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Conditional execution

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<h1><font color="black"><a name="htoc32">Chapter&nbsp;3</a>&nbsp;&nbsp;Conditional execution</font></h1>

<a name="toc31"></a>

<h2><font color="black"><a name="htoc33">3.1</a>&nbsp;&nbsp;Boolean expressions</font></h2>

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<font color="black">A <b>boolean expression</b> is an expression that is either true

or false. The following examples use the

operator <tt>==</tt>, which compares two operands and produces

<tt>True</tt> if they are equal and <tt>False</tt> otherwise:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; 5 == 5

True

&gt;&gt;&gt; 5 == 6

False

</font></pre><font color="black"><tt>True</tt> and <tt>False</tt> are special

values that belong to the type <tt>bool</tt>; they are not strings:</font><br />

<br />

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<pre><font size="4" color="blue">

&gt;&gt;&gt; type(True)

&lt;type 'bool'&gt;

&gt;&gt;&gt; type(False)

&lt;type 'bool'&gt;

</font></pre><font color="black">The <tt>==</tt> operator is one of the <b>comparison operators</b>; the

others are:

</font><pre><font size="4" color="blue">

x != y # x is not equal to y

x &gt; y # x is greater than y

x &lt; y # x is less than y

x &gt;= y # x is greater than or equal to y

x &lt;= 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

</font></pre><font color="black">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 (<tt>=</tt>) instead of a double equal sign

(<tt>==</tt>). Remember that <tt>=</tt> is an assignment operator and

<tt>==</tt> is a comparison operator. There is no such thing as

<tt>=&lt;</tt> or <tt>=&gt;</tt>.</font><br />

<br />

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<br />

<a name="toc32"></a>

<h2><font color="black"><a name="htoc34">3.2</a>&nbsp;&nbsp;Logical operators</font></h2>

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<font color="black">There are three <b>logical operators</b>: <tt>and</tt>, <tt>or</tt>, and <tt>not</tt>. The semantics (meaning) of these operators is

similar to their meaning in English. For example,<br />

<br />

<tt>x &gt; 0 and x &lt; 10</tt> <br />

<br />

is true only if <tt>x</tt> is greater than 0

<em>and</em> less than 10.<br />

<br />

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<br />

<tt>n%2 == 0 or n%3 == 0</tt> is true if <em>either</em> of the conditions

is true, that is, if the number is divisible by 2 <em>or</em> 3.<br />

<br />

Finally, the <tt>not</tt> operator negates a boolean

expression, so <tt>not (x &gt; y)</tt> is true if <tt>x &gt; y</tt> is false,

that is, if <tt>x</tt> is less than or equal to <tt>y</tt>.<br />

<br />

Strictly speaking, the operands of the logical operators should be

boolean expressions, but Python is not very strict.

Any nonzero number is interpreted as "true."

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; 17 and True

True

</font></pre><font color="black">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).</font><br />

<br />

<a name="toc33"></a>

<h2><font color="black"><a name="htoc35">3.3</a>&nbsp;&nbsp;Conditional execution</font></h2>

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<font color="black">In order to write useful programs, we almost always need the ability

to check conditions and change the behavior of the program

accordingly. <b>Conditional statements</b> give us this ability. The

simplest form is the <tt>if</tt> statement:

</font><pre><font size="4" color="blue">

if x &gt; 0 :

print 'x is positive'

</font></pre><font color="black">The boolean expression after the <tt>if</tt> statement is

called the <b>condition</b>. We end the <tt>if</tt>

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

after the if statement are indented. <br />

</font><div align="center"><font color="black"><img src="cfbook005.png" /></font></div><font color="black">

<br />

If the logical condition is true, then the indented

statement gets executed. If the logical condition is

false, the indented statement is skipped.<br />

<br />

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<br />

<tt>if</tt> statements have the same structure as function definitions

or <tt>for</tt> loops. The statement consists of a header line

that ends with the colon character (:)

followed by an indented block. Statements like this are

called <b>compound statements</b> because they stretch

across more than one line.<br />

<br />

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't written yet). In that

case, you can use the <tt>pass</tt> statement, which does nothing.</font><br />

<br />

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<pre><font size="4" color="blue">

if x &lt; 0 :

pass # need to handle negative values!

