Transcript CS1061 C Programming Lecture 2:A Few Simple Programs

CS1061 C Programming
Lecture 2: A Few Simple Programs
A. O’Riordan, 2004
First Program
The best way to get started with C is to actually look at a program.
main(){}
notes:

The word "main" is very important, and must appear once, and
only once in every C program. This is the point where execution is
begun when the program is run. We will see later that this does not
have to be the first statement in the program but it must exist as the
entry point.
First Program (continued)
notes:


Following the "main" program name is a pair of parentheses which
are an indication to the compiler that this is a function. The two
curly brackets, properly called braces, are used to define the limits of
the program itself. The actual program statements go between the
two braces and in this case, there are no statements because the
program does absolutely nothing.
You can compile and run this program, but since it has no executable
statements, it does nothing.
Second Program
Another sample C program is given below:
/*Sample program
*/
#include <stdio.h>
int main(){
printf(“C is fun!\n”);
return(0);
}
Second Program (continued)
notes:

This program outputs text to the monitor.

Notice the semi-colon at the end of the statements. C uses a semicolon as a statement terminator, so the semi-colon is required as a
signal to the compiler that this line is complete.

The first printf() outputs a line of text and returns the carriage.
printf() is a function used for displaying text. This function is part of
the C library. \n is a special newline character (see later).

The #include is a pre-processor directive which includes declarations
of library functions. The # character must appear as the first character
in a line. stdio.h is needed because of the printf().
Second Program (continued)


return() is a standard command that causes the program to ‘return’ a
value. Strictly speaking it is not needed here as it is the last line of
main() and the program will terminate anyway.
/* ... */ indicates a comment which will not be parsed by the compiler.
Comments are totally ignored by the compiler, they're just annotations
you can add to make it easier to read your code. The comment has the
following syntax: A comment begins with the two characters /* (no
space between them) and ends with the characters */. A comment can
span many lines. Comments cannot be nested.
A Word on Functions

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Functions are the basic unit of any C program, or any structured
programming language for that matter. Functions are called procedures
or subroutines in other languages.
If a function is well written it may be reusable from one project to the
next.
Every C program has to have at least one function called main(). This
is the first function that is executed. This function may then call other
functions. It is conventional to place the main() function at the start of
the program followed by all the other functions in the file.
It is considered good software engineering practice to keep functions
short.
A Word on Functions (continued)

A programmer can create his own functions
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Programmers will often use the C library functions
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Advantage: the programmer knows exactly how it works
Disadvantage: time consuming
Use these as building blocks
Avoid re-inventing the wheel
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If a pre-made function exists, generally best to use it rather than write
your own
Library functions carefully written, efficient, and portable
“Form” of Fucnction
A function has the form:
return_type function_name (parameters){
<local variable declarations>
<statements>
}
The important thing to note here is the order. Firstly we have the function
name (and return type and parameters). Any C function may have
parameters, which is a mechanism for passing extra information to a
function. Then we have the body of the function enclosed by curly
braces {}.
One last program
#include<stdio.h>
main(){
int index;
index = 13;
printf("The value of the index is %d\n",index);
index = 27;
printf("The value of the index is %d\n",index);
index = 10;
}
notes:

This uses a variable to store a numeric value.
One last program (continued)
notes:

int index; is used to define an integer variable named "index". The
"int" is a reserved word in C, and can therefore not be used for
anything else.

Variables are locations in memory where a value can be stored

int means the variables can hold integers (e.g. -1, 3, 0, 47)

there are three statements that assign a value to the variable "index",
but only one at a time. The first one assigns the value of 13 to index,
and its value is printed out. The second assignment gives index the
value 27 and the third 10.
 the syntax of printf() is discussed later.