If-else statement and loop

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5/2/2018

Decision Structures and Boolean Logic

Selected slides from Gaddis, “Starting out with Python”, 4th edition.

Topics

• The if Statement

• The if-else Statement

• Comparing Strings

• Nested Decision Structures and the if-elif-else Statement

• Logical Operators

• Boolean Variables

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The if Statement

• Control structure: logical design that controls order in which set of statements execute

• Sequence structure: set of statements that execute in the order they appear

• Decision structure: specific action(s) performed only if a condition exists • Also known as selection structure

The if Statement (cont’d.)

• In flowchart, diamond represents true/false condition that must be tested

• Actions can be conditionally executed • Performed only when a condition is true

• Single alternative decision structure: provides only one alternative path of execution • If condition is not true, exit the structure

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The if Statement (cont’d.)

• Python syntax: if condition:

Statement

• First line known as the if clause • Includes the keyword if followed by condition

• The condition can be true or false

• When the if statement executes, the condition is tested, and if it is true the block statements are executed. otherwise, block statements are skipped

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Boolean Expressions and Relational Operators

• Boolean expression: expression tested by if statement to determine if it is true or false • Example: a > b

• true if a is greater than b; false otherwise

• Relational operator: determines whether a specific relationship exists between two values • Example: greater than (>)

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Relational Operators

• In most cases, the boolean expression, used by the if statement, uses relational operators.

Relational Operator Meaning

> is greater than

< is less than

>= is greater than or equal to

<= is less than or equal to

== is equal to

!= is not equal to

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Boolean Expressions and Relational Operators (cont’d.) • >= and <= operators test more than one relationship

• It is enough for one of the relationships to exist for the expression to be true

• == operator determines whether the two operands are equal to one another • Do not confuse with assignment operator (=)

• != operator determines whether the two operands are not equal

Boolean Expressions and Relational Operators (cont’d.)

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Boolean Expressions and Relational Operators (cont’d.) • Using a Boolean expression with the > relational operator

Boolean Expressions and Relational Operators (cont’d.) • Any relational operator can be used in a decision block

• Example: if balance == 0

• Example: if payment != balance

• It is possible to have a block inside another block • Example: if statement inside a function

• Statements in inner block must be indented with respect to the outer block

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The if-else Statement

• Dual alternative decision structure: two possible paths of execution – One is taken if the condition is true, and the other if the condition is false

• Syntax: if condition: statements

else:

other statements

• if clause and else clause must be aligned

• Statements must be consistently indented

The if-else Statement (cont’d.)

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The if-else Statement (cont’d.)

Comparing Strings

• Strings can be compared using the == and != operators

• String comparisons are case sensitive

• Strings can be compared using >, <, >=, and <= • Compared character by character based on the ASCII values for each

character

• If shorter word is substring of longer word, longer word is greater than shorter word

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Comparing Strings (cont’d.)

Nested Decision Structures and the if-elif- else Statement

• A decision structure can be nested inside another decision structure • Commonly needed in programs

• Example: • Determine if someone qualifies for a loan, they must meet two conditions:

• Must earn at least $30,000/year

• Must have been employed for at least two years

• Check first condition, and if it is true, check second condition

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Nested Decision Structures and the if-elif-else Statement (cont’d.)

• Important to use proper indentation in a nested decision structure • Important for Python interpreter

• Makes code more readable for programmer

• Rules for writing nested if statements: • else clause should align with matching if clause

• Statements in each block must be consistently indented

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The if-elif-else Statement

• if-elif-else statement: special version of a decision structure • Makes logic of nested decision structures simpler to write

• Can include multiple elif statements

• Syntax:

if condition_1:

statement(s)

elif condition_2:

statement(s)

elif condition_3:

statement(s)

else

statement(s)

Insert as many elif clauses

as necessary.

The if-elif-else Statement (cont’d.) • Alignment used with if-elif-else statement:

• if, elif, and else clauses are all aligned

• Conditionally executed blocks are consistently indented

• if-elif-else statement is never required, but logic easier to follow • Can be accomplished by nested if-else

• Code can become complex, and indentation can cause problematic long lines

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Logical Operators

• Logical operators: operators that can be used to create complex Boolean expressions • and operator and or operator: binary operators, connect two Boolean

expressions into a compound Boolean expression

• not operator: unary operator, reverses the truth of its Boolean operand

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The and Operator

• Takes two Boolean expressions as operands • Creates compound Boolean expression that is true only when both sub

expressions are true

• Can be used to simplify nested decision structures

• Truth table for

the and operator Value of the Expression

Expression

falsefalse and false

falsefalse and true

falsetrue and false

truetrue and true

The or Operator

• Takes two Boolean expressions as operands • Creates compound Boolean expression that is true when either of the sub

expressions is true

• Can be used to simplify nested decision structures

• Truth table for

the or operator Value of the Expression

Expression

falsefalse and false

truefalse and true

truetrue and false

truetrue and true

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Short-Circuit Evaluation

• Short circuit evaluation: deciding the value of a compound Boolean expression after evaluating only one sub expression • Performed by the or and and operators

• For or operator: If left operand is true, compound expression is true. Otherwise, evaluate right operand

• For and operator: If left operand is false, compound expression is false. Otherwise, evaluate right operand

The not Operator

• Takes one Boolean expressions as operand and reverses its logical value • Sometimes it may be necessary to place parentheses around an expression to

clarify to what you are applying the not operator

• Truth table for the not operator

Value of the ExpressionExpression

falsetrue

truefalse

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Checking Numeric Ranges with Logical Operators • To determine whether a numeric value is within a specific range of

values, use and Example: x >= 10 and x <= 20

• To determine whether a numeric value is outside of a specific range of values, use or

Example: x < 10 or x > 20

Testing, Searching, and Manipulating Strings

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String Manipulation Functions

•.replace()

line = line.replace(old,new) #changes all instance of old to new

Example: line = "My CAR is cool, my CAR is awesome!" line = line.replace(“CAR”,”BUS”) print (line)

Application- Biological sequences

• Nucleic acid sequences are represented as strings of A,T,C,G,U (for RNA only) and N (unknown)

• Protein sequences are represented by strings from the standard IUB/IUPAC amino acid codes