >= 2 and y != 0 and (x/y) > 2

False

>>> x = 6

>>> y = 0

>>> x >= 2 and y != 0 and (x/y) > 2

False

>>> x >= 2 and (x/y) > 2 and y != 0

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ZeroDivisionError: integer division or modulo by zero

>>>

</pre>In the first logical expression, x >= 2 is False so the evaluation

stops at the and. In the second logical expression x >= 2 is True

but y != 0 is False so we never reach (x/y).In the third logical expression, the y != 0 is after the

(x/y) calculation so the expression fails with an error.In the second expression, we say that y != 0 acts as a guard

to insure that we only execute (x/y) if y is non-zero.

<h2 class="section" id="sec42">3.9&#XA0;&#XA0;Debugging</h2>

<a id="hevea_default167"></a>The traceback Python displays when an error occurs contains

a lot of information, but it can be overwhelming, especially

when there are many frames on the stack. The most

useful parts are usually:<ul class="itemize"><li class="li-itemize">What kind of error it was, and</li><li class="li-itemize">Where it occurred.</li></ul>Syntax errors are usually easy to find, but there are a few

gotchas. Whitespace errors can be tricky because spaces and

tabs are invisible and we are used to ignoring them.<a id="hevea_default168"></a><pre class="verbatim">>>> x = 5

>>> y = 6

File "<stdin>", line 1

y = 6

SyntaxError: invalid syntax

</pre>In this example, the problem is that the second line is indented by

one space. But the error message points to y, which is

misleading. In general, error messages indicate where the problem was

discovered, but the actual error might be earlier in the code,

sometimes on a previous line.<a id="hevea_default169"></a>

<a id="hevea_default170"></a>The same is true of runtime errors. Suppose you are trying

to compute a signal-to-noise ratio in decibels. The formula

is SNRdb = 10 log10 (Psignal / Pnoise). In Python,

you might write something like this:<pre class="verbatim">import math

signal_power = 9

noise_power = 10

ratio = signal_power / noise_power

decibels = 10 * math.log10(ratio)

print decibels

</pre>But when you run it, you get an error message<a id="text7" href="#note7">1</a>:<a id="hevea_default171"></a>

<a id="hevea_default172"></a><pre class="verbatim">Traceback (most recent call last):

File "snr.py", line 5, in ?

decibels = 10 * math.log10(ratio)

OverflowError: math range error

</pre>The error message indicates line 5, but there is nothing

wrong with that line. To find the real error, it might be

useful to print the value of ratio, which turns out to

be 0. The problem is in line 4, because dividing two integers

does floor division. The solution is to represent signal power

and noise power with floating-point values.<a id="hevea_default173"></a>

<a id="hevea_default174"></a>In general, error messages tell you where the problem was discovered,

but that is often not where it was caused.

<h2 class="section" id="sec43">3.10&#XA0;&#XA0;Glossary</h2>

<dl class="description"><dt class="dt-description">body:</dt><dd class="dd-description"> The sequence of statements within a compound statement.

<a id="hevea_default175"></a></dd><dt class="dt-description">boolean expression:</dt><dd class="dd-description"> An expression whose value is either

True or False.

<a id="hevea_default177"></a></dd><dt class="dt-description">branch:</dt><dd class="dd-description"> One of the alternative sequences of statements in

a conditional statement.

<a id="hevea_default178"></a></dd><dt class="dt-description">chained conditional:</dt><dd class="dd-description"> A conditional statement with a series

of alternative branches.

<a id="hevea_default180"></a></dd><dt class="dt-description">comparison operator:</dt><dd class="dd-description"> One of the operators that compares

its operands: ==, !=, >, <, >=, and <=.</dd><dt class="dt-description">conditional statement:</dt><dd class="dd-description"> A statement that controls the flow of

execution depending on some condition.

<a id="hevea_default182"></a></dd><dt class="dt-description">condition:</dt><dd class="dd-description"> The boolean expression in a conditional statement

that determines which branch is executed.

<a id="hevea_default183"></a></dd><dt class="dt-description">compound statement:</dt><dd class="dd-description"> A statement that consists of a header

and a body. The header ends with a colon (:). The body is indented

relative to the header.

<a id="hevea_default184"></a></dd><dt class="dt-description">guardian pattern:</dt><dd class="dd-description"> Where we construct a logical expression

with additional

comparisons to take advantage of the short circuit behavior.

<a id="hevea_default186"></a></dd><dt class="dt-description">logical operator:</dt><dd class="dd-description"> One of the operators that combines boolean

expressions: and, or, and not.</dd><dt class="dt-description">nested conditional:</dt><dd class="dd-description"> A conditional statement that appears

in one of the branches of another conditional statement.

<a id="hevea_default188"></a></dd><dt class="dt-description">traceback:</dt><dd class="dd-description"> A list of the functions that are executing,

printed when an exception occurs.

<a id="hevea_default189"></a></dd><dt class="dt-description">short circuit:</dt><dd class="dd-description"> When Python is part-way through evaluating a

logical expression and stops the evaluation because Python

knows the final value for the expression

without needing to evaluate the rest of the expression.

<h2 class="section" id="sec44">3.11&#XA0;&#XA0;Exercises</h2>

<div class="theorem">Exercise&#XA0;1&#XA0;&#XA0;

Rewrite your pay computation to give the employee 1.5

times the hourly rate for

hours worked above 40 hours.<pre class="verbatim">Enter Hours: 45

Enter Rate: 10

Pay: 475.0

</pre></div><div class="theorem">Exercise&#XA0;2&#XA0;&#XA0;

Rewrite your pay program using try and except

so that your program handles non-numeric input gracefully

by printing a message and exiting the program.

The following shows two executions of the program:<pre class="verbatim">Enter Hours: 20

Enter Rate: nine

Error, please enter numeric input

Enter Hours: forty

Error, please enter numeric input

</pre></div><div class="theorem">Exercise&#XA0;3&#XA0;&#XA0;

Write a program to prompt for a score between 0.0 and 1.0.

If the score is out of range print an error. If the score

is between 0.0 and 1.0, print a grade using the following

table:<pre class="verbatim">Score Grade

>= 0.9 A

>= 0.8 B

>= 0.7 C

>= 0.6 D

< 0.6 F

Enter score: 0.95

Enter score: perfect

Bad score

Enter score: 10.0

Bad score

Enter score: 0.75

Enter score: 0.5

</pre>Run the program repeatedly as shown above to test the

various different values for input.

</div>

<a id="note7" href="#text7">1</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">In Python 3.0,

you no longer get an error message; the division operator performs

floating-point division even with integer operands.</div>

</dd></dl>

<hr />

</body>

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