</font></pre><font color="black">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:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; x = 3

&gt;&gt;&gt; if x &lt; 10:

... print 'Small'

...

Small

&gt;&gt;&gt;

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<h2><font color="black"><a name="htoc36">3.4</a>&nbsp;&nbsp;Alternative execution</font></h2>

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<font color="black">A second form of the <tt>if</tt> statement is <b>alternative execution</b>,

in which there are two possibilities and the condition determines

which one gets executed. The syntax looks like this:

</font><pre><font size="4" color="blue">

if x%2 == 0 :

print 'x is even'

else :

print 'x is odd'

</font></pre><font color="black">If the remainder when <tt>x</tt> is divided by 2 is 0, then we

know that <tt>x</tt> is even, and the program displays a message to that

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

executed. <br />

</font><div align="center"><font color="black"><img src="cfbook006.png" /></font></div><font color="black">

<br />

Since the condition must be true or false, exactly one of

the alternatives will be executed. The alternatives are called

<b>branches</b>, because they are branches in the flow of execution.</font><br />

<br />

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<br />

<a name="toc35"></a>

<h2><font color="black"><a name="htoc37">3.5</a>&nbsp;&nbsp;Chained conditionals</font></h2>

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<font color="black">Sometimes there are more than two possibilities and we need more than

two branches. One way to express a computation like that is a <b>chained conditional</b>:

</font><pre><font size="4" color="blue">

if x &lt; y:

print 'x is less than y'

elif x &gt; y:

print 'x is greater than y'

else:

print 'x and y are equal'

</font></pre><font color="black"><tt>elif</tt> is an abbreviation of "else if." Again, exactly one

branch will be executed. <br />

</font><div align="center"><font color="black"><img src="cfbook007.png" /></font></div><font color="black">

<br />

There is no limit on the number of <tt>elif</tt> statements. If there is an <tt>else</tt> clause, it has to be

at the end, but there doesn't have to be one.</font><br />

<br />

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<pre><font size="4" color="blue">

if choice == 'a':

print 'Bad guess'

elif choice == 'b':

print 'Good guess'

elif choice == 'c':

print 'Close, but not correct'

</font></pre><font color="black">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. </font><br />

<br />

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<h2><font color="black"><a name="htoc38">3.6</a>&nbsp;&nbsp;Nested conditionals</font></h2>

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<font color="black">One conditional can also be nested within another. We could have

written the trichotomy example like this:

</font><pre><font size="4" color="blue">

if x == y:

print 'x and y are equal'

else:

if x &lt; y:

print 'x is less than y'

else:

print 'x is greater than y'

</font></pre><font color="black">The outer conditional contains two branches. The

first branch contains a simple statement. The second branch

contains another <tt>if</tt> statement, which has two branches of its

own. Those two branches are both simple statements,

although they could have been conditional statements as well.<br />

</font><div align="center"><font color="black"><img src="cfbook008.png" /></font></div><font color="black">

<br />

Although the indentation of the statements makes the structure

apparent, <b>nested conditionals</b> become difficult to read very

quickly. In general, it is a good idea to avoid them when you can.<br />

<br />

Logical operators often provide a way to simplify nested conditional

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

single conditional:

</font><pre><font size="4" color="blue">

if 0 &lt; x:

if x &lt; 10:

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

</font></pre><font color="black">The <tt>print</tt> statement is executed only if we make it past both

conditionals, so we can get the same effect with the <tt>and</tt> operator:

</font><pre><font size="4" color="blue">

if 0 &lt; x and x &lt; 10:

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

<a name="toc37"></a>

<h2><font color="black"><a name="htoc39">3.7</a>&nbsp;&nbsp;Catching exceptions using try and except</font></h2>

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<font color="black">Earlier we saw a code segment where we used the <code>raw_input</code> and

<tt>int</tt> functions to read and parse an integer number entered by

the user. We also saw how treacherous doing this could be:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; speed = raw_input(prompt)

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

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

&gt;&gt;&gt; int(speed)

ValueError: invalid literal for int()

&gt;&gt;&gt;

</font></pre><font color="black">When we are executing these statements in the Python interpreter,

we get a new prompt from the interpreter, think "oops" and move

on to our next statement. <br />

<br />

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.

