Program and Programming StyleGoals of this LectureOverviewPart 1: Program StyleProgram StyleSlide 6Structure: SpacingStructure: Indentation (cont.)Slide 9Structure: “Paragraphs”Slide 11Structure: ExpressionsStructure: Expressions (cont.)Slide 14Slide 15C IdiomsNamingCommentsComments (cont.)Slide 20Function CommentsFunction Comments (cont.)Slide 23ModularityPart 2: Programming StyleBottom-Up Design is BadTop-Down Design is GoodTop-Down Design in RealityExample: Text FormattingExample Input and OutputThinking About the ProblemWriting the ProgramThe Top LevelReading a WordReading a Word (cont.)Slide 36Saving a WordSaving a Word (cont.)Printing the Last LineDeciding When to PrintPrinting with JustificationPrinting with Justification (cont.)Slide 43Clearing the LineModularity: Summary of ExampleSummaryAppendix: The “justify” ProgramSlide 48Slide 49Slide 50Slide 51Slide 521Program andProgramming StyleThe material for this lecture is drawn, in part, fromThe Practice of Programming (Kernighan & Pike) Chapter 1Professor Jennifer Rexfordhttp://www.cs.princeton.edu/~jrex2Goals of this Lecture•Help you learn about:•Good program (noun) style•Good programming (verb) style•Why?•A large program that is well styled is •easier to maintain and•more likely to be correct than a large program that is poorly styled•A power programmer knows the qualities of a well styled program, and how to go about developing one3Overview•Program style: qualities of a good program•Well structured•Uses common idioms•Uses descriptive names•Contains proper comments•Modular•Programming style: how to create a good program•Top-down design•Successive refinement•Example: left and right justifying text4Part 1: Program Style5Program Style•Who reads your code?•The compiler•Other programmerstypedef struct{double x,y,z}vec;vec U,black,amb={.02,.02,.02};struct sphere{ vec cen,color;double rad,kd,ks,kt,kl,ir}*s,*best,sph[]={0.,6.,.5,1.,1.,1.,.9, .05,.2,.85,0.,1.7,-1.,8.,-.5,1.,.5,.2,1.,.7,.3,0.,.05,1.2,1.,8.,-.5,.1,.8,.8, 1.,.3,.7,0.,0.,1.2,3.,-6.,15.,1.,.8,1.,7.,0.,0.,0.,.6,1.5,-3.,-3.,12.,.8,1., 1.,5.,0.,0.,0.,.5,1.5,};yx;double u,b,tmin,sqrt(),tan();double vdot(A,B)vec A ,B;{return A.x*B.x+A.y*B.y+A.z*B.z;}vec vcomb(a,A,B)double a;vec A,B;{B.x+=a* A.x;B.y+=a*A.y;B.z+=a*A.z;return B;}vec vunit(A)vec A;{return vcomb(1./sqrt( vdot(A,A)),A,black);}struct sphere*intersect(P,D)vec P,D;{best=0;tmin=1e30;s= sph+5;while(s--sph)b=vdot(D,U=vcomb(-1.,P,s-cen)),u=b*b-vdot(U,U)+s-rad*s -rad,u=u0?sqrt(u):1e31,u=b-u1e-7?b-u:b+u,tmin=u=1e-7&&u<tmin?best=s,u: tmin;return best;}vec trace(level,P,D)vec P,D;{double d,eta,e;vec N,color; struct sphere*s,*l;if(!level--)return black;if(s=intersect(P,D));else return amb;color=amb;eta=s-ir;d= -vdot(D,N=vunit(vcomb(-1.,P=vcomb(tmin,D,P),s-cen )));if(d<0)N=vcomb(-1.,N,black),eta=1/eta,d= -d;l=sph+5;while(l--sph)if((e=l -kl*vdot(N,U=vunit(vcomb(-1.,P,l-cen))))0&&intersect(P,U)==l)color=vcomb(e ,l-color,color);U=s-color;color.x*=U.x;color.y*=U.y;color.z*=U.z;e=1-eta* eta*(1-d*d);return vcomb(s-kt,e0?trace(level,P,vcomb(eta,D,vcomb(eta*d-sqrt (e),N,black))):black,vcomb(s-ks,trace(level,P,vcomb(2*d,N,D)),vcomb(s-kd, color,vcomb(s-kl,U,black))));}main(){printf("%d %d\n",32,32);while(yx<32*32) U.x=yx%32-32/2,U.z=32/2-yx++/32,U.y=32/2/tan(25/114.5915590261),U=vcomb(255., trace(3,black,vunit(U)),black),printf("%.0f %.0f %.0f\n",U);}This is a working ray tracer! (courtesy of Paul Heckbert)6Program Style•Why does program style matter?•Bugs often caused by programmer’s misunderstanding •What does this variable do?•How is this function called?•Good code = human readable code•How can code become easier for humans to read?•Convey program structure•Use common idioms•Choose descriptive names•Compose proper comments•Use modularity7Structure: Spacing•Use readable/consistent spacing•Example: Assign each array element a[j] to the value j.•Bad code•Good code•Can often rely on auto-indenting feature in editorfor (j=0;j<100;j++) a[j]=j;for (j=0; j<100; j++) a[j] = j;8Structure: Indentation (cont.)•Use readable/consistent indentation•Example: Checking for leap year (does Feb 29 exist?)if (month == FEB) { if (year % 4 == 0) if (day > 29) legal = FALSE; else if (day > 28) legal = FALSE;} if (month == FEB) { if (year % 4 == 0) { if (day > 29) legal = FALSE; } else { if (day > 28) legal = FALSE; }} Wrong code (else matches “if day > 29”)Right code9Structure: Indentation (cont.)•Use “else-if” for multi-way decision structures•Example: Comparison step in a binary search.•Bad code•Good codeif (x < v[mid]) high = mid – 1; else if (x > v[mid]) low = mid + 1; else return mid; if (x < v[mid]) high = mid – 1;else if (x > v[mid]) low = mid + 1; else return mid;245781017low=0high=6mid=310xv10Structure: “Paragraphs”•Use blank lines to divide the code into key parts#include <stdio.h>#include <stdlib.h>int main(void)/* Read a circle's radius from stdin, and compute and write its diameter and circumference to stdout. Return 0 if successful. */{ const double PI = 3.14159; int radius; int diam; double circum; printf("Enter the circle's radius:\n"); if (scanf("%d", &radius) != 1) { fprintf(stderr, "Error: Not a number\n"); exit(EXIT_FAILURE); /* or: return EXIT_FAILURE; */ }…11Structure: “Paragraphs”•Use blank lines to divide the code into key parts diam = 2 * radius; circum = PI * (double)diam; printf("A circle with radius %d has diameter %d\n", radius, diam); printf("and circumference %f.\n", circum); return 0;}12Structure: Expressions•Use natural form of expressions•Example: Check if integer n satisfies j < n < k•Bad code•Good code•Conditions should read as you’d say them aloud•Not “Conditions shouldn’t read as you’d never say them aloud”!if (!(n >= k) && !(n <= j))if ((j < n) && (n < k))13Structure: Expressions (cont.)•Parenthesize to resolve ambiguity•Example: Check if integer n satisfies j < n < k•Moderately bad code•Moderately better code•Good to make the groupings explicit•Relational operators (e.g., “>”) have precedence over logical operators (e.g., “&&”), but who can remember these things?if ((j < n) && (n < k)) if (j < n && n < k)14Structure: Expressions (cont.)•Parenthesize to resolve ambiguity (cont.)•Example: read and print character until the
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