Lecture 16-While Loop

Lecture 16-While Loop - CS1110 Lecture 16, 26 Oct 2010...

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Unformatted text preview: CS1110 Lecture 16, 26 Oct 2010 Reading for next time: Ch. 8.1-8.3 (arrays) While-loops Beyond ranges of integers: the while loop while (<condition>) { sequence of statements } boolean expression. = "there's still stuff to do" the <repetend> Prelim 2: Tu Nov 9th, 7:30-9pm. Last name A-Lewis: Olin 155 Last name Li-Z: Olin 255 Conflicts? Submit CMS "assignment" "P2 conflicts" by today. Review session: Sun Nov 7th, 1-3pm, Phillips 101. (Set your clocks back the night before!) Reminder: A5 due Sat. Oct 30th. See assignments pg for hints on snowflake geometry. condition true repetend false A mystery (due to von Neumann): suppose you have a biased coin with unknown probability of heads p, 0 < p < 1. How can you use this coin to simulate the output of a fair coin? (Answer: while-loops …) 1 In comparison to for-loops: we get a broader notion of “there’s still stuff to do” (not tied to integer ranges). But we must ensure that “condition” stops holding, since there’s no explicit increment. 2 Canonical while loops // Process b..c for (int k= b; k <= c; k= k+1) { Process k; } Scope note: since k happened to be declared "within" the loop, k can’t be used after the loop. Here’s one way to use the while loop // Process b..c int k= b; while (k <= c) { Process k; k= k+1; } Interesting while loops (showing why they can be hard to understand) /** Von Neumann’s “fair coin” from unfair coin, assuming heads prob not 0 or 1. Encode heads/tails as true/false. = "output is heads". */ public static boolean fairFlip() { while (true) { // loop "forever"… boolean f1= new unfair flip; boolean f2= new unfair flip; if (f1 && !f2) { // HT return true; // escape the loop } else if (!f1 && f2) { // TH return false; } } 3 /** open question in mathematics: is there an n such that this function never returns a value (i.e., n doesn't "return to 1")? Precondition: n >=1. */ public static boolean collatz(int n) { while (n != 1) { } return true; } 4 if (n%2 == 0) { } else { } n= 3*n +1; n= n/2; /* process a sequence of inputs where you don't know how many need to be taken care of */ <initialization>; while (<still input to deal with>) { Process next piece of input; make ready for the next piece of input; } } How to analyze loops: understanding assertions about lists 0 1 2 3 4 5 6 7 8 v X Y Z X A D C C C v is (the name of) a list of Characters. (We aren't showing v as a variable to save space.) This is an assertion about v and k, thus explaining the meaning of these variables. It is true because v[0..3] are not ‘C’ and v[6..8] are ‘C’s. This falsely asserts that v[0..3] aren't C's, v[5..8] are ‘C’s. True assertion that v[0..-1] aren't C's (nothing in the empty list is a C) True assertion: v[0..3] are not C's, v[4] is A, v[5] is D, v[6..8] are C's.) 5 Counting characters. Store in n the number of '/'s in string s. // Store in n to truthify diagram R k= 0; n= 0; // inv: See diagram P, below while ( k!= s.length() ) { 1. How does it start? ((how) does init. make inv true?) the invariant and the falsity of loop condition, deduce that result holds.) k ? s.length() n is # of '/'s here s.length() 0 3 v not C's k ? all C’s 8 k 6 if (s.charAt(k)=='/') {n= n + 1;} 2. When does it stop? (From k= k + 1; } 0 3 v not C 's 0 not C's k ? k ? k AD all C's 8 k 5 8 k 8 all C’s k 4 0 v 0 P: s n is # of '/'s here 0 R: s 3. (How) does it make progress toward termination? 4. How does repetend keep invariant true? 6 0 v not C's 1 Suppose we are thinking of this while loop: initialization; while ( B ) { repetend } We add the postcondition and also show where the invariant must be true: initialization; // invariant: P while ( B ) { // { P and B} repetend // { P } } // { P and !B } // { Result R } The four loopy questions Second box helps us develop four loopy questions for developing or understanding a loop: 1. How does loop start? Initialization must truthify invariant P. 2. When does loop stop? At end, P and !B are true, and these must imply R. Find !B that satisfies P && !B => R. 3. Make progress toward termination? Put something in repetend to ensure this. 4. How to keep invariant true? Put something in repetend to ensure this. 7 Linear search. Character c is in String s. Find its first position. // Store in k to truthify diagram R k= 0; // inv: See diagram P, below while ( s.charAt(k) != c ) { Idea: Start at beginning of s, looking for c; stop when found. How to express as an invariant? 1. How does it start? ((how) does init. make inv true?) 2. When does it stop? (From the invariant and the falsity of loop condition, deduce that result holds.) k= k + 1; } 0 P : s c not here 0 R: s c not here k ? k c s.length() 3. (How) does it make progress toward termination? s.length() 4. How does repetend keep ? invariant true? 8 Appendix examples: Develop loop to store in x the sum of 1..100. Roach infestation /** = number of weeks it takes roaches to fill the apartment --see p 244 of text*/ public static int roaches() { double roachVol= .001; // Space one roach takes double aptVol= 20*20*8; // Apartment volume double growthRate= 1.25; // Population growth rate per week int w= 0; // number of weeks int pop= 100; // roach population after w weeks // inv: pop = roach population after w weeks AND // before week w, volume of the roaches < aptVol while (aptVol > pop * roachVol ) { pop= (int) (pop * growthRate); w= w + 1; } return w; } 9 10 We’ll keep this definition of x and k true:  x = sum of 1..k–1 1. How should the loop start? Make range 1..k–1 Four loopy empty: k= 1; x= 0; questions 2. When can loop stop? What condition lets us  know that x has desired result? When k == 101 3. How can repetend make progress toward termination? k= k+1; 4. How do we keep def of x and k true? x= x + k; k= 1; x= 0; // invariant: x = sum of 1..(k–1) while ( k != 101) { x= x + k; k= k + 1; } // { x = sum of 1..100 } Iterative version of logarithmic algorithm to calculate b**c (we’ve seen a recursive version before). /** set z to b**c, given c ≥ 0 */ int x= b; int y= c; int z= 1; // invariant: z * x**y = b**c and 0 ≤ y ≤ c while (y != 0) { if (y % 2 == 0) { x= x * x; y= y/2; } else { z= z * x; y= y – 1; } } // { z = b**c } Calculate quotient and remainder when dividing x by y x/y = q + r/y 21/4= 4 + 3/4 Property: x = q * y + r and 0 ≤ r < y /** Set q to quotient and r to remainder. Note: x >= 0 and y > 0 */ int q= 0; int r= x; // invariant: x = q * y + r and 0 ≤ r while (r >= y) { r= r – y; q= q + 1; } // { x = q * y + r and 0 ≤ r < y } 11 12 2 ...
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This note was uploaded on 11/27/2010 for the course CS 9339 at Cornell.

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