CSci 160 Lecture 13

1. CSci 160Lecture 13 Martin van Bommel

2. Simple Statements • Expression followed by semicolon • Assignments total = n1 + n2; • Function calls printf(”Hello.\n”); • Useless statements n1 + n2;

3. Embedded Assignments • Assignment expression can be used as part of a larger expression in a statement • Its value is the value assigned z = (x = 6) + y; • x is assigned value 6, then z assigned 6 + y • Difficult to read • Used rarely and only when makes sense

4. Multiple Assignments • Embedded assignments useful to set several variables to the same value n1 = n2 = n3 = 0; • Assignment operator evaluated right to left • Avoid mixed types; e.g. double d and int i d = i = 1.5; • Assigns i value 1, thus 1 assigned to d

5. Statement Blocks • Sequence of statements that specify a coherent unit enclosed in curly braces • E.g. if, for, while statement bodies { statement1 statement2 . . . }

6. Boolean Data • Conditionals test expressions whose values are either TRUE or FALSE • George Boole - developed algebra for T/F • Standard C has no Boolean type • We use the integer 0 for FALSE and the integer 1 for TRUE

7. Relational Operators • Compare two atomic values (not strings) • Precedence after +/- > Greater than < Less than >= Greater or equal <= Less or equal • Next level of precedence == Equal != Not equal

8. Logical Operators • Operate on other Boolean values (in order of precedence) ! Not (TRUE if operand FALSE) && And (TRUE if both TRUE) || Or (TRUE if either TRUE)

9. Example 1 • “x is not equal to either 2 or 3” if (x != 2 || x != 3) • No, it should be if (!(x == 2 || x == 3)) • or if (x != 2 && x != 3)

10. Example 2 • “x is in the range from 0 to 10 exclusive” if (0 < x < 10) • No, it should be if (0 < x && x < 10)

11. Short-Circuit Evaluation • Evaluating exp1 && exp2 or exp1 || exp2 evaluates expressions from left to right • If answer known before both evaluated, second expression not evaluated (x != 0) && (y % x == 0)

12. Flags • Variables used for testing truth values int done; • Assign value to indicate state of flag done = 0; or done = 1;

13. Setting Flags • To set a flag on a condition, could use if (input == -1) { done = 1; } else { done = 0; } • Better to simply use done = (input == -1);

14. Testing a Flag • To test whether done has value TRUE, could use if (done == 1) • Better to simply use if (done) • Test for truth value 1 is redundant

15. Boolean Example • Leap year every fourth year, except centuries, then just every fourth century • year is divisible by 4 but not by 100, or • year is divisible by 400 • Try ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0)