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<LI><A
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<HR>
<H1><A name=SECTION001200000000000000000>Boolean Expressions</A> </H1>Here is a
little example of boolean expressions (you don't have to type it in): <PRE>a = 6
b = 7
c = 42
print 1, a == 6
print 2, a == 7
print 3,a == 6 and b == 7
print 4,a == 7 and b == 7
print 5,not a == 7 and b == 7
print 6,a == 7 or b == 7
print 7,a == 7 or b == 6
print 8,not (a == 7 and b == 6)
print 9,not a == 7 and b == 6
</PRE>With the output being: <PRE>1 1
2 0
3 1
4 0
5 1
6 1
7 0
8 1
9 0
</PRE>What is going on? The program consists of a bunch of funny looking
<TT>print</TT> statements. Each <TT>print</TT> statement prints a number and a
expression. The number is to help keep track of which statement I am dealing
with. Notice how each expression ends up being either 0 or 1. In Python false is
written as 0 and true is written as 1. The lines: <PRE>print 1, a == 6
print 2, a == 7
</PRE>print out a 1 and a 0 respectively just as expected since the first is
true and the second is false. The third print, <CODE>print 3,a == 6 and b ==
7</CODE>, is a little different. The operator <TT>and</TT> means if both the
statement before and the statement after are true then the whole expression is
true otherwise the whole expression is false. The next line, <CODE>print 4,a ==
7 and b == 7</CODE>, shows how if part of an <TT>and</TT> expression is false,
the whole thing is false. The behavior of <TT>and</TT> can be summarized as
follows:
<P>
<TABLE border=1 cellPadding=3>
<TBODY>
<TR>
<TD align=left>expression</TD>
<TD align=left>result</TD></TR>
<TR>
<TD align=left>true and true</TD>
<TD align=left>true</TD></TR>
<TR>
<TD align=left>true and false</TD>
<TD align=left>false</TD></TR>
<TR>
<TD align=left>false and true</TD>
<TD align=left>false</TD></TR>
<TR>
<TD align=left>false and false</TD>
<TD align=left>false</TD></TR></TBODY></TABLE>
<P>Notice that if the first expression is false Python does not check the second
expression since it knows the whole expression is false.
<P>The next line, <CODE>print 5,not a == 7 and b == 7</CODE>, uses the
<TT>not</TT> operator. <TT>not</TT> just gives the opposite of the expression
(The expression could be rewritten as <TT>print 5,a != 7 and b == 7</TT>). Heres
the table:
<P>
<TABLE border=1 cellPadding=3>
<TBODY>
<TR>
<TD align=left>expression</TD>
<TD align=left>result</TD></TR>
<TR>
<TD align=left>not true</TD>
<TD align=left>false</TD></TR>
<TR>
<TD align=left>not false</TD>
<TD align=left>true</TD></TR></TBODY></TABLE>
<P>The two following lines, <CODE>print 6,a == 7 or b == 7</CODE> and
<CODE>print 7,a == 7 or b == 6</CODE>, use the <TT>or</TT> operator. The
<TT>or</TT> operator returns true if the first expression is true, or if the
second expression is true or both are true. If neither are true it returns
false. Here's the table:
<P>
<TABLE border=1 cellPadding=3>
<TBODY>
<TR>
<TD align=left>expression</TD>
<TD align=left>result</TD></TR>
<TR>
<TD align=left>true or true</TD>
<TD align=left>true</TD></TR>
<TR>
<TD align=left>true or false</TD>
<TD align=left>true</TD></TR>
<TR>
<TD align=left>false or true</TD>
<TD align=left>true</TD></TR>
<TR>
<TD align=left>false or false</TD>
<TD align=left>false</TD></TR></TBODY></TABLE>
<P>Notice that if the first expression is true Python doesn't check the second
expression since it knows the whole expression is true. This works since
<TT>or</TT> is true if at least one half of the expression is true. The first
part is true so the second part could be either false or true, but the whole
expression is still true.
<P>The next two lines, <CODE>print 8,not (a == 7 and b == 6)</CODE> and
<CODE>print 9,not a == 7 and b == 6</CODE>, show that parentheses can be used to
group expressions and force one part to be evaluated first. Notice that the
parentheses changed the expression from false to true. This occurred since the
parentheses forced the <TT>not</TT> to apply to the whole expression instead of
just the <TT>a == 7</TT> portion.
<P>Here is an example of using a boolean expression: <PRE>list = ["Life","The Universe","Everything","Jack","Jill","Life","Jill"]
#make a copy of the list
copy = list[:]
#sort the copy
copy.sort()
prev = copy[0]
del copy[0]
count = 0
#go through the list searching for a match
while count < len(copy) and copy[count] != prev:
prev = copy[count]
count = count + 1
#If a match was not found then count can't be < len
#since the while loop continues while count is < len
#and no match is found
if count < len(copy):
print "First Match: ",prev
</PRE>
<P>And here is the output: <PRE>First Match: Jill
</PRE>
<P>This program works by continuing to check for match <TT>while count <
len(copy and copy[count]</TT>. When either <TT>count</TT> is greater than the
last index of <TT>copy</TT> or a match has been found the <TT>and</TT> is no
longer true so the loop exits. The <TT>if</TT> simply checks to make sure that
the <TT>while</TT> exited because a match was found.
<P>The other `trick' of <TT>and</TT> is used in this example. If you look at the
table for <TT>and</TT> notice that the third entry is ``false and won't check''.
If <TT>count >= len(copy)</TT> (in other words <TT>count < len(copy)</TT>
is false) then copy[count] is never looked at. This is because Python knows that
if the first is false then they both can't be true. This is known as a short
circuit and is useful if the second half of the <TT>and</TT> will cause an error
if something is wrong. I used the first expression (<TT>count <
len(copy)</TT>) to check and see if <TT>count</TT> was a valid index for
<TT>copy</TT>. (If you don't believe me remove the matches `Jill' and `Life',
check that it still works and then reverse the order of <TT>count < len(copy)
and copy[count] != prev</TT> to <TT>copy[count] != prev and count <
len(copy)</TT>.)
<P>Boolean expressions can be used when you need to check two or more different
things at once.
<P>
<H1><A name=SECTION001210000000000000000>Examples</A> </H1>
<P>password1.py <PRE>## This programs asks a user for a name and a password.
# It then checks them to make sure the the user is allowed in.
name = raw_input("What is your name? ")
password = raw_input("What is the password? ")
if name == "Josh" and password == "Friday":
print "Welcome Josh"
elif name == "Fred" and password == "Rock":
print "Welcome Fred"
else:
print "I don't know you."
</PRE>
<P>Sample runs <PRE>What is your name? Josh
What is the password? Friday
Welcome Josh
What is your name? Bill
What is the password? Money
I don't know you.
</PRE>
<P>
<H1><A name=SECTION001220000000000000000>Exercises</A> </H1>
<P>Write a program that has a user guess your name, but they only get 3 chances
to do so until the program quits.
<P>
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<ADDRESS>Josh Cogliati <A
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