Lecture #7: Declarations

CSE 5A

Bob Pitha

bpitha@cs.ucsd.edu

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

administrivia page 1

• Class web page has been updated recently

– homework #1

– course outline

• Homework issues

– you can use the lab

– you can use your home computer

– you can use your friend’s computer

– you can use any computer you like

• but if it’s not the lab’s computer, it’s up to you to get and use the software

administrivia page 2

• Homework (continued)

– how to turn homework in (again)

• best option: leave it in the directory I specified in the assignment

– H:\Homework\HW1

• second best option: e-mail the files to cs5a@ieng9.ucsd.edu

• third best option: floppy disk

– marked with your name and class account name

administrivia page 3

• Quiz

– Friday, not Wednesday, during class

– will cover everything up through today

• general programming concepts

• data types

• constants

• declarations

declarations

• What is an declaration?

– how we introduce an identifier

– specifies the interpretation and attributes of an identifier

• in other words, tells the computer what an identifier means

– variable

– function

– various other things

• let’s focus on variables right now

variables

• So, what is a variable?

• Sometimes called an object

– though less so these days, since it’s not the same as the objects in “object-oriented” programming

– your text defines object as “a named region of storage”

– ANSI standard:

“A region of data storage in the execution environment, the contents of which can represent values. … objects are composed of contiguous sequences of one or more bytes, the number, order, and encoding of which are either explicitly specified or implementation-defined.”

variables

• So variables are memory regions you can use to hold data while your program is running.

• For your convenience, you give them names (instead of having to know the address)

• Because different types of data are different sizes, the computer needs to know what type each variable is

– so it can reserve the memory you need

back to declarations

• So all this means that before you can use a variable, you need to tell the compiler:

– its name

– its data type

– anything else that the compiler needs to know

• storage class

• right now, we’ll just stick to the first two

why declare variables?

• Easier for the compiler

– it doesn’t have to guess what you want

– it can reserve the right amount of memory

– machine code can be different for some different data types

• Discipline

– forces you to think about what you need

– this is a subtle issue, we’ll revisit it

declaration rules

• Each declaration consists of:

– a data type

• int, double, char…

– a list of variable names

• remember the rules for identifiers

– letters, digits, underscore

– can’t start with a digit

– case sensitive

– optional initialization

– ends with a semicolon

simple declaration examples

Text Box: char c;
int i;
double d;
int lower, upper, step;

initialization?

• Sets the initial value of the variable

– this doesn’t mean it can’t change subsequently

– must be a constant expression

• a constant

• an expression whose value is constant

Text Box: char c = ‘x’;
int i = 37;
double d = 12.34;
int lower=0, upper=300, step=20;

initialization and data types

• The initializer must be:

– of the same data type as the variable

– of a data type the compiler can convert into the data type of the variable

Good:

Text Box: int i = 3;
double a = 19.38;
double b = 12;

Less good:

placement of declarations

• Placement in program is significant

– At beginning of statement block

• this is what we’ve seen so far

– Outside a function - global variable

– Parameter declaration - in function declaration

variables in statement blocks

• What statement blocks have we seen so far?

– after all, we haven’t yet even talked about statement blocks!

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

– statement blocks are surrounded by { }

another statement block

$Text Box: /* * FtoC1 – print a Fahrenheit-to-Celsius * conversion table */ #include <stdio.h> int main (void) { int fahr; int celsius; int lower = 0; int upper = 300; int step = 20; fahr = lower; while (fahr <= upper) { celsius = 5 * (fahr – 32) / 9; printf (“%d\t%d\n”, fahr, celsius); fahr = fahr + step; } return 0; }$