Lecture #10: Operators, Expressions, and Statements

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

equality operators

== !=

• Test whether operands are equal or not equal

• Return 1 or 0

precedence and associativity

equality operator issues

• Don’t count on testing floating-point types with ==

– why?

• real numbers in the computer can differ by an insignificant amount

• == tests for exactly equal

– so instead, test the difference between them

important!

• Don’t confuse == with =

– the worst part is, this is legal C code; it just doesn’t do what you (probably) want

• the expression i = 0

– it sets the value of i to 0

– its value is 0

• the expression i == 0

– doesn’t set the value of anything

– its value is 1 or 0 depending on whether i is 0

– This can cause bugs that are very hard to find!

logical operators

&& ||

• Logical AND and OR

(a && b) is true if both a and b are true

(a || b) is true if either a or b are true, or both.

• Evaluation stops when truth value is known

((a != b) && (c <= d))

precedence and associativity

expressions

what is an expression?

• Nobody has a good, clear definition.

• A sequence of operators and operands that has a value and/or does something interesting

• There are a bunch of types

how are expressions used?

• As operands in larger expressions

Text Box: •i = j++
•5 * (fahrenheit – 32) / 9
•(a < b) || (c > d)

• As parts of statements

Text Box: while (i > 0)
for (i = 0; i < 10; ++i)

side effects in expressions

• Some operators have “side effects”

– change the value of their operands

• You must be careful not to combine side effects

– no expression should modify the same object twice

Text Box: i = i++;
i = (j = 5) + j++;

we’ve finally finished chapter 2!

• 2.1 Variable Names

• 2.2 Data types

• 2.3 Constants

• 2.4 Declarations

• 2.5 Arithmetic operators

• 2.6 Relational operators

• 2.7 Type conversions

• 2.8 Increment/decrement operators

• 2.10 Assignment operators

• 2.12 Precedence

statements

what’s a statement?

• A specification of some action to be performed

• Executed sequentially

what kinds of statements do we have?

• Expression statements

• Compound statements

• Selection statements

• Iteration statements

• Jump statements

what kinds of statements do we have?

• Expression statements

• Compound statements

• Selection statements

• Iteration statements

• Jump statements

expression statements

• Any expression can be a statement

– just stick a semicolon (;) on the end

– but they’re only evaluated for their side effects

• the expression’s value is ignored

Text Box: i = 5;
i++;
++i;
celsius = 5 * (fahrenheit – 32) / 9;

expression statements

• Since the value is ignored, expressions that only have a value are not meaningful statements

– they’re not illegal, just useless

– some compilers will warn you about them

• the assumption is that you forgot something

• Special case: the null statement

– an empty statement – just a semicolon

– used when the syntax requires a statement, but you don’t want to do anything.

what kinds of statements do we have?

• Expression statements

• Compound statements

• Selection statements

• Iteration statements

• Jump statements

compound statements

• Also called statement blocks

• Groups one or more statements into a unit

– syntactically equivalent to a single statement

– that means that wherever it’s legal to have a single statement, you can have a compound statement.

• Can have declarations and initializations

– but only at the beginning of a block

compound statement syntax

• Statements surrounded by braces { }

– no semicolon at the end

– although the statements and declarations in the block use semicolons as normal

use of compound statements

• Impossible to use C without using compound statements

• We’ve already seen them a lot

$Text Box: /* * HelloWorld: A simple C program */ #include <stdio.h> int main (void) { printf (“Hello, world!\n”); return 0; }$