COSC 1306 COMPUTER SCIENCE AND PROGRAMMING Jehan-François Pâris jfparis@uh.edu Fall 2017 1

THE ONLINE BOOK CHAPTER VII SELECTIONS

Motivation We will learn how programs can make choices: Boolean variables Boolean expressions if and if/else

Boolean type Two values: True and False Created as Result of a comparison: ==, !=, >, >=, <, <= Conversion using bool(…) function Zero values and anything empty converts into a False Anything else converts into a True value

Examples >>> a = 3 >>> b = 5 >>> p = a < b >>> q = b <= a >>> print("p =", p, "and q =", q) p = True and q = False

A Boolean function def isDivisible(x, y): return x % y == 0 Returns True if x is divisible by y

Boolean operations (I) AND: True if both operands are true This is called a truth table a b a and b F T

Boolean operations (II) OR: True unless both operands are false a b a or b F T

Boolean operations (III) NOT: True if operand is false a not a F T

Boolean operations (IV) Equality/Mutual Implication: True when both operands are equal Represented by a == sign a b a==b F T

Boolean operations (V) Exclusive OR (XOR): True when operands are not equal Represented by a != sign a b a!=b F T

Notes XOR is used for Generating parity bits Hardware hashing

Examples Assuming p is True and q is False >>> p or q True >>> p and q False >>> not p >>> p != q

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Precedence Rules Level Category Operators 7(high) exponent ** 6 multiplication *, /, //, % 5 addition +, - 4 relational ==, !=, <=, >=, >, < 3 logical not not 2 logical and and 1(low) logical or or

Examples a < b + c or c > 0 same as (a < (b+c)) or c >0) a and b or c same as (a and b) or c a and not b or c same as (a and (not b)) or c

Conversely We must write a = (p or q) and s b = (c and d) != (e or f) …

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Binary selection if BOOLEAN EXPRESSION : STATEMENTS_1 # when condition is True else: STATEMENTS_2 # when condition is False Observe The columns after if and else The indentation

Binary selection If condition: something else: other stuff False True other stuff something If condition: something else: other stuff

Flowchart conventions Graphic representation of flow of control Loops represented by loops Ifs have two branches ??? False True

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Omitting the else clause if BOOLEAN EXPRESSION: STATEMENTS_1 # if condition is True # Continue normal flow Observe The column after if The indentation

Omitting the else clause If condition : something If tank_empty : get_gas True condition something False

Main usage if + else To make a choice between two options if To handle a special case Zero value, negative value Work is done

A problem with letter grades Semester Average Letter Grade ≥ 90 A ≥ 80 but < 90 B ≥ 70 but < 80 C ≥ 60 but < 70 D < 60 F

An ugly solution if average >= 90 : grade = 'A' if average >= 80 and average < 90 : grade = 'B' if average >= 70 and average < 80 : grade = 'C' if average >= 60 and average < 70 : grade = 'D' if average < 60 : grade = 'F'

The right way if average >= 90 : grade = 'A' else : if average >= 80: grade = 'B' else : if average >= 70 : grade = 'C' else : if average >= 60 : grade = 'D' else: grade = 'F'

Using an elif (I) Too many nested ifs else … if cond-1 : … else if cond-2 : … else if cond-3 : … else …

Using an elif (II) With elif, lines align better if cond-1 : … elif cond-2 : … elif cond-3 : … else : …

The right way if average >= 90 : grade = 'A' elif average >= 80: grade = 'B' elif average >= 70 : grade = 'C' elif average >= 60 : grade = 'D' else: grade = 'F'

A beginner's trap if average >= 90 : grade = 'A' elif average >= 80 and average < 90 : grade = 'B' elif average >= 70 and average < 80 : grade = 'C' elif average >= 60 and average < 70: grade = 'D' elif average < 60: grade = 'F'

Useless comparisons if average >= 90 : grade = 'A' elif average >= 80 and average < 90 : grade = 'B' elif average >= 70 and average < 80 : grade = 'C' elif average >= 60 and average < 70: grade = 'D' elif average < 60: # use else instead grade = 'F'

Indenting advice Python attaches great importance to indenting You cannot indent anything that is not inside An if, a while, a for, … A function declaration or a main function Your indentation must be consistent Do not mix spaces and tabs Four spaces is neither too small nor too big

Floating point values Differ from integer values Cannot handle extremely large or extremely small values Limited precision Can manipulate trillions of dollars but results are not exact to the cent Must specify number of decimals in outputs

Limited precision # precision.py """ floating-point computations can be inaccurate """ biggie = 5E9 epsilon = 5E-9 sum = biggie + epsilon delta = sum - biggie print ('epsilon =', epsilon, 'but delta =', delta)

The output epsilon = 5e-9 but delta = 0.0 Why? Computations are carried out with a limited number of significant bits Result has a limited number of significant digits Be careful when computing very small differences between two numbers

Note Limitations on floating point number size and precision are Hardware dependent Shared by most programming languages Unlike Python, most programming languages do not support arbitrary-size integers Python is better

Displaying floating point numbers Easiest way to specify number of decimal places is string interpolation 'Answer is %.nf ' %x does two things It inserts the value of x inside the string It specifies that the string representation of x should have n decimal places

Example (I) pi = 3.1415917 print('pi equals %.1f' % pi ) print('pi equals %.2f' % pi ) print('pi equals %.3f' % pi ) print('pi equals %.4f' % pi )

Example (II) Program prints pi equals 3.1 pi equals 3.14 pi equals 3.142 pi equals 3.1416 Values are nicely rounded