Selection in Python
If statements
Control Structures




Sequence
Selection
Repetition
Module
Selection
If or if / else statement
 choosing between mutually
exclusive possibilities
 Two forms

see next slide
Selection

Two forms
if logical expression:
statement
 if logical expression:

statement
else:
statement
Logical Expressions

Relational Operators
==, !=, <=, >=, >, <



They are binary operators – have two
operands
work on any type of data, be careful about
matching types
produce a bool result (True or False)
Syntax of if statement




if logical expression:
statement
Indentation required
if expression is true, then statement is
executed
if expression is false, statement is NOT
executed
Syntax of if statement


if x > 0:
y = sqrt(x)
if x > y:
t=x
x=y
y=t
If-Then Statement
Determine whether or not to execute a statement (which can be
a single statement or an entire block)
TRUE
statement
expression
FALSE
Examples





output larger of two numbers
don't allow sqrt of negative number
don't allow overflow
determining even or odd
give student another chance at a question
Visual Aid

Decision tree




“if person is 18 or over and state is KY, they can
have regular license”
“if person is 16 or over and under 18 and state is
TN or VA, they can have learner’s permit”
“if person is under 16, in any state, they can’t
have license”
“if person is over 65 in VA, they have modified
license”
Logical Operators






and or not
and, or are binary operators, not is unary
used to combine bool values
produce a bool result
truth tables
operator precedence


not then and then or
not, and, or are very low, below other operators
Precedence of operators


See
http://docs.python.org/py3k/reference/expres
sions.html#evaluation-order
Note that this table is “upside down”, lowest
precedence is on top!
Converting English to logic



"0 is less than x is less than 5"
"x is 5 or 6"
"x is bigger than 5 and less than 10“


5 < x < 10 IS valid in Python!
impossible situations "dead code"


always true – “x < x + 1” “x > 5 or x < 8”
always false – “x < 5 and x > 10”
Write an expression for each
taxRate is over 25% and income is less than
$20000
temperature is less than or equal to 75 or
humidity is less than 70%
age is over 21 and age is less than 60
age is 21 or 22
Some Answers
(taxRate > .25) and (income < 20000)
(temperature <= 75) or (humidity < .70)
(age > 21) and (age < 60)
(age == 21) or (age == 22)
Nested if's


the statement to be executed in an if
statement can be another if
an else branch goes with the if which
matches its indentation
what’s the difference?

if a > 5:
if b < 10:
print(“green”)
else:
print(“blue”)

if a > 5:
if b < 10:
print(“green”)
else:
print(“blue”)
if .. else provides two-way selection
between executing one of 2 clauses (the if
clause or the else clause)
TRUE
if clause
expression
FALSE
else clause
Example

Given x, y on Cartesian plane, which quadrant is it
in? (I, II, III, IV)
if x > 0:
if y > 0:
quadrant = 1
else:
quadrant = 4
else:
if y > 0: # NOT a redundant test!
quadrant = 2
else:
quadrant = 3
Another form of nested if's
if condition:
statement;
elif condition:
statement
elif condition:
statement
else:
statement
# good for mutually exclusive and exhaustive
#conditions

Comparing Strings

Two objects of type string (or a string object and a C string)
can be compared using the relational operators

A character-by-character comparison is made using the
ASCII character set values



If all the characters are equal, then the 2 strings are equal.
Otherwise, the string with the character with smaller ASCII
value is the “lesser” string
space < digits < uppercase < lowercase
“ “ < “0” < … < “9” < “A” < “B” < … < “Z” < “a” < … < “z”
string myState;
string yourState;
myState = “Texas”;
yourState = “Maryland”;
Expression
Value
myState == yourState
false
myState > yourState
true
myState == “Texas”
true
myState < “texas”
true
Comparing Real Values
Do not compare floating point values for equality,
compare them for near-equality.
a=0
for i in range(5000000):
a = a + 0.001;
if abs(a – 5000) < 0.00001:
print( “They are close enough!”)