CS 100Lecture 51 CS100J Lecture 5 n Previous Lecture –Programming Concepts n Rules of thumb –learn and use patterns –inspiration from hand-working problem –boundary conditions –validation n Pattern for processing input values up to (but not including) a stopping signal –Example n Processing exam grades –Java Constructs n Casts and Rounding –Reading, Lewis & Loftus, Section 3.9 n This Lecture –Programming Concepts n Programming by stepwise refinement –a pattern –sequential refinement –case analysis –iterative refinement n Use of comments as higher-level statements

CS 100Lecture 52 Programming By Stepwise Refinement n An “algorithm” for you to follow when writing a program that solves problem P: if ( P is simple enough to code immediately ) if ( P is simple enough to code immediately ) Write the code that solves P; Write the code that solves P; else { else { Refine P into subproblems; Refine P into subproblems; Write the code that solves each subproblem; Write the code that solves each subproblem; } n Stepwise refinement is an example of –a divide-and-conquer algorithm, because it breaks problems down into simpler parts, –a recursive algorithm, because you use the same “algorithm” to write the code for any given subproblem. n The refinement of P into subproblems must include a description of how the code segments solving the subproblems combine to form code that solves P. n You can write the code segments that solve the subproblems in any order.

CS 100Lecture 53 Ways to Refine P into Subproblems n A program pattern –Do whatever n times –Process input values up until (but not including) a stopping value. n A sequential refinement n A case analysis n An iterative refinement

CS 100Lecture 54 Sequential Refinement n The refinement is structured so that solving P 1 through P n in sequence solves P. /* Solve problem P */ { /* Solve subproblem P 1 */ /* Solve subproblem P 1 */ /* Solve subproblem P 2 */ /* Solve subproblem P 2 */ /* Solve subproblem P n */ /* Solve subproblem P n */ }

CS 100Lecture 55 Sequential Refinement, cont. n Example 1: /* Drive from Ithaca to NYC. */ { /* Drive from Ithaca to Binghamton */ /* Drive from Ithaca to Binghamton */ /* Drive from Binghamton to NYC */ /* Drive from Binghamton to NYC */ } n Example 2: /* Drive from Ithaca to NYC. */ { /* Drive from Ithaca to Albany. */ /* Drive from Ithaca to Albany. */ /* Drive from Albany to NYC. */ /* Drive from Albany to NYC. */ }

CS 100Lecture 56 Sequential Refinement, cont. n Example 1, continued /* Drive from Ithaca to NYC. */ { /* Drive from Ithaca to Binghamton. */ /* Drive from Ithaca to Binghamton. */ /* Drive from Binghamton to NYC. */ /* Drive from Binghamton to NYC. */ /* Drive from Binghamton to /* Drive from Binghamton to Stroudsburg. */ Stroudsburg. */ /* Drive from Stroudsburg to /* Drive from Stroudsburg to NYC. */ NYC. */ }

CS 100Lecture 57 Sequential Refinement, cont. n Example 3 Let X 1,…,X n be an ordered sequence of variables. Let X 1,…,X n be an ordered sequence of variables. Let k be an integer between 1 and n. Let k be an integer between 1 and n. /* Rotate the values in X 1,…,X n left k places, where values shifted off the left end reenter at the right. */ E.g., suppose k = 3, n = 7, and the x’s contain letters. Before: x 1 x 2 x 3 x 4 x 5 x 6 x 7 x 1 x 2 x 3 x 4 x 5 x 6 x 7 a b c d e f g a b c d e f g After: x 1 x 2 x 3 x 4 x 5 x 6 x 7 x 1 x 2 x 3 x 4 x 5 x 6 x 7 d e f g a b c d e f g a b c

CS 100Lecture 58 Sequential Refinement, cont. /* Rotate the values in X 1,…,X n left k places, where values shifted off the left end reenter at the right. */ { /* Reverse the order of X 1,…,X k */ /* Reverse the order of X 1,…,X k */ /* Reverse the order of X k+1,…,X n */ /* Reverse the order of X k+1,…,X n */ /* Reverse the order of X 1,…,X n */ /* Reverse the order of X 1,…,X n */ }

CS 100Lecture 59 Sequential Refinement, cont. n Example 3, continued E.g., suppose k = 3 and n = 7 x 1 x 2 x 3 x 4 x 5 x 6 x 7 x 1 x 2 x 3 x 4 x 5 x 6 x 7 a b c d e f g a b c d e f g c b a d e f g c b a d e f g c b a g f e d c b a g f e d d e f g a b c d e f g a b c time 

