Python Loops & Conditionals: Relational, Assignment & Boolean Operators, Syntax Rules, Study notes of Logic

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Loops and

Conditionals

HORT 59000 Lecture 11 Instructor: Kranthi Varala

Relational Operators

• These operators compare the value of two ‘expressions’ and returns a Boolean value.
• Beware of comparing across data types, especially when reading values in from command line or files.

Assignment Operators

• A += B increase A by value of B
• A - = B decrease A by value of B
• A *= B multiply A by B and assign value to A
• A /= B divide A by B and assign value to A
• A **=B raise value of A to the power of B
• A %= B modulus of A by B, assigned to A
• A //= B floor of A divided by B, assigned to A
• String context:
  • S 1 += S2 add string on right to the one on left
  • S1 *= A Make A copies of S1 and concatenate them to S

Boolean Operators

Combines two or more statements that return a Boolean value. A and B True if both A and B are true A or B True if either A or B is true not A reverse the Boolean given by A xor(A,B) True if only one of A or B is True A B A and B A or B Not A xor(A,B) TRUE TRUE TRUE TRUE FALSE FALSE TRUE FALSE FALSE TRUE FALSE TRUE FALSE TRUE FALSE TRUE TRUE TRUE FALSE FALSE FALSE FALSE TRUE FALSE

Branching logic

• Used to implement alternate paths for the logic flow. https://upload.wikimedia.org/wikipedia/commons/4/44/LampFlowchart.png

If/elif/else statements

if test1: statement 1 elif test2: statement 2 else: statement 3

• Both the elif and else blocks are optional.

Lamp flowchart with if/else

Accepts input from user, as a string

Truth and Boolean tests in Python

• All objects in python have an inherent true or false value.
• Any nonempty object is true.
  • For Integers : Any non-zero number is true
• Zero, empty objects and special object ‘None’ are false.
• Comparisons return the values True or False

while loops

• while condition: statement 1 statement 2 . .
• Most generic form of loop, that checks whether the condition is true at the start of each iteration.
• Expects the condition to become false at some point during the iterations of the loop.
• If condition is never changed, this creates an ‘infinite’ loop. i.e., the program will get stuck in this loop for ever.

Example while loops

Altering while loops

for loops

• for item in sequence: statement 1 statement 2 . .
• Generic iterator for items in a ordered sequence such as lists, tuples etc.
• On each iteration retrieves one item from the list and assigns it to the variable specified.
• Automatically moves to the next item in the order.
• Value of variable may be altered within the for loop, but change is not made in the list.

Looping over Strings and Lists

• List is a general sequence object while String is a character sequence object.
• Both can be iterated over by a for loop:

Looping over lists with and without index

• Looping with an index allows accessing the item within the list and changing it.