1. CS1061 C Programming Lecture 6: Operators and Expressions A. O’Riordan, 2004

2. Two Short Programs We begin with some example programs to illustrate what we have covered so far. Problem 1: Write a program that takes in a value for the radius of a circle, computes the area and outputs this to the terminal. Problem 2: Write a program that takes in a distance in miles and yards, converts this to kilometers and outputs the metric distance to the terminal. These problems are straightforward so we jump rightaway to the code.

3. Problem 1 /* calculate the area of a circle */ #include int main(){ float radius; float area; printf("Please enter the length of the radius: "); scanf ("%f", &radius); area = 3.14*radius*radius; printf("The area is %f\n", area); return 0; }

4. Problem 2 #include #define yardsTomiles 1760.0 #define milesTokms 1.609 void main(){ int miles, yards; float kilometers; printf(“Enter number of miles and yards: “); scanf(“%f %f”,&miles, &yards); kilometers = milesTokms * (miles + yards/yardsTomiles); printf(“%d miles and %d yards is %f kilometers”, miles, yards, kilometers); }

5. Assignment Operators We have seen the assignment statement already. Assignment operators abbreviate assignment expressions. c = c + 3; can be abbreviated as c += 3; using the addition- assignment operator Examples of other assignment operators: d -= 4 (d = d - 4) e *= 5 (e = e * 5) f /= 3 (f = f / 3) g %= 9 (g = g % 9)

6. Expressions In C almost everything is an expression. For example, assignment = “returns” the value of its right-hand side. As a “side-effect” it also sets the value of the left-hand variable. Thus, int x; x = 12; sets the value of x to 12. Since the assignment is also an expression, we can combine several assignments as illustrated in the following example: int x, y, z; x = y = z = 12;

7. Expression Associativity The first assignment assigns z the value of its right-hand side, 12. This is the value that is then assigned to y, etc. Note the right- to-left associativity. We could rewrite this statement to make the order of execution explicit. x= (y = (z = 12))); Statements may be split across multiple lines in a file. This is so because the new-line character is considered to be a white-space character, the same as the space and tabulation characters. This feature can aid readability. The null statement is also valid C, but of little use: ; /* The null statement */

8. Increment/Decrement Operators Increment operator ( ++ ) can be used instead of c+=1 Decrement operator ( -- ) can be used instead of c-=1 Pre-increment (++c) Operator is used before the variable/expression i.e. variable is changed before the expression it is in is evaluated. Post-increment (c++) Operator is used after the variable/expression, i.e. expression evaluated before the variable is changed. The pre-decrement and post-decrement work analogously.

9. Increment/Decrement Operators (2) Examples: If c equals 5, then printf( "%d", ++c); prints 6 and printf( "%d", c++); prints 5 If c equals 5, then ++c; evaluates to 6 and c++; also evaluates to 6. When variable not in an expression pre-incrementing and post- incrementing have the same effect.

10. Logical Operators Logical operators are useful in compound expressions. example: print value of x if x is less than 10 and is also an even number: if ((x < 10) && (x%2 == 0)) printf(“The value of x is %d\n”, x); The && is a logical operator. It takes two Boolean expressions as operands and returns a Boolean value, i.e. either true or false. Another example with negation (! operator): if (!false) printf(“Always displayed”);

11. Logical Operators(2) In C, non-zero expressions evaluate to true (and 0 evaluates to false). Thus you will often see code like this: if (my_int){ /* do something here */ } which is equivalent to: if (my_int != 0){ /* do something here */ }

12. Logical Operators(3) && (logical AND) Returns true if both conditions are true || ( logical OR) Returns true if either of its conditions are true ! (logical NOT, logical negation ) Reverses the truth/falsity of its condition ! is a unary operator, meaning has only one operand These are all useful as conditions in loops, e.g.: ExpressionResult true && falsefalse true || falsetrue !falsetrue

13. Truth Tables The meaning of logical expressions can be given in a truth table: opd1opd2ANDORNOT (uses only op1) false falsefalsefalsetrue truefalsefalsetruefalse falsetruefalsetruetrue truetruetruetruefalse

14. Comma Operator The comma operator is used for specifying two expressions, which are evaluated in turn, left to right, e.g.: count = 0, max = 0; is equivalent is count = 0; max = 0; We will see an important use for this later.

15. Note on expressions with operators Multiple operators cannot be combined in a single expression. For example the follow line of code would cause a compiler error: /* check if x is between 0 and 1.0 */ if (0 <= x <= 1.0) /* illegal - causes error */ The correct way to implement this is: if (0 <= x && x <= 10)

16. Operator Precedence All of the operators have a defined precedence. Each row in the table below is a different precedence level. The first row gives the operators with the highest precedence. the second the second-most, etc. (Associativity on far right). -> [] ()left ++ -- not –(unary) *(de-ref) &sizeofight * / %left + -left >left >=left == !=left &left ^left |left andleft orleft ?:right