An introduction to C programming intended for people who have done some Applesoft Basic programming on an Apple //.

1. A Quick Taste of C
For Applesoft programmers

2. Why Use C on an Apple //?
• Your code will run faster if you code in C versus
AppleSoft
• It is easier to code in C than assembly language
(in my opinion)
• If you are careful, you can write C code which
works on an Apple // and on modern OS’s (Mac
OS X, Windows, Linux, etc)
• There are good cross-compiling tools for C

3. C Compiler vs Applesoft
Interpreter
• Applesoft is an interpreter
• This means that the Apple // is running a program which
reads and executes your program.
• An interpreter introduces a significant performance penalty
• C is a compiled language
• This means your code is turned into machine code which
runs “on the metal” on your Apple //
• But if you make a mistake in your code, it can lead to
crashes which are hard to figure out

4. Compilation Process
This is what happens when your C code is turned into a
binary which you can run.
file1.c
file2.c
file1.oCompiler
Compiler
file2.o
BinaryLinker
Mac OS X
BinaryCopy
Real or
emulated
Apple //

5. Comments in Your Code
• You can add comments in your Applesoft code:
10
REM
THIS
IS
A
COMMENT
IN
BASIC
• There are two ways to add comments in C:
/*
This
is
a
multiline
comment
which
starts
from
a
slash
star
and
goes
to
a
star
slash
*/
//
This
is
a
single
line
comment

6. No Line Numbers?
• Unlike Applesoft, there are no line numbers in C code
• But how does this work?
• Although C does have a goto statement, it jumps to
a location with a name, not a line number.
• Instead of gosub, you create and call functions
referenced by name, not a line number.
• Loops and if statements can enclose a block of
statements. We will see examples later.

7. Where does the program
start?
• In Applesoft, execution starts from the lowest
numbered line of code (line 10 perhaps)
• But C does not have line numbers so where does
execution start?
• C defines a special function called “main” which is
where execution begins.
• When a C executable is launched, the main function is
called and the program ends when the main function
returns.

8. But What Is A Function?
• The previous slide introduced the “main function” without defining functions.
• In C, a function is similar to a subroutine in Applesoft.
• But, a function is more structured than an Applesoft subroutine:
• Every function has a name. You use the name to call the function.
• Every function has their own “local variables”. Variables in Applesoft are
“global”. C has global variables but C functions can have their own
variables which are private.
• Every function takes zero or more inputs that are special local variables.
They are set to a value which comes from the caller of the function.
• Every function can return a value back to the caller.

9. The main() Function
• Here is a pretty standard way to define the main function
int
main(void)
{
return
0;
}
• The main function here:
• Takes no input arguments. That is what “void” means.
• Returns an integer to the caller. That is what “int” means.
• The code for this function is between the curly braces.

10. Another function
• Here is another function in C:
int
square(int
x)
{
return
x
*
x;
}
• This function:
• Takes an integer from the caller and puts it into a local variable
called “x”.
• Returns that number multiplied by itself back to the caller.

11. What About The Semi-
Colons?
• In Applesoft, you can use a semi-colon at the end of a
print statement to prevent it from outputting a new line
at the end.
• In C, every statement in code must end with a semi-
colon character.
• If you forget it, the C compiler won’t understand your
code and it will give you errors.
• The C compiler needs the semi-colon to tell where
each statement ends and a new one starts.

12. Variables In Applesoft
• You can just start using a variable. The first time
you reference it, it is created.
• The variable name can be as long as you like
but only the first two characters are relevant.
• The type of the variable is encoded in the name
with special characters.

13. Variables in C
• You must “declare” your variables before you can use them. They are not just
implicitly created when first used.
• A variable can have a name that is as long as you like. All characters of the
name are used, not just the first two.
• You can use lots of kinds of characters in the name:
• They must start with a lower or upper case letter or an underscore
character.
• After the first character, you can use letters, numbers or the underscore
character.
• C is case sensitive
• The declaration tells you the type of variable.

14. Variable Types
Applesoft C
Integers X% = 5 int x = 5;
Floating point Y = 1.5 float y = 1.5;
Strings Z$ = “HELLO” char z[] = “hello”;
Note: cc65 does not support floating point!

15. Strings
• The previous slide made it seem like C has a string variable type but
that really is not true.
• C has a “char” variable type which is a variable which holds a single
character.
• You can do arrays of char’s in order to create a string.
• In C, the standard is that a string is zero or more characters followed by
a “null terminator” which is ASCII zero (ie CHR$(0) in Applesoft).
• Or a string can be a “pointer to a char” - I will not try to explain that
right now…
• Strings, arrays and pointers are all intertwined and if you understand
them, you pretty much understand C.

16. Printing Text
• To print text in C, we need to use the “standard I/O
functions”.
• But all functions (like all variables) need to be declared.
• There is a short-hand to declare all of the standard I/O
functions. You just tell the compiler to “include” them as
though you typed them:
#include
<stdio.h>
• The “stdio.h” file will be inserted as though you typed it in
your source file. That way, you can get all of the standard I/O
declarations.

17. Printing Text
• To print text, use the “printf” function. Here is a complete example:
#include
<stdio.h>
int
main
(void)
{
printf(“HELLO,
WORLD!n”);
return
0;
}
• This is the first function call you have seen. We are calling the function
printf() with a single value. That value is a string (inside quotes).
• The “n” in the string is replaced by the C compiler with new-line character.
So, you get a new line at the end of the printed text.

