lecture 20 – boolean operators dr. patricia j. riddle

Lecture 20 – Boolean OperatorsDr. Patricia J. Riddle

COMPSCI 101Principles of Programming

2COMPSCI 101 - Principles of Programming

At the end of this lecture, students should be able to:

Use and, or and not in conditional conditions

Understand basic truth tables

Use short circuit evaluation

Learning outcomes


Booleans represent truth values True False

Relational operators compare two different things Evaluate to Boolean values == < <= > >= !=

Review of Booleans


Boolean Operators are and, or and not:

if palindrome("word")

if not palindrome("word")

if palindrome("word") and heterogram("word")

if palindrome("word") or heterogram("word")

Boolean Operator Examples


P Q not P P and Q P or Q

True True False True True

True False False False True

False True True False True

False False True False False

Boolean Operator Truth Tables


Write a function named is_a_leap_year() that accepts a year as a parameter and returns True if the year is a leap year and False otherwise.

A year is a leap year if it is divisible by 400, or divisible by 4 but not by 100.

Exercise


From lecture 12, slide 19

def is_leap_year(year): if year % 400 == 0: return True elif year % 100 == 0: return False elif year % 4 == 0: return True else: return False

Original Leap Year Code


def is_a_leap_year(year): if not isinstance(year,int): print("year is not an integer") return False if (year % 400) == 0 or ((year % 4) == 0

and (year % 100) != 0): return True return False

Answer


If A and B B is not evaluated unless A is True

If A or B B is not evaluated unless A is False

Examples

If divisor != 0 and numerator/divisor > 4:

If divisor = 0 or numerator/divisor > 4:

Short Circuit Evaluation


Write a function named find_names_with() that accepts a letter, a location and a list of names, and returns the list of names that have the letter in the specified location.

find_names_with("r", 3, ["Sara","Fred","Al","Tar"])['Sara', 'Tar']

Exercise


def find_names_with(letter, place, names_list): name_list = [] for name in names_list: if len(name) >= place and

name[place-1] == letter: name_list += [name] return name_list

Answer


Write a function named pangram() that accepts a string parameter and returns True if the string is a pangram and False otherwise.

A pangram is a sentence which contains every letter in the alphabet.

“The quick brown fox jumps over the lazy dog”

Perfect panagrams “TV quiz jock, Mr. PhD, bags few lynx”“Glum Schwartzkopf vex'd by NJ IQ”“Blowzy night-frumps vex'd Jack Q”

Exercise


def pangram(sentence): if not isinstance(sentence,str): print("sentence is not a string") return False alphabet = ["a", "b", "c", "d", "e", "f",

"g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

for letter in alphabet: if letter not in sentence: return False return True

Answer


Write a function named pangram2() that accepts a string parameter and returns True if it is a Pangram and False otherwise, and works for both uppercase and lowercase letters

Perfect panagrams “TV quiz jock, Mr. PhD, bags few lynx”“Glum Schwartzkopf vex'd by NJ IQ”“Blowzy night-frumps vex'd Jack Q”

Exercise


def pangram2(sentence): if not isinstance(sentence,str): print("sentence is not a string") return False alphabet = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j",

"k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

capital_alphabet = ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y”, "Z"]

alphabet_range = range(0,len(alphabet)) for index in alphabet_range: if alphabet[index] not in sentence and capital_alphabet[index] not in sentence: return False return True

Answer


http://en.wikipedia.org/wiki/Conway%27s_Game_of_Life

The universe of the Game of Life is an infinite two-dimensional orthogonal grid of square cells, each of which is in one of two possible states, alive or dead. Every cell interacts with its 8 neighbours, which are the cells that are horizontally, vertically, or diagonally adjacent. At each step in time, the following transitions occur: Any live cell with fewer than two live neighbours dies, as if caused by under-

population. Any live cell with two or three live neighbours lives on to the next generation. Any live cell with more than three live neighbours dies, as if by overcrowding. Any dead cell with exactly three live neighbours becomes a live cell, as if by

reproduction. Wikipedia

The Game of Life

http://en.wikipedia.org/wiki/Conway's_Game_of_Life


Write a function named is_alive() that accepts 2 parameters: a Boolean value (alive) and a number between 1 and 8 (live_neighbours) and returns True if the cell should turn on and False otherwise

>>> is_alive(True,1)False

>>> is_alive(True,3)True

>>> is_alive(False,3)True

>>> is_alive(False,4)False

Exercise


Nested if statements can be hard to read

def is_alive(alive,neighbours_alive): if alive: if neighbours_alive < 2: return False if neighbours_alive > 3: return False return True if neighbours_alive == 3: return True return False

Answer


def is_alive2(alive,neighbours_alive): its_dead = False its_alive = True if alive and (neighbours_alive < 2 or

neighbours_alive > 3): return its_dead if not alive and neighbours_alive != 3:

return its_dead return its_alive

Answer


Basic Truth tables and or not

Use of Complex conditions: Use of “and”, “not”, and “or”

Benefits of Short Circuit evaluation: Allows you to write code that will not trigger error messages

Summary


Complex String Processing

Tomorrow

lecture 20 – boolean operators dr. patricia j. riddle

Documents