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 else:

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

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

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/expressions.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 FALSE expression if clause 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 if y > 0: # NOT a redundant test! quadrant = 2 quadrant = 3

Another form of nested if's if condition: statement; elif condition: statement else: # 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”

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

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!”)