18. Printing Integers
• Here is an example of printing an integer variable
#include
<stdio.h>
int
main(void)
{
int
x
=
10;
printf(“x
=
%dn”,
x);
return
0;
}
• The %d character is special to printf(). When it sees a %d, it looks at the
next input argument. It expects to find an integer and it replaces the %d
with the value. In this case it prints “x = 10”.

19. Printing Characters
• Here is an example of printing a character variable
#include
<stdio.h>
int
main(void)
{
char
x
=
‘e’;
printf(“x
=
%cn”,
x);
return
0;
}
• The %c character is special to printf(). When it sees a %c, it looks at the
next input argument. It expects to find an character and it replaces the %c
with the value. In this case, it prints “x = e”.

20. Printing Strings
• Here is an example of printing a character variable
#include
<stdio.h>
int
main(void)
{
char
x[]
=
“hello”;
printf(“x
=
%sn”,
x);
return
0;
}
• The %s character is special to printf(). When it sees a %s, it looks at the
next input argument. It expects to find a string and it replaces the %s with
the value. In this case, it prints “x = hello”.

21. Getting Input
• To read a string from the user, you need to define a
string buffer to read into.
• In Applesoft, strings grow or shrink as required to hold
them.
• In C, you need to manage the size of your own strings
and make sure you don’t put more data into a string
than it will hold
• This is the well known “buffer overflow” problem that
has lead to so many security problems.

22. Getting Input
• Here is an example of reading a line of text:
#include
<stdio.h>
int
main(void)
{
char
buffer[80];
//
80
bytes
fgets(buffer,
sizeof(buffer),
stdin);
printf(“You
typed
=
%sn”,
buffer);
return
0;
}
• The fgets() function gets a string from a file (File GET String or fgets).
• The “file” it reads from is the special file “stdin” which is short for standard input which
is generally the keyboard.

23. If Statements
• Here is what an if statement looks like:
int
x
=
10;
if
(x
==
10)
{
printf(“x
is
10!n”);
}
• Note that the curly braces are optional if only one statement exists in the
body of the if.
• The indenting is not required but can help you and others read and
understand your code.
• Note the double equal signs which means “equals” where single equal
signs means “assignment”.

24. If Statements
• Your if statements can have multiple statements inside the curly braces:
int
x
=
10;
if
(x
==
10)
{
x
=
x
*
2;
printf(“x
was
10!n”);
}
• Here if x is 10, the two statements in the curly braces will be exectued. After
that, execution continues after the end of the curly braces.
• If x is not 10, then those two statements are skipped. Execution continues
after the end of the curly braces.

25. If Statements
• If statements can have “else” clauses.
int
x
=
10;
if
(x
==
10)
{
x
=
x
*
2;
printf(“x
was
10!n”);
}
else
{
printf(“x
was
not
10!n”);
}
• Here if x is 10, the two statements in the curly braces will be exectued. After that,
execution skips the printf() in the else clause and continues after the end of the else
curly braces.
• If x is not 10, then those two statements are skipped. The printf() in the else curly brace
is executed instead and execution continues after the else curly braces.

26. If Statements
• If statements can be nested:
int
x
=
10;
int
y
=
5;
if
(x
==
10)
{
if
(y
==
5)
{
printf(“x
is
10
and
y
is
5n”);
}
else
{
printf(“x
is
10
and
y
is
not
5n”);
}
}
else
{
if
(y
==
5)
{
printf(“x
is
not
10
and
y
is
5n”);
}
else
{
printf(“x
is
not
10
and
y
is
not
5n”);
}
}

27. For Loops
• Here is the classic “count from 1 to 100” in C using a for loop:
#include
<stdio.h>
int
main(void)
{
int
i;
for
(i
=
1;
i
<=
100;
i++)
{
printf(“%dn”,
i);
}
return
0;
}
• “i++” is a shortcut which is the same as saying “i = i + 1”

28. For Loops
• A for loop in C has four different components:
for
(A;
B;
C)
{
D;
}
• The “A” above is the initialization phase of the for loop. It is executed only once before the
first execution through the loop.
• The next thing which happens is that “B” is executed. If B is a statement which returns true,
then the loop executes.
• At this point “D” executes. D is one or more statements which make up the body of the loop.
• After that, “C” executes. This is generally where the loop counter is incremented. But, you
can do just about what ever you want in this statement.
• Again, “B” is executed and if it again returns true, the loop executes again. As soon as B
returns false, the for loop is done and execution continues after the curly braces.

29. While Loops
• Here is the classic “count from 1 to 100” in C using a while loop:
#include
<stdio.h>
int
main(void)
{
int
i
=
1;
while
(i
<=
100)
{
printf(“%dn”,
i);
i++;
}
return
0;
}

30. While Loops
• A while loop in C has two different components:
while
(A)
{
B;
}
• The first thing which happens is that “A” is executed. If A is a
statement which returns true, then the loop executes.
• At this point “B” executes. B is one or more statements which make
up the body of the loop.
• Again, “A” is executed and if it again returns true, the loop executes
again. As soon as A returns false, the while loop is done and
execution continues after the curly braces.

31. Putting It Together
• Here is a C program to calculate the squares from 1 to 100:
#include
<stdio.h>
int
square(int
x)
{
return
x
*
x;
}
int
main(void)
{
int
i;
for
(i
=
1;
i
<=
100;
i++)
{
printf(“%d
squared
is
%dn”,
i,
square(i));
}
return
0;
}

32. Now Go and Write Some C Code!