英格兰萨里大学：《C语言》课程教学资源（讲义）Lecture 6 - Operators, Expressions and Statements

rodaLecture 6ArithmeticOperators.C has a number of arithmetic operators which are used tocombine variables and constants into expressions- Unary operators+,-e.g *x, -xLaopinBinary operators+, , .1, %-Aese.g. +y'zdedIntegerexpressionexamplesing1+2=3nO3'4=121715 = 317%5= 2Note that integer division discards any fractional part.The modulus operator can be useful for many thingse.g. x%2 is 0ifx is even, 1if xis oddOperatorPrecedenceintarith.c.Ifwehaveseveraloperatorsinan88988expression,whatorderaretheyprocessedine.g. does 1+2*3 evaluate to 9 or 7?Chasfixedrulesforthis,seepg22ofnotes(. ,1, %, +,.Wecanforcetheorderusingparentheseswhichhavetoppriority.Foroperatorsofequalprecedencetheevaluation is from leftto rightFloating point expressionsarithmetic with floats ordoubles works moreor less asTraps fortheunwaryexpected, however we cant use % operator'Eximple: floating point arithmetic /include stdio.h>loat r1 - 22000:n : 100flost E.r2.13)-2*33*1)n"-a7.2nn",r)1/±2+1/3)1m2fn

1 1. Introduction 2. Binary Representation 3. Hardware and Software 4. High Level Languages 5. Standard input and output 6. Operators, expression and statements 7. Making Decisions 8. Looping 9. Arrays 10. Basics of pointers 11. Strings 12. Basics of functions 13. More about functions 14. Files 14. Data Structures 16. Case study: lottery number generator Lecture 6 Arithmetic Operators • C has a number of arithmetic operators which are used to combine variables and constants into expressions – Unary operators +, - • e.g. +x, -x – Binary operators +, -, *, /, % • e.g. x+y*z • Integer expression examples 1+2 = 3 3*4 = 12 17/5 = 3 17%5 = 2 • Note that integer division discards any fractional part. The modulus operator can be useful for many things e.g. x%2 is 0 if x is even, 1 if x is odd Operator Precedence • If we have several operators in an expression, what order are they processed in e.g. does 1+2*3 evaluate to 9 or 7 ? • C has fixed rules for this, see pg 22 of notes ( ), *, /, %, +, - • We can force the order using parentheses which have top priority • For operators of equal precedence the evaluation is from left to right intarith.c /* Example: integer arithmetic operators */ /* Old Imperial measures: miles, furlongs, chains, yards, feet and inches. 1 mi = 8 fur; 1 fur = 10 ch; 1 ch = 22 yd; 1 yd = 3 ft; 1 ft = 12 in */ #include #define FUR_PER_MI 8L #define CH_PER_FUR 10L #define YD_PER_CH 22L #define FT_PER_YD 3L #define IN_PER_FT 12L /* Note: these are longs to avoid overflow later. (The suffix L means the value should be treated as a long.) */ main() { int a = 5, b = 3; long x, remainder; int inches, feet, yards, chains, furlongs, miles; printf("a = %i, b = %i\n\n", a, b); printf("a + b = %i\n", a + b); printf("a - b = %i\n", a - b); printf("a * b = %i\n", a * b); printf("a / b = %i\n", a / b); printf("a %% b = %i\n", a % b); printf("\nEnter a number of inches: "); scanf("%li", &x); miles = x / IN_PER_FT / FT_PER_YD / YD_PER_CH / CH_PER_FUR / FUR_PER_MI; remainder = x % (IN_PER_FT * FT_PER_YD * YD_PER_CH * CH_PER_FUR * FUR_PER_MI); /* Note: if the above constants are defined as ints, overflow might occur when their product is computed. This is because the product of ints is itself an int. */ furlongs = remainder / IN_PER_FT / FT_PER_YD / YD_PER_CH / CH_PER_FUR; remainder %= (IN_PER_FT * FT_PER_YD * YD_PER_CH * CH_PER_FUR); chains = remainder / IN_PER_FT / FT_PER_YD / YD_PER_CH; remainder %= (IN_PER_FT * FT_PER_YD * YD_PER_CH); yards = remainder / IN_PER_FT / FT_PER_YD; remainder %= (IN_PER_FT * FT_PER_YD); feet = remainder / IN_PER_FT; inches = remainder % IN_PER_FT; puts("\nConverted to old fashioned units:"); printf("%li inches = %i mi, ", x, miles); printf("%i fur, %i ch, ", furlongs, chains); printf("%i yd, %i ft, %i in\n", yards, feet, inches); } Traps for the unwary /* BUG ZONE!!! Example: integer arithmetic */ #include main() { int r1 = 22000; int r2 = 10000; int r3 = 15000; int r; puts("Three resistors in parallel"); printf("%i || %i || %i\n\n", r1, r2, r3); puts("Method 1: r = (r1 * r2 * r3)/(r1*r2 + r2*r3 + r3*r1)\n"); r = (r1 * r2 * r3)/(r1*r2 + r2*r3 + r3*r1); /* BUG */ printf("Total resistance = %i\n\n", r); puts("Method 2: r = 1/(1/r1 + 1/r2 + 1/r3)\n"); r = 1/(1/r1 + 1/r2 + 1/r3); /* BUG */ printf("Total resistance = %i\n", r); } /* BUG ZONE!!! Example: integer arithmetic */ #include main() { int StudentsInClass = 116; int MaxGroupSize = 5; int NumberOfGroups; int passes; int GirlsWithBrownEyes; NumberOfGroups = StudentsInClass/MaxGroupSize; /* BUG */ printf("%i students at %i max. per group means %i groups\n\n", StudentsInClass, MaxGroupSize, NumberOfGroups); /* Three-quarters of the class pass the exam: */ passes = 3/4 * StudentsInClass; /* BUG */ printf("%i students passed the exam!\n\n", passes); /* Half the students are female, and half of these have brown eyes */ GirlsWithBrownEyes = StudentsInClass / 2*2; /* BUG */ printf("There are %i brown-eyed girls in the class\n", GirlsWithBrownEyes); } Floating point expressions – arithmetic with floats or doubles works more or less as expected, however we cant use % operator /* Example: floating point arithmetic */ #include main() { float r1 = 22000; float r2 = 10000; float r3 = 15000; float r; puts("Three resistors in parallel"); printf("%5.0f || %5.0f || %5.0f\n\n", r1, r2, r3); puts("Method 1: r = (r1 * r2 * r3)/(r1*r2 + r2*r3 + r3*r1)\n"); r = (r1 * r2 * r3)/(r1*r2 + r2*r3 + r3*r1); printf("Total resistance = %7.2f\n\n", r); puts("Method 2: r = 1/(1/r1 + 1/r2 + 1/r3)\n"); r = 1/(1/r1 + 1/r2 + 1/r3); printf("Total resistance = %7.2f\n", r); }