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<br />

Here is a sample program to convert a Fahrenheit temperature

to a Celsius temperature:

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<pre><font size="4" color="blue">

inp = raw_input('Enter Fahrenheit Temperature:')

fahr = float(inp)

cel = (fahr - 32.0) * 5.0 / 9.0

print cel

</font></pre><font color="black">If we execute this code and give it invalid input, it simply fails

with an unfriendly error message:

</font><pre><font size="4" color="blue">

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 &lt;module&gt;

fahr = float(inp)

ValueError: invalid literal for float(): fred

</font></pre><font color="black">There is a conditional execution structure built into

Python to handle these types of expected and unexpected

errors called "try / except". The idea of <tt>try</tt>

and <tt>except</tt> 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.<br />

<br />

You can think of the <tt>try</tt> and <tt>except</tt> feature

in Python as an "insurance policy" on a sequence of

statements.<br />

<br />

We can rewrite our temperature converter as follows:

</font><pre><font size="4" color="blue">

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'

<font color="black">Python starts by executing the

sequence of statements in the

<tt>try</tt> block. If all goes

well, it skips the <tt>except</tt> block and proceeds. If an

exception occurs in the <tt>try</tt> block,

Python jumps out of the <tt>try</tt> block and

executes the sequence of statements in the <tt>except</tt> block.

</font><pre><font size="4" color="blue">

python fahren2.py

Enter Fahrenheit Temperature:72

22.2222222222

python fahren2.py

Enter Fahrenheit Temperature:fred

Please enter a number

<font color="black">Handling an exception with a <tt>try</tt> statement is called <b>catching</b> an exception. In this example, the <tt>except</tt> 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.</font><br />

<br />

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<h2><font color="black"><a name="htoc40">3.8</a>&nbsp;&nbsp;Short circuit evaluation of logical expressions</font></h2>

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<font color="black">When Python is processing a logical expression such as

<tt>x &gt;= 2 and (x/y) &gt; 2</tt>, it evaluates the expression

from left-to-right. Because of the definition of <tt>and</tt>,

if <tt>x</tt> is less than 2, the expression <tt>x &gt;= 2</tt> is

<tt>False</tt> and so the whole expression is <tt>False</tt> regardless

of whether <tt>(x/y) &gt; 2</tt> evaluates to <tt>True</tt> or <tt>False</tt>.<br />

<br />

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 <b>short-circuiting</b>

the evaluation.<br />

<br />

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

leads to a clever technique called the <b>guardian pattern</b>.

Consider the following code sequence in the Python interpreter:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; x = 6

&gt;&gt;&gt; y = 2

&gt;&gt;&gt; x &gt;= 2 and (x/y) &gt; 2

True

&gt;&gt;&gt; x = 1

&gt;&gt;&gt; y = 0

&gt;&gt;&gt; x &gt;= 2 and (x/y) &gt; 2

False

&gt;&gt;&gt; x = 6

&gt;&gt;&gt; y = 0

&gt;&gt;&gt; x &gt;= 2 and (x/y) &gt; 2

Traceback (most recent call last):

File "&lt;stdin&gt;", line 1, in &lt;module&gt;

ZeroDivisionError: integer division or modulo by zero

&gt;&gt;&gt;

</font></pre><font color="black">The third calculation failed because Python was evaluating <tt>(x/y)</tt>

and <tt>y</tt> was zero which causes a runtime error. But the second example

did <em>not</em> fail because the first part of the expression <tt>x &gt;= 2</tt>

