1. Selection in Python
If statements

2. Control Structures
Sequence
Selection
Repetition
Module

3. Selection If or if / else statement
choosing between mutually exclusive possibilities
Two forms
see next slide

4. Selection
Two forms
if logical expression:
statement
else:

5. 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)

6. 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

7. Syntax of if statement if x > 0: y = sqrt(x) if x > y: t = x

8. If-Then Statement TRUE expression FALSE statement
Determine whether or not to execute a statement (which can be a single statement or an entire block)
TRUE
expression
FALSE
statement

9. 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

10. 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”

11. 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

12. Precedence of operators
See
Note that this table is “upside down”, lowest precedence is on top!

13. 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”

14. 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

15. Some Answers (taxRate > .25) and (income < 20000)
(temperature <= 75) or (humidity < .70)
(age > 21) and (age < 60)
(age == 21) or (age == 22)

16. 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

17. 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”)

18. if .. else provides two-way selection
between executing one of 2 clauses (the if clause or the else clause)
TRUE
FALSE
expression
if clause
else clause

19. 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
if y > 0: # NOT a redundant test!
quadrant = 2
quadrant = 3

20. Another form of nested if's
if condition:
statement;
elif condition:
statement
else:
# good for mutually exclusive and exhaustive #conditions

21. 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”

22. myState > yourState true
string myState;
string yourState;
myState = “Texas”;
yourState = “Maryland”;
Expression Value
myState == yourState false
myState > yourState true
myState == “Texas” true
myState < “texas” true

23. Comparing Real Values
Do not compare floating point values for equality, compare them for near-equality.
a = 0
for i in range( ):
a = a ;
if abs(a – 5000) < :
print( “They are close enough!”)