While Loops Announcements for This Lecture Assignments Prelim 2 - PowerPoint PPT Presentation

Lecture 22 While Loops

Announcements for This Lecture Assignments Prelim 2 • A6 due on Wednesday • TONIGHT , 5:15 or 7:30 § First two should be done § K – Z at 5:15pm § Start Algorithm by weekend § A – J at 7:30 pm § Next Week : Partition/Update § See website for room § Conflicts received e-mail • A7 will be last assignment § Will talk about next week • Will have 4-5 questions § Posted on Wednesday § Might drop short answer • There is lab next week § Similar to previous years § No lab week of Turkey Day • Graded by the weekend 11/8/18 While-Loops 2

Recall: For Loops # Print contents of seq The for-loop: x = seq[0] for x in seq: print(x) print(x) x = seq[1] print(x) … • Key Concepts x = seq[len(seq)-1] § loop sequence: seq print(x) § loop variable : x § body : print(x) § Also called repetend 11/8/18 While-Loops 3

Important Concept in CS: Doing Things Repeatedly 1. Process each item in a sequence § Compute aggregate statistics for a dataset, for x in sequence: such as the mean, median, standard deviation, etc. process x § Send everyone in a Facebook group an appointment time 2. Perform n trials or get n samples. § A4: draw a triangle six times to make a hexagon for x in range(n): Run a protein-folding simulation for 10 6 time steps § do next thing 3. Do something an unknown number of times ???? § CUAUV team, vehicle keeps moving until reached its goal 11/8/18 While-Loops 4

Beyond Sequences: The while-loop while < condition >: statement 1 repetend or body … statement n • Relationship to for-loop § Broader notion of “still stuff to do” § Must explicitly ensure true condition repetend condition becomes false § You explicitly manage false what changes per iteration 11/8/18 While-Loops 5

While-Loops and Flow print('Before while') Output: count = 0 Before while i = 0 Start loop 0 while i < 3: End loop print('Start loop '+str(i)) Start loop 1 count = count + i End loop i = i + 1 Start loop 2 print('End loop ') End loop print('After while') After while 11/8/18 While-Loops 6

while Versus for # process range b..c-1 # process range b..c-1 for k in range(b,c) k = b while k < c: process k process k Must remember to increment k = k+1 # process range b..c # process range b..c for k in range(b,c+1) k = b while k <= c: process k process k k = k+1 11/8/18 While-Loops 7

Range Notation • m..n is a range containing n+1-m values § 2..5 contains 2, 3, 4, 5. Contains 5+1 – 2 = 4 values § 2..4 contains 2, 3, 4. Contains 4+1 – 2 = 3 values § 2..3 contains 2, 3. Contains 3+1 – 2 = 2 values § 2..2 contains 2. Contains 2+1 – 2 = 1 values § 2..1 contains ??? A: nothing B: 2,1 C: 1 What does 2..1 contain? D: 2 E: something else 11/8/18 While-Loops 8

Range Notation • m..n is a range containing n+1-m values § 2..5 contains 2, 3, 4, 5. Contains 5+1 – 2 = 4 values § 2..4 contains 2, 3, 4. Contains 4+1 – 2 = 3 values § 2..3 contains 2, 3. Contains 3+1 – 2 = 2 values § 2..2 contains 2. Contains 2+1 – 2 = 1 values § 2..1 contains ??? • The notation m..n, always implies that m <= n+1 § So you can assume that even if we do not say it § If m = n+1, the range has 0 values 11/8/18 While-Loops 9

while Versus for # incr seq elements # incr seq elements for k in range(len(seq)): k = 0 while k < len(seq): seq[k] = seq[k]+1 seq[k] = seq[k]+1 k = k+1 Makes a range object . while is more flexible, but requires more code to use 11/8/18 While-Loops 10

Patterns for Processing Integers range a..b-1 range c..d i = a i= c while i < b: while i <= d: process integer i process integer i i= i + 1 i = i + 1 # store in count # of '/'s in String s # Store in double var. v the sum count = 0 # 1/1 + 1/2 + …+ 1/n i = 0 v = 0; # call this 1/0 for today while i < len(s): i = 1 if s[i] == '/': while i <= n: count= count + 1 v = v + 1.0 / i i= i +1 i= i +1 # count is # of '/'s in s[0..s.length()-1] # v= 1/1 + 1/2 + …+ 1/n 11/8/18 While-Loops 11

while Versus for # table of squares to N # table of squares to N seq = [] seq = [] n = floor(sqrt(N)) + 1 k = 0 for k in range(n): while k*k < N: seq.append(k*k) seq.append(k*k) k = k+1 A while loop can use A for-loop requires that complex expressions to you know where to stop check if the loop is done the loop ahead of time 11/8/18 While-Loops 12

while Versus for Fibonacci numbers: F 0 = 1 F 1 = 1 F n = F n –1 + F n –2 # Table of n Fibonacci nums # Table of n Fibonacci nums fib = [1, 1] fib = [1, 1] for k in range(2,n): while len(fib) < n: fib.append(fib[-1] + fib[-2]) fib.append(fib[-1] + fib[-2]) Sometimes you do not use Do not need to have a loop the loop variable at all variable if you don’t need one 11/8/18 While-Loops 13

Cases to Use while Great for when you must modify the loop variable # Remove all 3's from list t # Remove all 3's from list t while 3 in t: i = 0 while i < len(t): t.remove(3) # no 3’s in t[0..i–1] if t[i] == 3: del t[i] else: i = i+1 11/8/18 While-Loops 14

Cases to Use while Great for when you must modify the loop variable # Remove all 3's from list t # Remove all 3's from list t while 3 in t: i = 0 while i < len(t): t.remove(3) # no 3’s in t[0..i–1] if t[i] == 3: The stopping condition is not del t[i] a numerical counter this time. Stopping else: Simplifies code a lot. point keeps i = i+1 changing. 11/8/18 While-Loops 15

Cases to Use while def sqrt(c): • Want square root of c """Return: square root of c § Make poly f (x) = x 2 - c Uses Newton’s method § Want root of the poly Pre: c >= 0 (int or float)""" ( x such that f ( x ) is 0) x = c/2 • Use Newton’s Method # Check for convergence § x 0 = GUESS ( c /2??) while abs(x*x – c) > 1e-6: § x n +1 = x n – f ( x n )/ f ' ( x n ) # Get x n+1 from x n = x n – ( x n x n - c )/(2 x n ) x = x / 2 + c / (2*x) = x n – x n /2 + c /2 x n return x = x n /2 + c /2 x n § Stop when x n good enough 11/8/18 While-Loops 16

Recall Lab 9 Welcome to CS 1110 Blackjack. Rules: Face cards are 10 points. Aces are 11 points. All other cards are at face value. Your hand: 2 of Spades 10 of Clubs Dealer's hand: Play until player 5 of Clubs stops or busts Type h for new card, s to stop: 11/8/18 While-Loops 17

Recall Lab 9 Welcome to CS 1110 Blackjack. Rules: Face cards are 10 points. Aces are 11 points. All other cards are at face value. Your hand: How do we design 2 of Spades this as a loop? 10 of Clubs Dealer's hand: Play until player 5 of Clubs stops or busts Type h for new card, s to stop: 11/8/18 While-Loops 18

Recall Lab 9 halted = False while not game.playerBust() and not halted: # ri: input received from player ri = input('Type h for new card, s to stop: ') halted = (ri == 's') if (ri == 'h'): game.playerHand.append(game.deck.pop(0) print('You drew the ' + str(game.playerHand[-1]) +'\n') 11/8/18 While-Loops 19

Recall Lab 9 halted = False Explicit loop variable while not game.playerBust() and not halted: # ri: input received from player ri = input('Type h for new card, s to stop: ') halted = (ri == 's') Set to False to break the loop if (ri == 'h'): game.playerHand.append(game.deck.pop(0) print('You drew the ' + str(game.playerHand[-1]) +'\n') 11/8/18 While-Loops 20

Recall Lab 9 halted = False More than one way to stop while not game.playerBust() and not halted: # ri: input received from player ri = input('Type h for new card, s to stop: ') halted = (ri == 's') if (ri == 'h'): game.playerHand.append(game.deck.pop(0) print('You drew the ' + str(game.playerHand[-1]) +'\n') 11/8/18 While-Loops 21

Using while-loops Instead of for-loops Advantages Disadvantages • Better for modifying data • Performance is slower § More natural than range § Python optimizes for -loops § Works better with deletion § Cannot optimize while • Better for convergent tasks • Infinite loops more likely § Loop until calculation done § Easy to forget loop vars § Exact steps are unknown § Or get stop condition wrong • Easier to stop early • Debugging is harder § Just set loop var to False § Will see why in later lectures 11/8/18 While-Loops 22

Our Goal From Here: Sorting 0 i 0 i 2 5 6 3 4 2 3 5 6 4 0 i 0 i 2 5 3 6 5 2 4 5 4 6 Will see how to do this with while -loops 0 i 0 i 2 3 5 6 5 2 3 4 5 6 0 i 0 2 3 5 6 4 2 3 4 5 6 11/8/18 While-Loops 23