英格兰萨里大学：《C语言》课程教学资源（讲义）Lecture 7 - Making Decisions

rrodcscLecture 7MakingDecisions.This lecture is about program “branching', where differentactions can occur depending upon the state of data.The simplest of which is the conditional expression or.query-colon ?:C has a conditional operator, ?:which is used in the formexpr1? expr2:expr3ooeaession? itmue : itabeRemember that in C, non-zero means true, zero means.?false.Ifexpr1 is true, the valueof theexpression isthatofexpr2; if expr1 is false then it is that of expr3Conditional Expression contIfStatementsBecausethis is anexpression,we can use itThe if statement is the bread and butter ofon theRHS of an assignment, eg.mostprogrammersacrossmanylanguagesy=(x>=7) 25: (17-x) ;.Cprovidesanifstatement,initssimplestformSoif (expr) ifx is 19, yis 5statement; ifxis 2, y is 15e-gif (x-y++,Asthe?:canmakecodea lttedifficulto read it isbestforshortexpressions:if exprevaluatestotrue (non zero),statementis executed; if not, statement is skipped.IfStatementsIfStatements? As with the conditional:Note that if several statements are to beoperator,itspossibletohaveexecuted,theymustbegroupedtwobranchestogethertoformablock,usingbraces0ir (expr)lude<atdio.hoTe Beple:wlt nnstatenentl:. e.g.1f (x--3)elseaciade (stdio.b)237statenent2,aint:Again, statement1 andint ajstatement2canbeblocksFeinttS

1 1. Introduction 2. Binary Representation 3. HardwareandSoftware 4. HighLevel Languages 5. Standard inputand output 6. Operators, expression and statements 7. MakingDecisions 8. Looping 9. Arrays 10. Basics of pointers 11. Strings 12. Basics of functions 13. Moreabout functions 14. Files 14. DataStructures 16. Casestudy:lottery number generator Lecture 7 Making Decisions • This lecture is about program “branching”, where different actions can occur depending upon the state of data. • The simplest of which is the conditional expression or query-colon ?: • C has a conditional operator, ?: which is used in the form expr1 ? expr2 : expr3 boolean expression ? if true : if false • Remember that in C, non-zero means true, zero means false. If expr1 is true, the value of the expression is that of expr2; if expr1 is false then it is that of expr3 Conditional Expression cont • Because this is an expression, we can use it on the RHS of an assignment, eg. y=(x>=7)?5:(17-x); • So – if x is 19, y is 5 – if x is 2, y is 15 • As the ?: can make code a little difficult to read it is best for short expressions /* Example: conditional expressions */ #include main() { int n, m, abs, max; printf("Enter a positive or negative integer: "); scanf("%i", &n); printf("\nYou entered %i.\n", n); abs = n m ? n : m; printf("%i is the larger value.\n", max); } If Statements • The if statement is the bread and butter of most programmers across many languages • C provides an if statement, in its simplest form if (expr) statement; e.g. if (x==3) y++; • if expr evaluates to true (non zero), statement is executed; if not, statement is skipped. expr statement T F If Statements • Note that if several statements are to be executed, they must be grouped together to form a block, using braces {} • e.g. if (x==3){ y++; z*=37; } /* Example: simple if statement */ #include main() { int n; printf("Enter an integer: "); scanf("%i", &n); printf("\nYou entered %i", n); if (n >= 0) printf(", which isn't negative.\n"); } If Statements • As with the conditional operator, its possible to have two branches if (expr) statement1; else statement2; • Again, statement1 and statement2 can be blocks expr statement1 statement2 /* Example: if.else statement */ #include main() { int n; printf("Enter an integer: "); scanf("%i", &n); printf("\nYou entered %i", n); if (n >= 0) printf(", which isn't negative.\n"); else printf(", which is negative.\n"); } T F

