1. Q and A for Section 4.4 CS 106, Fall 2015

2. If statement syntax Q: The syntax for an if statement (or conditional statement) is: if _______________ : _____________ A: if : When is this used? When you have to do something when a certain condition is true, and nothing in all other cases.

3. Use an if statement Q: Write code that prints out "Brilliant!" if the value in variable food is "spam". A: if food == "spam": print "Brilliant!"

4. Q3 Q: What happens if the value of food is "bubble and squeak"? A: Nothing.

5. Compound bool expression Q: Write code to print "Brilliant!" if food is "spam" and how_prepared is "on toast". A: if food == "spam" and how_prepared == "on toast": print "Brilliant!” If food is not “spam”, then short-circuiting is used: how_prepared won’t even be evaluated

6. if-else syntax Q: An if-else statement has this format: if _____________ : ___________ else: ___________ A: ; ; When is this used?: when there are only two choices/options, and you have to do something in each case.

7. Use if-else Q: Suppose you have a function is_prime(x) that returns True if x is a prime number. Write code to add x to a list primes if x is prime and to a list non_primes otherwise. A: if is_prime(x): primes.append(x) else: non_primes.append(x)

8. More practice Q: Write code that prints "NaN" if the list nums is empty or, otherwise, computes and prints the average of the list of floats. A: if len(nums) == 0: print "NaN" else: print sum(nums) / len(nums)

9. if-elif syntax An if-else statement has this format: if : elif : # elif : can repeat elif part # else: # else is optional When is this used? When there are > 2 choices to match and code has to do something different for each.

10. Counting Q: You are given a long string gen containing only As, Gs, Ts, and Cs. Write code to count the number of As and the number of Gs in the string. countAs = 0 countGs = 0 for ch in gen: if ch == 'A': countAs += 1 elif ch == 'G': countGs += 1

11. Evaluate a number Given a variable x referring to an number, print “negative”, “0”, or “positive”, as appropriate. A: if x < 0: print “negative” elif x == 0: print “0” else: print “positive”

12. Letter grades Q: Write code to print out a student's letter grade, based on the value in score. Pick your own scale. A: if score > 92: print 'A' elif score > 85: print 'B' elif score > 77: print 'C' elif score > 70: print 'D' else: print 'F'

13. Q11 Q: You have 2 operands, op1 and op2, and an operator, op, that is one of '+', '-', '*', '/'. Write code to compute the correct result into res. A: if op == '+': res = op1 + op2 elif op == '-': res = op1 - op2 elif op == '*': res = op1 * op2 elif op == '/': res = op1 / op2

14. Nested ifs Q: Write code to print "Brilliant!" if food is "spam" and how_prepared is "on toast", and to print "Awesome!" if how_prepared is "in a casserole". A: if food == "spam": if how_prepared == "on toast": print "Brilliant!" if how_prepared == "in a casserole": print "Awesome!"

15. Rewrite nested if as if-elif Q: Rewrite this code using if-elif: if i < 7: do_something(i) else: if j > 9: do_something_else(j) A: if i < 7: do_something(i) elif j > 9: do_something_else(j)

16. Nested if vs. elif Q: When would you prefer an if nested inside an else, over using an elif. A: An if-else is a binary decision: there are two choices. An if-elif-elif-else is an n-way decision: some variable can belong to n different “groups” or “classes”. If you are comparing a variable n to something, and then in one of the results have to “classify” some other variable, use a nested if.

17. Compute the max Given a list of positive numbers nums, write code that leaves the variable maxSeen referring to the largest number in nums. Don’t use the built-in max() function. maxSeen = 0 for num in nums: if num > maxSeen: maxSeen = num

18. Data cleansing data is a list of weather readings. Each reading is a list with 10 elements. The 3th value is a temperature reading. But sometimes our thermometer goes haywire and reports the temp as -999. In this case, we want to throw out the entire reading. Write a for loop to do this, building a new list called good_data that contains only the good readings. E.g.: data = [ [ 0, 1, 2, 78, 4, 5, 6, 7, 8, 9 ], [ 0, 1, 2, -999, 4, 5, 6, 7, 8, 9 ] ]

19. Answer good_data = [] for aReading in data: # aReading is a list with 10 elements. # Check if the temp is not bogus. if aReading[3] != -999: good_data.append(aReading) # now good_data is clean. Now, add code to also filter out humidity readings (in the 7 th field) when they are negative. Now, add code to also filter out any readings that don’t have 10 fields in them.

20. Non-booleans used as booleans Q: In the “for the guru” on page 142, there is code if not dinner:. Can you explain that? A: Why, yes. Yes, I can. The pattern is if : So, if the expression given is not a boolean ( dinner is a string), then it is converted to a boolean, as if bool(dinner) were called. In general, if a value is empty ( “” or [] ), the value is False; else, True.

21. Q10 Q: You have a list of Room objects, rooms. A Room object has a method anybody_home() that returns True if anyone is there. Write code to count how many rooms are not empty. A: someone_home = 0 for room in rooms: if room.anybody_home(): someone_home += 1 This is like a filter: do something only for certain elems in the sequence

22. Checking if a value is one of many Q: Write code that prints "Let's go" if the variable response equals either 'y' or 'yes'. A: if response in ('y', 'yes'): print "Let's go" (not if response == 'y' or 'yes' !!)

23. What kind of triangle? Suppose we have 3 values, s1, s2, and s3, that represent the lengths of the 3 sides of a triangle. Print out “equilateral” if the sides represent an equilateral triangle, “isosceles” for isosceles, or “scalene” otherwise.

24. Simulating bouncing particles Suppose we are simulating n particles bouncing off the inside of a container’s walls. We have 3 lists: turts is a list of Turtle objects; xvels is a list of x velocities; yvels is a list of y velocities. The next location of a turt is computed by: (turts[i].getX() + xvels[i], turts[i].getY() + yvels[i]) Write code to switch the velocities (from positive to negative or vice-versa) if turts[i] is currently within 3 pixels of the walls of a 600x400 canvas.

25. Q12 Q: What can we do to the previous code to print out "Oops!" if op contains a bad value? A: Add else: print 'Oops!'