Lecture #4: The C Language

CSE 5A

Bob Pitha

bpitha@cs.ucsd.edu

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

class business

• No office hours this afternoon

– will start on Wednesday

• A course outline will be posted on the web page by Monday.

• Discussion section is not mandatory, only recommended

a second example

• Here’s another example C program

– it’s not much more sophisticated

– it also doesn’t do much of any practical use

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

new concepts

• Variables

• Loops

variables

• How we manipulate data in our program

• Name associated with some memory

• Has a specific data type

• In this program:

– a variable named i is used to hold an integer value

– as the program runs, i’s value goes from 0 to 9

$Text Box: #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

loops

• A way of repeating the same instructions over and over

• C provides some flexible ways of deciding how many times to loop, or when to exit a loop

second example

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

• So let’s look at this example again…

second example

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

• Here’s that include directive again.

– like before, this allows printf to work

– we’ll just do it now, without worrying about why

second example

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

• And like before, this line introduces our main function.

– remember that main is where the program starts running

second example

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

• These braces enclose blocks of statements.

– a function is an example of a block

second example

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

• This line introduces a variable.

– it’s called a declaration

– it tells the compiler that we’ll be using a variable

• named i

• it holds an integer

second example

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

• Here is a loop

– it will execute its contents 10 times

– each time, i will have a different value

• from 0 to 9

second example

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i); return 0; }$

• This statement is the “contents” of the loop

– so it gets executed 10 times

i is 0

i is 1

i is 2

etc.

we could have written it like this…

$Text Box: /* * This program prints a list of * integers, from 0 to 9. */ #include <stdio.h> int main (void) { int i; printf (“i is 0\n”); printf (“i is 1\n”); printf (“i is 2\n”); printf (“i is 3\n”); printf (“i is 4\n”); printf (“i is 5\n”); printf (“i is 6\n”); printf (“i is 7\n”); printf (“i is 8\n”); printf (“i is 9\n”); return 0; }$

ok, what’s going on here?

• We’ve just seen 2 simple programs

• These are C programs

– not machine language

• the C compiler is used to turn them into something the machine understands

• The C compiler is itself a program

– it reads the C program you write

– it writes out (hopefully) a program the computer can run

remember what a program is

• The real program is just numbers

• A C program is a sequence of C instructions

– we can define variables to hold our data

– we write C statements to do things with our data

– we can group our statements into functions

so what’s a compiler?

• A compiler is a program that converts a C program into machine language

– remember, the computer can only run machine language

so let’s look at the whole process

• Write a program

– text editor

• Compile the program

– C compiler

– Will do one of two things:

• print error messages

• produce an executable program

• Run the program

let’s talk about this variable concept…

• Variables are central to almost any C program

• They are your program’s data

– the information your program works with

• If you want to store any data for use elsewhere in the program, you need to use a variable