CS 100Lecture 510 Case Analysis n The refinement is structured so that solving one of the subproblems P 1,…,P n solves P. Which P i is solved is determined during execution by testing conditions. /* Solve problem P */ if (P is an instance of case 1) /* Solve subproblem 1 */ /* Solve subproblem 1 */ else if (P is an instance of case 2) /* Solve subproblem 2 */ /* Solve subproblem 2 */ else if (P is an instance of case 3) /* Solve subproblem 3 */ /* Solve subproblem 3 */ else /* P is an instance of case n */ /* Solve subproblem n */ /* Solve subproblem n */......

CS 100Lecture 511 Case Analysis, cont. n Example 1 /* Let x be the absolute value of y. */ if ( y < 0 ) if ( y < 0 ) /* Let x be -y. */ /* Let x be -y. */ else else /* Let x be y. */ /* Let x be y. */ n Example 2 /* Let x be the absolute value of y. */ x = Math.abs(y); x = Math.abs(y); I.e., sometimes case analysis is counter-productive and there is a uniform way to solve the problem.

CS 100Lecture 512 Iterative Refinement n The refinement of P is structured so that repeated solution of subproblem P’ eventually solves P. /* Solve problem P */ while (P has not yet been solved) while (P has not yet been solved) /* Solve subproblem P’ */ /* Solve subproblem P’ */ n Question: How can repeatedly doing P’ solve P ? n Answer: P’ must be parameterized in terms of some state. Each execution of P’ must change the state so that progress is made, i.e., with each iteration, we move to a state that is “closer” to a solution for P. n The notion of “distance” must be well-founded, i.e., it must converge to 0 in a finite number of steps that get “closer”. n Different notions of “distance” lead to different programs.

CS 100Lecture 513 Iterative Refinement, cont. n Example: Running a maze /* There are n 2 rooms arranged in an n-by-n grid. Some adjacent rooms have connecting doors. No doors lead outside. You are in the upper-leftmost room facing left. A sequence of doors leads to the lower- rightmost room. Get there. */ n Rule of thumb: Work some test cases by hand. Inspiration: Keep your left hand on the wall and keep walking (the left-hand rule).

CS 100Lecture 514 Iterative Refinement, cont. /* There are n 2 rooms arranged in an n-by-n grid. Some adjacent rooms have connecting doors. No doors lead outside. You are in the upper- leftmost room facing left. A sequence of doors leads to the lower-rightmost room. Get there. */ while ( you are not in lower rightmost room ) while ( you are not in lower rightmost room ) /* Get “closer” to lower rightmost room. */ /* Get “closer” to lower rightmost room. */ n Different notions of “closer” lead to different versions of /* Get “closer” to lower rightmost room. */ /* Get “closer” to lower rightmost room. */ Notion A: wall length away, using the “left-hand rule”. Notion B: # of rooms away, using the “left-hand rule”.

CS 100Lecture 515 Iterative Refinement, cont. n Refinement A (using wall-length distance) /* There are n 2 rooms arranged in an n-by-n grid. Some adjacent rooms have connecting doors. No doors lead outside. You are in the upper- leftmost room facing left. A sequence of doors leads to the lower-rightmost room. Get there. */ while ( you are not in lower rightmost room ) while ( you are not in lower rightmost room ) /* Get at least one wall “closer” to /* Get at least one wall “closer” to the lower rightmost room. */ the lower rightmost room. */ if (you are facing a door){ if (you are facing a door){ Go through the door; Go through the door; Turn 90 degrees counter-clockwise; Turn 90 degrees counter-clockwise; } else else Turn 90 degrees clockwise; Turn 90 degrees clockwise;

CS 100Lecture 516 Iterative Refinement, cont. n Refinement B (using #rooms distance) /* There are n 2 rooms arranged in an n-by-n grid. Some adjacent rooms have connecting doors. No doors lead outside. You are in the upper- leftmost room facing left. A sequence of doors leads to the lower-rightmost room. Get there. */ while ( you are not in lower rightmost room ) while ( you are not in lower rightmost room ) { /* Get at least one room “closer” to /* Get at least one room “closer” to the lower rightmost room. */ the lower rightmost room. */ while (you are not facing a door) while (you are not facing a door) Turn 90 degrees clockwise; Turn 90 degrees clockwise; Go through door; Go through door; Turn 90 degrees counter-clockwise; Turn 90 degrees counter-clockwise; }