1 Expressions –Arithmetic Operators (+, -, *, /, %, ++, --) Preprocessor and defined Constants –#define directive Today’s Material

2 What’s an Expression? A mathematical formula that computes a value –formed by operators (+, -, *, /, %, …) celsius = (fahrenheit-32)/1.8; area = 3.14*radius*radius; circumference = 2*3.14*radius; volume = length*width*height; sum = sum + 5; …

3 Arithmetic Operators Basic operators for performing arithmetic are the same in many programming languages: Modulus (remainder)% Division/ Multiplication* Subtraction- Addition+ Binary operators Decrement-- Increment++ Unary operators

4 Arithmetic Operators: Example 14sum = sum % 3%Modulus 24sum = sum / 2/Divide 84sum = sum * 2*Multiply 24sum = sum – 2-Subtraction 64sum = sum + 2+Addition Value of sum after Value of Sum before EquationOperatorOperation 34--sum--Decrement 54++sum++Increment Value of sum after Value of sum before EquationOperatorOperation

5 Notes on / Operator If both operands are integers, / performs integer division int a = 5; int b = 4; int result = a/b; /* > 1 */ printf(“result: %d\n”, result); /* will print 1 */

6 Mixed Operands If one of the operands is a float/double, the other operand is implicitly converted to float/double before the expression is evaluated –If the l-value is an integer, the result is typecast to int before assignment –If the l-value is a float/double, the result is typecast to float/double before assignment int result; float fresult; result = 5/4.0; /* > 1 */ fresult = 5/4.0; /* > 1.25 */ printf(“result: %d, fresult: %.2f\n”, result, fresult);

7 More on Unary Operators result = ++a; /* increment first, then assign */ Equivalent to: a = a+1; result = a; result = --a; /* decrement first, then assign */ Equivalent to: a = a-1; result = a;

8 More on Unary Operators result = a++; /* assign, then increment */ Equivalent to: result = a; a = a+1; result = a--; /* assign, then decrement */ Equivalent to: result = a; a = a-1; Unary operators can be used with both int & float/double operands. But typically applied to ints

9 Compound Assignment Operators Compound assignment operators: += -= *= /= %= … For example: sum += data; /* sum = sum + data */ a -= 6; /* a = a – 6; */ product *= data; /* product = product*data */ a %= 2; /* a = a%2 */ a /= b+c; /* a = a/(b+c) */ …

10 Associativity & Precedence When an expression contains more than one operator of the same type, associativity rules apply result = a+b+c;  result = (a+b)+c; /* left associativity */ a = b = c;  a = (b=c); /* right associativity */ When an expression contains different operators together, precedence rules apply result = a+b*c;  result = a+(b*c); /* * has higher precedence than + */

11 Associativity & Precedence Rules right=, +=, -=,.. 4 left+, -3 left*, /, %2 right-, ++, --1 AssociativityOperatorPrecedence It is difficult to remember all associativity and precedence rules So it is best to parenthesize the expression yourself to avoid ambiguity

12 Expression Examples(1) int a = 10, b = 22; int result = a+b*3; printf("a: %d, b: %d, : %d\n", a, b, result); int a = 10, b = 22; int result = (a+b)*3; printf("a: %d, b: %d, : %d\n", a, b, result); a: 10, b: 22, : 76 a: 10, b: 22, : 96

13 Expression Examples(2) int a = 7, b = 9, c = 4; int result = (a+10)%b/c; printf("a: %d, b: %d, c: %d, : %d\n", a, b, c, result); int a = 7, b = 9, c = 4; int result = (a+10)%(b/c); printf("a: %d, b: %d, c: %d, : %d\n", a, b, c, result); a: 7, b: 9, c: 4, : 2 a: 7, b: 9, c: 4, : 1

14 Expression Examples(3) int a = 7, b = 8; printf("a(before): %d, ", a); a*=b+1; printf("a(after): %d, b: %d\n", a, b); a = b = c = 1; printf("a(before): %d, b(before): %d, ", a, b); a+=b+=c; printf("a(after): %d, b(after): %d, c: %d\n", a, b, c); a(before): 7, a(after): 63, b: 8 a(before): 1, b(before): 1, a(after): 3, b(after): 2, c: 1

15 Expression Examples(4) a = 10; b = 5; printf("a(before): %d, b(before): %d, ", a, b); result = (a b); printf("a(after): %d, b(after): %d, result: %d\n", a, b,result); a = 10; b = 5; printf("a(before): %d, b(before): %d, ", a, b); result = (++a + ++b); printf("a(after): %d, b(after): %d, result: %d\n", a, b,result); a(before): 10, b(before): 5, a(after): 11, b(after): 6, result: 16 a(before): 10, b(before): 5, a(after): 11, b(after): 6, result: 17

16 Example C Program (1) #include /* Convert fahrenheit to celsius */ main() { float fahrenheit, celsius; printf(“Enter a temp in fahrenheit: “); scanf(“%f”, &fahrenheit); celsius = (fahrenheit-32)/1.8; printf(“%f degrees fahrenheit equals %f degrees celsius\n”, fahrenheit, celsius); } Prompt the user and get the fahrenheit temperature to convert celsius = (fahrenheit-32)/1.8 Print the fahrenheit and celsius degrees Start End

17 Example C Program(2) #include /* Compute sum, product, avg of 2 ints */ main() { int number1, number2; int sum, product, average; printf(“Enter 2 integers: “); scanf(“%d%d”, &number1, &number2); sum = number1+number2; product = number1*number2; average = sum/2; printf(“sum: %d, product: %d, avg:%d\n”, sum, product, average); } /* end-main */ Prompt the user and get number1 and number2 sum = number1 + number2 Print sum, product and average product = number1 * number2 average =sum/2 Start End

18 Example C Program(3) #include /* Computes the circumference and area of a circle */ main() { float radius, circumference, area; printf(“Enter the radius: “); scanf(“%f”, &radius); circumference = 2* *radius; area = *radius*radius; printf(“Circumference: %f, area: %f\n”, circumference, area); } /* end-main */ Prompt the user to enter the radius circumference = 2* *radius Print circumference and area of the circle area = *radius*radius; Start End Get radius of the circle

19 The Need for Symbolic Constants In the previous C program, we twice used the number PI = inside the program Using such magic numbers inside of C programs usually leads to bugs –We may type one of the PI numbers wrong! –If we want to add more precision to PI, we need to change all occurrences of the PI numbers, which may also lead to bugs Question: Is there a solution? –Yes: Symbolic constants

20 Defining Symbolic Constants Defined using the preprocessor directive #define Example: #define TRUE 1 #define FALSE 0 #define PI Using symbolic constants: –circumference = 2*PI*radius; –area = PI*radius*radius;

21 Example C Program(3) #include #define PI /* Computes the circumference and area of a circle */ main() { float radius, circumference, area; printf(“Enter the radius: “); scanf(“%f”, &radius); circumference = 2*PI*radius; area = PI*radius*radius; printf(“Circumference: %f, area: %f\n”, circumference, area); } /* end-main */

22 Resolving Symbolic Constants Symbolic constants are resolved by the preprocessor before compilation begins –Preprocessors works like find/replace –Replaces every occurrence of PI with its definition –Preprocessor also removes comments from the code so that the compiler does not have to deal with them prog.c COMPILER ASSEMBLER LINKER LOADER + OS prog.s prog.o printf.o scanf.o prog.exe program running PREPROCESSOR prog.i

23 A Common #define Bug Find the error here: #define SIZE 5; int main() { printf(“Size is %d\n”, SIZE); } int main() { printf(“Size is %d\n”, 5;); } After find/replace by preprocessor: