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Q and A for Section 5.1

Q and A for Section 5.1 . CS 106, Fall 2012. While loop syntax. Q: The syntax for a while statement is: while _______________ : _____________ A: while <boolean expression>: <body>. or, condition. Difference from for loop.

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Q and A for Section 5.1

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  1. Q and A for Section 5.1 CS 106, Fall 2012

  2. While loop syntax Q: The syntax for a while statement is: while _______________ : _____________ A: while <boolean expression>: <body> or, condition

  3. Difference from for loop Q: How often does a while loop execute its <body>? A: until the boolean condition is False. Note: that’s how it differs from for loop.

  4. Changing the condition Q: Suppose you have a while loop like this: while var1 <op> var2: <body> Is it good/bad/ugly to alter the value of var1 or var2 in the body of the loop? A: You almost *always* alter the value of var1 or var2 in the body, or you would have an infinite loop.

  5. Newton’s Method getSqrtOf = float(raw_input(“Enter a number: “)) # Set oldEst and newEst to any initial values oldEst = 1.0 newEst = 4.0 # while last 2 estimates are quite different while abs(newEst - oldEst) > 0.0000001: # store previous estimate in oldEst oldEst = newEst # compute a new estimate and print it newEst = (oldEst + getSqrtOf/ oldEst) / 2.0

  6. Q: Thwarting an infinite loop Q: How would we make sure the following code does not run forever (without changing do_stuff() or update_x())? while some_cond(x): do_stuff() x = update_x(x)

  7. A: Thwarting an infinite loop num_runs= 0 while some_cond(x) and num_runs < 10000: do_stuff() x = update_x(x) num_runs += 1

  8. continue statement • Used only within a loop body • in loop body itself, or within a body within the loop body. • for while loops and for loops. • Makes control go back to the top of the loop. • Often used when code detects that nothing else needs to be done with the current element of the loop.

  9. One use of continue: to filter # Plot each earthquake, skipping comment lines # and the header line. for line in input_lines: fields = line.split() if len(fields) == 0: # skip blank line continue if fields[0].startsWith("#"): # skip comment lines continue if fields[0] == 'Src': continue # do stuff with fields.

  10. break statement • also used only in loop body -- for loop or while loop. • causes control to pass to first line after end of loop body. • i.e., you "break" out of the loop. • useful for when searching through a sequence until you find what you are looking for. • allows you to make a “mid-loop test”.

  11. Mid-loop test • while statement is pre-loop test • Other languages have post-loop test: repeat: do_stuff() until <boolexpris False> • Using while True and break, you can do mid-loop test. (Useful for getting user input.) while True: do_stuff() if <boolexpr>: break do_other_stuff()

  12. Example of use of break # find if num is a prime isPrime = True # assume it is prime for div in range(2, num / 2 + 1): if num % div == 0: # divides with no remainder # found a divisor so numis not prime isPrime = False break # isPrime is True mean num is a prime number.

  13. Example of use of break # looking for first room with someone in it. found = False for room in rooms: if not room.is_empty(): found = True break if found: room.invite_to_dance_the_night_away()

  14. Practice while loops Problem: Convert the following into an index-based loop: for g in groceries: print g Answer: for i in range(len(groceries)): print groceries[i]

  15. Practice while loops Problem: Convert the following into a while loop: for g in groceries: print g Answer: i = 0 while i < len(groceries): print groceries[i] i += 1

  16. More practice: robots and obstacles Problem: Given a function seeObstacle() that returns True when an obstacle is in front of a robot, write an algorithm to have the robot detect and print out distance to the 8 obstacles around it at N, NE, E, SE, S, SW, W, NW.

  17. More practice: robots and obstacles Solution: for dir in range(8): # 8 directions dist = 0 while not seeObstacle(): goForward(5) # cm. dist += 5 print “distance is”, dist goBackward(dist) turnRight(45) # degrees

  18. Average of numbers Problem: write code to prompt the user for a number, until the user enters “q”. When the user enters “q”, print the average of the numbers that were entered.

  19. Average of numbers total = 0 numNums = 0 while True: num = raw_input(“Enter a number, q=quit:”) if num == “q”: break total += float(num) numNums += 1 if numNums == 0: print “No numbers entered” else: print “Average is “, float(total) / numNums

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