IfStatementsnestedif.cif1.bugif2.bug:What if we wish to have a 3-way(or more) choice ?Wecanusea"nested"if...elsestatementstatement2Nested basically means one insideanotheif(exprl)aoRRelseir(expr2)stateaent2Logical OperatorsLogicalOperatorsThis would makean equallycomplicated C program:ConsiderthefollowingFortunately C provides three logical operators to getaround this.II-OR and !-NOT&&-AND,"lonlyattend lectures on Monday at 10orns both a and b must be true for the result to betrua&&bronThursdayat10,otherwise/sayinbed"allbmeans either a or b (or both) must be true for the result to be truemeans amustbefalsefortheresult tobetrue&& has higher precedence thanl.This could be described by a rather·Sowecouldcodeitasla0--30complicatedflow diagram involving four.c."if"boxes:atam_la_bed(1;Sometimesif (a==0)iswrittenif (!a)leapyear.cSwitchStatementsyA?Cprovidesa statement formultiplechoices,based on the value of an integer variable:+4*9:breakcausestheprogramtojumptotheendoftheswitchstatement2

2 If Statements • What if we wish to have a 3-way (or more) choice ? • We can use a “nested” if.else statement • Nested basically means one inside another if (expr1) statement1; else if (expr2) statement2; else statement n; expr1 statement1 expr1 statement2 statement n T T F /* Example: nested if.else statements */ #include main() { int n; printf("Enter an integer: "); scanf("%i", &n); printf("\nYou entered %i", n); if (n > 0) printf(", which is positive.\n"); else if (n main() { int lucky; printf("Enter your lucky number: "); scanf("%i", &lucky); if (lucky = 7) /* BUG */ printf("Your lucky number is %i, same as mine!\n", lucky); else printf("Your lucky number is %i\n", lucky); } /* BUG ZONE!!! Example: if statement */ #include main() { int dialled = 123; printf("You dialled %i\n", dialled); if (dialled == 999); /* BUG */ puts("Emergency services."); puts("Which service do you require?"); /* BUG */ puts("Police, ambulance, fire or coastguard?"); } nestedif.c if1.bug if2.bug Logical Operators • Consider the following “I only attend lectures on Monday at 10 or on Thursday at 10, otherwise I say in bed” • This could be described by a rather complicated flow diagram involving four “if” boxes: Logical Operators • This would make an equally complicated C program. Fortunately C provides three logical operators to get around this. && - AND, || - OR and ! - NOT a&&bmeans both a and b must be true for the result to be true a||bmeans either a or b (or both) must be true for the result to be true !a means a must be false for the result to be true && has higher precedence than || • So we could code it as if (( day==MON || day==TUE ) && time==10) go_to_lecture(); else stay_in_bed(); • Sometimes if(a==0) is written if(!a) /* Example: determining leap years */ #include main() { /* A leap year is one that is divisible by 4, but not divisible by 100. Overriding that, a year divisible by 400 is a leap year. */ int year, leap, div4, div100, div400; printf("Enter a year (e.g. 1996): "); scanf("%i", &year); div4 = year % 4 == 0; /* true if divisible by 4 */ /* Could also write div 4 = !(year % 4) */ div100 = year % 100 == 0; div400 = year % 400 == 0; leap = (div4 && !div100) || div400; /* Could also write leap = div4 && !(div100 && !div400); */ if (leap) puts("This is a leap year"); else puts("This is not a leap year"); } leapyear.c Switch Statements • C provides a statement for multiple choices, based on the value of an integer variable: switch (integer_expression){ case value1: statement(s); break; case value2: statement(s); break; case value n; statement(s); break; default : statement(s); } • break causes the program to jump to the end of the switch statement

switch.cisIh-18 *1"lwr--2switch2.c3

3 /* Example: switch statement */ #include main() { int d; printf("Enter a number from 1 to 9: "); scanf("%i", &d); putchar('\n'); switch (d) { case 1: puts("A stitch in time saves nine."); break; case 2: case 6: case 9: puts("Handsome is as handsome does."); break; case 3: puts("Too many cooks spoil the broth."); break; case 4: case 7: puts("Pride comes before a fall."); break; case 5: case 8: puts("Many hands make light work."); break; default: puts("Very clever. Try again."); } } /* Example: switch statement without breaks */ #include main() { int v; printf("Enter a number from 1 to 10: "); scanf("%i", &v); putchar('\n'); if (v 10) { puts("Very clever. I'll use 10.\n"); v = 10; } switch (v) { case 10: puts("Ten."); case 9: puts("Nine."); case 8: puts("Eight."); case 7: puts("Seven."); case 6: puts("Six."); case 5: puts("Five."); case 4: puts("Four."); case 3: puts("Three."); case 2: puts("Two."); case 1: puts("One."); puts("ZERO!!!"); } } switch.c switch2.c