Today

1. Today • We have a TA now – Nancy Hwang. Please attend the lab today to meet her. She is ready for your questions! • Quiz next week in the lab. More detail on next slide. • Boolean operators • Conditional expressions CISC101 - Prof. McLeod

2. Quiz 1 • A sample quiz 1 is linked to the grading page on the course web site. • Everything up to and including today’s lecture except that conditionals (“if” statements) are not on the quiz. Exercises 1 and 2 are fair game. You will need to understand conditionals for the assignment. • 45 minutes on paper – no aids. Pen or pencil. First part of lab. Tell Nancy if you cannot write in the first hour so she will expect you later in the lab. • T/F, short answer, read code (“what is the output”), write code (obtain something, calculate something else, display result). • Returned in class, marks in Moodle (eventually!). CISC101 - Prof. McLeod

3. Also… • Videos are now being streamed through links in our Moodle site. • Let me know if you have any problems! CISC101 - Prof. McLeod

4. Unary Boolean Operator • One unary boolean operator: not • Sets True to False, and False to True (just like a NOT gate!) CISC101 - Prof. McLeod

5. Binary Boolean Operators > < >= <= == != greater than less than greater than or equal to less than or equal to equals not equals All of these result in a True or Falsebool value. and or a logical AND a logical OR CISC101 - Prof. McLeod

6. Binary Boolean Operators, Cont. > < >= <= == != greater than less than greater than or equal to less than or equal to equals not equals All of these must have a number on both sides or a string on both sides. These must have a bool value on both sides. and or a logical AND a logical OR CISC101 - Prof. McLeod

7. Binary Boolean Operators, Cont. • The interpreter is going to want both sides to be either numeric or both sides to be strings. • You will get an error if you try to compare a string to a number. CISC101 - Prof. McLeod

8. Precedence Rules • Precedence rules are expanded! • Unary operations (not and negation) first. • Math operators in usual order: ** * / // % + - • Binary boolean comparison operators: > >= < <= == != • Logical operator: and • Logical operator: or • Assignment operator: = CISC101 - Prof. McLeod

9. Precedence Rules, Cont. • Round brackets are still used to control precedence. • And, function invocations and variable substitution take place before all of the operators listed on the previous slide. • Can brackets force the assignment operator to go first? CISC101 - Prof. McLeod

10. Boolean Expression Examples 5 > 2 4 < 3 or 7 > 1 7 != 8 6 + 2 > 9 or 4 == 3 + 1 7 == 7 and 5 > 2 and 6 != 3 5 * 5 >= 5 ** 2 128 % 2 == 0 CISC101 - Prof. McLeod

11. Another Example – Evaluate: not(5 * 4 > 3) or 2 + 3 <= 5 and 6 == 2 not(20 > 3) or 2 + 3 <= 5 and 6 == 2 not True or 2 + 3 <= 5 and 6 == 2 False or 2 + 3 <= 5 and 6 == 2 False or 5 <= 5 and 6 == 2 False or True and 6 == 2 False or True and False False or False False CISC101 - Prof. McLeod

12. Expressions, Cont. • We’ve discussed literals, variables and operators. • We still have function/method calls, keywords and punctuation to discuss. • I’ll talk about conditionals first – so you can work on the assignment and Exercise 3. CISC101 - Prof. McLeod

13. So Far… • Without conditionals programs are linear: Obtain data from user Calculations… Display results etc. CISC101 - Prof. McLeod

14. Branching Algorithms • If a program could test a condition, then it would have a choice as to its path of execution: if true if false etc. CISC101 - Prof. McLeod

15. Conditionals or “Selection Statements” • Python has if, if-elseand chained if (if-elif-else) statements. • if statement syntax: if boolean_expression: statement1_when_true statement2_when_true statement3_when_true … • Example: if capacitance < 0 : print("Illegal capacitance") CISC101 - Prof. McLeod

16. if-else Statement • Syntax of if-elsestatement: if boolean_expression: statement(s)_when_true else : statement(s)_when_false • Example: if stress > maxStress / 1.5 : result = "failure" else : result = "pass" CISC101 - Prof. McLeod

17. if Statement, Cont. • You will often have to nest if statements: etc. CISC101 - Prof. McLeod

18. Nested if Statements • For example, if you have three numbers, a, b, & c and you want to print them to the screen in order: T F a < b T F T F b < c b < c T F T F a < c a < c abc cba acb cab bac bca See SortThree.py CISC101 - Prof. McLeod

19. if-elif Statements • In the code in SortThree.py, you might have noticed this construct: else : if a < c : • This kind of thing occurs so often that it can be shortened to: elif a < c : CISC101 - Prof. McLeod

20. if-elif Statements, Cont. • This leads to a common code structure often called a “chained if” construct: if condition1 : statement(s) elifcondition2 : statement(s) elifcondition3 : statement(s) else : statement(s) You can have as many elifs as you want. The else part is optional. CISC101 - Prof. McLeod

21. if-elif Statements, Cont. • There is nothing in this construct that you could not make with normal if-else statements. • For some kinds of conditionals, the if-elif might be easier to put together. • See Part B of assignment 1, for example. CISC101 - Prof. McLeod

22. “Linear” Program Demo • Write a program that uses just input, output and simple calculations. • How robust is the resulting program? CISC101 - Prof. McLeod

23. Area of a Triangle • Algorithm: • Obtain three side lengths from the user – side1, side2, side3. • Calculate the area of the triangle. • Display the result. CISC101 - Prof. McLeod

24. Area of a Triangle, Cont. • The area can be calculated using what is commonly called "Heron's Rule" after Heron of Alexandria, even though the theorem was first developed by Archimedes.  Given the three sides, a, b and c: CISC101 - Prof. McLeod

25. Area of a Triangle, Cont. • See TriangleArea.py CISC101 - Prof. McLeod