Embed Size (px)
Citation preview
A Quick Taste of CFor Applesoft programmers
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
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
Compilation ProcessThis 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 //
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
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.
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.
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.
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.
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.
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.
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.
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.
Variable TypesApplesoft 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!
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.
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.
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.
Printing Integers• Here is an example of printing an integer variable
#include <stdio.h>
int main(void)
{
int x = 10;
printf(“x = %d\n”, 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”.
Printing Characters• Here is an example of printing a character variable
#include <stdio.h>
int main(void)
{
char x = ‘e’;
printf(“x = %c\n”, 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”.
Printing Strings• Here is an example of printing a character variable
#include <stdio.h>
int main(void)
{
char x[] = “hello”;
printf(“x = %s\n”, 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”.
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.
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 = %s\n”, 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.
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”.
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.
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.
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 5\n”);
} else {
printf(“x is 10 and y is not 5\n”);
}
} else {
if (y == 5) {
printf(“x is not 10 and y is 5\n”);
} else {
printf(“x is not 10 and y is not 5\n”);
}
}
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(“%d\n”, i);
}
return 0;
}
• “i++” is a shortcut which is the same as saying “i = i + 1”
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.
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(“%d\n”, i);
i++;
}
return 0;
}
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.
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 %d\n”, i, square(i));
}
return 0;
}
Now Go and Write Some C Code!