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

due to the <b>short circuit</b> rule and there was no error.<br />

<br />

We can construct the logical expression to strategically place a <b>guard</b>

evaluation just before the evaluation that might cause an error as follows:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; x = 1

&gt;&gt;&gt; y = 0

&gt;&gt;&gt; x &gt;= 2 and y != 0 and (x/y) &gt; 2

False

&gt;&gt;&gt; x = 6

&gt;&gt;&gt; y = 0

&gt;&gt;&gt; x &gt;= 2 and y != 0 and (x/y) &gt; 2

False

&gt;&gt;&gt; x &gt;= 2 and (x/y) &gt; 2 and y != 0

Traceback (most recent call last):

File "&lt;stdin&gt;", line 1, in &lt;module&gt;

ZeroDivisionError: integer division or modulo by zero

&gt;&gt;&gt;

</font></pre><font color="black">In the first logical expression, <tt>x &gt;= 2</tt> is <tt>False</tt> so the evaluation

stops at the <tt>and</tt>. In the second logical expression <tt>x &gt;= 2</tt> is <tt>True</tt>

but <tt>y != 0</tt> is <tt>False</tt> so we never reach <tt>(x/y)</tt>.<br />

<br />

In the third logical expression, the <tt>y != 0</tt> is <em>after</em> the

<tt>(x/y) </tt> calculation so the expression fails with an error.<br />

<br />

In the second expression, we say that <tt>y != 0</tt> acts as a <b>guard</b>

to insure that we only execute <tt>(x/y)</tt> if <tt>y</tt> is non-zero.</font><br />

<br />

<a name="toc39"></a>

<h2><font color="black"><a name="htoc41">3.9</a>&nbsp;&nbsp;Debugging</font></h2>

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<font color="black">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:

</font><ul><li><font color="black">What kind of error it was, and</font><br />

<br />

</li><li><font color="black">Where it occurred.</font></li></ul>

<font color="black">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.</font><br />

<br />

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<pre><font size="4" color="blue">

&gt;&gt;&gt; x = 5

&gt;&gt;&gt; y = 6

File "&lt;stdin&gt;", line 1

y = 6

^

SyntaxError: invalid syntax

</font></pre><font color="black">In this example, the problem is that the second line is indented by

one space. But the error message points to <tt>y</tt>, 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.<br />

<br />

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<br />

The same is true of runtime errors. Suppose you are trying

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

is <i>SNR<font size="2">_db</font></i> = 10 log<font size="2">₁₀</font> (<i>P<font size="2">_signal</font></i> / <i>P<font size="2">_noise</font></i>). In Python,

you might write something like this:

</font><pre><font size="4" color="blue">

import math

signal_power = 9

noise_power = 10

ratio = signal_power / noise_power

decibels = 10 * math.log10(ratio)

print decibels

</font></pre><font color="black">But when you run it, you get an error message^{<a name="text7" href="#note7">1</a>}:</font><br />

<br />

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<pre><font size="4" color="blue">

Traceback (most recent call last):

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

decibels = 10 * math.log10(ratio)

OverflowError: math range error

</font></pre><font color="black">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 <tt>ratio</tt>, 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.<br />

<br />

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<br />

In general, error messages tell you where the problem was discovered,

but that is often not where it was caused.</font><br />

<br />

<a name="toc40"></a>

<h2><font color="black"><a name="htoc42">3.10</a>&nbsp;&nbsp;Glossary</font></h2>

<dl compact="compact"><dt><font color="black"><b>body:</b></font></dt><dd><font color="black"> The sequence of statements within a compound statement.

</font><a name="@default179"></a><br />

<br />

</dd><dt><font color="black"><b>boolean expression:</b></font></dt><dd><font color="black"> An expression whose value is either

<tt>True</tt> or <tt>False</tt>.

</font><a name="@default180"></a>

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<br />

</dd><dt><font color="black"><b>branch:</b></font></dt><dd><font color="black"> One of the alternative sequences of statements in

a conditional statement.