TypeCastingtypecast.cConversion between data types (eg int to float) iscalled casting. It can be : implict (done automatically)int x=1.34 explicit (we force it to happen)int x=(int)1.34).Note:int > 1oat is straightforward, e.g. 32-> 32.000E1oat -> int involves truncation, e.g. 32.73-> 32AssignmentOperatorsIncrementandDecrementOperatorsWe have already seen the basic assignment operator e.g. xy* We could write x=x+1 (this is valid C)C also has some useful shorthand's1111But shorthand versions are:11131 pre increment :++x post increment : x++:The difference is that with x++the value of xis usedSomething that can be used on the LHS of an assignment is calledfirst then incremented; the reverse for ++xan Ivalue e.g.x is an valueeg int a=5, biint a=5,bib=a++}b=++aix/2, 3+y, a+b are not/-Nowais6, b is5-//nowais6,bis6/Because assignments are operators several can be combined in asingle statement:There is also a decrement operator --which followsxay2-0;the same rulesRelational Operatorsrelation.c+ They are >, , 21 is true 27 false++"els es.--- anything else -> trueHowever,theserelational operators give1for trueu wnd ens t2

2 Type Casting • Conversion between data types (eg int to float) is called casting. It can be : – implicit (done automatically) int x=1.34; – explicit (we force it to happen) int x=(int)1.34; • Note: int -> float is straightforward, e.g. 32 -> 32.000 float -> int involves truncation, e.g. 32.73 -> 32 typecast.c /* Example: type casting */ #include main() { float x = 34.256, y; int n = 27, m; char c = 'A'; /* Implicit casting: */ m = x * n; /* implicit (int) cast */ y = n/2; /* implicit (float) cast */ printf("m = %i, y = %6.2f\n\n", m, y); m = c * n; /* implicit (int) cast */ printf("m = %i\n\n", m); /* Explicit casting: */ m = (int)x * n; /* explict (int) cast */ y = (float)n/2; /* explicit (float) cast */ printf("m = %i, y = %6.2f\n\n", m, y); y = (float)c + x; printf("y = %6.3f\n\n", y); /* Explicit and implicit casting: */ m = (float)n/2; y = ((int)x + 1)/2 + x; printf("m = %i, y = %6.3f\n\n", m, y); } Increment and Decrement Operators • We could write x=x+1 (this is valid C) • But shorthand versions are: – pre increment : ++x – post increment : x++ • The difference is that with x++ the value of x is used first then incremented; the reverse for ++x eg int a=5, b; int a=5, b; b=a++; b=++a; /*Now a is 6, b is 5*/ /*Now a is 6, b is 6*/ • There is also a decrement operator - which follows the same rules Assignment Operators • We have already seen the basic assignment operator ‘=‘ e.g. x=y • C also has some useful shorthand's a+=b means a=a+b a-=b means a=a-b a*=b means a=a*b a/=b means a=a/b a%=b means a=a%b • Something that can be used on the LHS of an assignment is called an lvalue e.g. x is an lvalue x/2, 3+y, a+b are not • Because assignments are operators several can be combined in a single statement: x=y=z=0; Relational Operators • They are >, =, 21 is true – 27 false – anything else -> true • However, these relational operators give 1 for true relation.c /* Example: relational and logical operators */ #include main() { int a = 5, b = 6, c = 7; puts("int a = 5, b = 6, c = 7;\n"); printf("The value of a > b is \t%i\n\n", a > b); printf("The value of b = c is \t%i\n\n", a + b >= c); printf("The value of a - b <= b - c is\t%i\n\n", a - b <= b - c); printf("The value of b - a == b - c is\t%i\n\n", b - a == b - c); printf("The value of a * b != c * c is\t%i\n\n", a * b < c * c); }

StatementsBlocks (orCompoundStatements)·Expressions canbemade into statementsbyThese are simply one ormore statements*asuffixing semicolonenclosedwithinbraces ()togroupthemtogether.1·Statementscanbesimpleorcomplexx=31e.g.*y=x+p;y++;++x:tal1=height>180;poly=a*x*x+b*x+c;The compound statement is treated as asingle entity,as well see in the next 2 lectures3

3 Statements • Expressions can be made into statements by a suffixing semicolon • Statements can be simple or complex e.g. x; y++; tall=height>180; poly=a*x*x+b*x+c; Blocks (or Compound Statements) • These are simply one or more statements enclosed within braces { } to group them together. { x=3; y=x+p; ++x; } • The compound statement is treated as a single entity, as well see in the next 2 lectures