</font><a name="@default182"></a><br />

<br />

</dd><dt><font color="black"><b>chained conditional:</b></font></dt><dd><font color="black"> A conditional statement with a series

of alternative branches.

</font><a name="@default183"></a>

<a name="@default184"></a><br />

<br />

</dd><dt><font color="black"><b>comparison operator:</b></font></dt><dd><font color="black"> One of the operators that compares

its operands: <tt>==</tt>, <tt>!=</tt>, <tt>&gt;</tt>, <tt>&lt;</tt>, <tt>&gt;=</tt>, and <tt>&lt;=</tt>.</font><br />

<br />

</dd><dt><font color="black"><b>conditional statement:</b></font></dt><dd><font color="black"> A statement that controls the flow of

execution depending on some condition.

</font><a name="@default185"></a>

<a name="@default186"></a><br />

<br />

</dd><dt><font color="black"><b>condition:</b></font></dt><dd><font color="black"> The boolean expression in a conditional statement

that determines which branch is executed.

</font><a name="@default187"></a><br />

<br />

</dd><dt><font color="black"><b>compound statement:</b></font></dt><dd><font color="black"> A statement that consists of a header

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

relative to the header.

</font><a name="@default188"></a><br />

<br />

</dd><dt><font color="black"><b>guardian pattern:</b></font></dt><dd><font color="black"> Where we construct a logical expression

with additional

comparisons to take advantage of the short circuit behavior.

</font><a name="@default189"></a>

<a name="@default190"></a><br />

<br />

</dd><dt><font color="black"><b>logical operator:</b></font></dt><dd><font color="black"> One of the operators that combines boolean

expressions: <tt>and</tt>, <tt>or</tt>, and <tt>not</tt>.</font><br />

<br />

</dd><dt><font color="black"><b>nested conditional:</b></font></dt><dd><font color="black"> A conditional statement that appears

in one of the branches of another conditional statement.

</font><a name="@default191"></a>

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<br />

</dd><dt><font color="black"><b>traceback:</b></font></dt><dd><font color="black"> A list of the functions that are executing,

printed when an exception occurs.

</font><a name="@default193"></a><br />

<br />

</dd><dt><font color="black"><b>short circuit:</b></font></dt><dd><font color="black"> 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.

</font><a name="@default194"></a></dd></dl>

<a name="toc41"></a>

<h2><font color="black"><a name="htoc43">3.11</a>&nbsp;&nbsp;Exercises</font></h2><br />

<div align="left"><font color="black"><b>Exercise&nbsp;1</b>&nbsp;&nbsp;<em>

Rewrite your pay computation to give the employee 1.5

times the hourly rate for

hours worked above 40 hours.

</em></font><pre><font color="black"><em>

Enter Hours: 45

Enter Rate: 10

Pay: 475.0

</em></font></pre></div><br />

<div align="left"><font color="black"><b>Exercise&nbsp;2</b>&nbsp;&nbsp;<em>

Rewrite your pay program using <tt>try</tt> and <tt>except</tt>

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:

</em></font><pre><font color="black"><em>

Enter Hours: 20

Enter Rate: nine

Error, please enter numeric input

Enter Hours: forty

Error, please enter numeric input

</em></font></pre></div><br />

<div align="left"><font color="black"><b>Exercise&nbsp;3</b>&nbsp;&nbsp;<em>

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:

</em></font><pre><font color="black"><em>

Score Grade

&gt;= 0.9 A

&gt;= 0.8 B

&gt;= 0.7 C

&gt;= 0.6 D

&lt; 0.6 F

Enter score: 0.95

A

Enter score: perfect

Bad score

Enter score: 10.0

Bad score

Enter score: 0.75

C

Enter score: 0.5

F

<font color="black"><em>Run the program repeatedly as shown above to test the

various different values for input.

</em></font></div><br />

<hr width="50%" size="1" /><dl><dt><font size="5" color="black"><a name="note7" href="#text7">1</a></font></dt><dd><font color="black">In Python 3.0,

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

floating-point division even with integer operands.

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