Programming in C

Lecture 1: INTRODUCING C (1)

CHRIS STAFF
Dept. of Computer Science and Artificial Intelligence
University of Malta

Next Lecture: Introducing C (2)

Lecture Outline

Aims and Objectives

Of the course

Assessment is through a single assignment only

Of the lecture

Introduction to C
The Structure of a C program
Some basic C syntax

What is C?

C is a general purpose programming language
C contains some features that are normally associated with low-level programming languages
C is basis for Java, C++
C# shares properties with C, C++, Java, and Delphi
C deals with the same objects that are manipulated by computers: single characters, numbers and memory addresses
Any other type of object is created, by the programmer, by combining those objects (e.g., character strings, arrays, records, fields, etc.)
C does not have any built-in functions to perform complex input and output! Again, the programmer must explicitly write these functions.
Luckily, ANSI C contains a set of prepared functions to perform some routine functions, BUT THESE ARE NOT AN OFFICIAL PART OF THE C PROGRAMMING LANGUAGE!
To start with, we will ignore the fact that some of these functions exist, so that we can learn how C operates...

Preamble

THIS IS NOT A CONVERSION COURSE!
I will assume that you will be using the C compiler on one of the UNIX host computers (e.g., zeus, babe). You will write your programs using a text editor (e.g., vi), and you will use the gcc C compiler.
You must save your program as prog.c, otherwise the gcc compiler won't work
To compile your program, type

gcc prog.c
Your compiled program will be stored in a.out
a.out is a binary or executable file
To run your program, type

a.out

The first C Program

	#include <stdio.h>

	main()
	{
		printf("hello, world\n");
	}

The program simply prints the words "hello, world" on the screen (standard output)
C commands are all lower case

The Program line by line

	#include <stdio.h>

Tells the C compiler to include the library header file stdio.h in the program. This file contains several functions to do with input and output. The function printf() is in this library routine. If stdio.h was not included in the program, C would not be able to find the printf() function

	main()
	{
	}

main() is a the name of a user-defined function. C looks for a function with this name to start execution. If main() is missing from the program, C will generate an error message at compile time
The curly brackets (braces) indicate to C the statements that make up the function

The braces do not have to be on separate lines

	#include <stdio.h>

	main(){printf("hello, world\n");}

But , be clear - as programs get longer the above makes then unreadable!

	printf("hello, world\n");

A library function that instructs C to print the character string on the screen (standard output)
All statements are separated by ;
\n is an escape character - it represents the newline character
C does not generate newlines automatically. We could just as easily have written
```
	printf("hello, ");
	printf("world");
	printf("\n");
```
This would have produced the same output

Writing more complex programs

In order to write a more complex program, some more of the C syntax must be introduced

Variables

A variable is a placeholder for data.
Variables must be declared before they can be used
Declaring a variable serves two purposes: C knows what data type the variable is, and C reserves space for the data

Some Variable types

char - a single character (one byte)

int - an integer

short - a short integer

long - a long integer

float - a floating point number

double - a double-precision float

The size of the numeric data types is dependent on the machine architecture

Assignment

To set a variable to a value

variable = expression

a = b + 5

Conditionals

expression operator expression

a <= b

Looping constructs (simple)

	while ( conditional ) {
		statements ;
	}

while the conditional is true (evaluates to anything other than 0) perform statements
If there is only one statement, the braces can be omitted, but it is better to leave them just in case later you want to add more statements and you forget to put the braces in (defensive programming!)

Comments

	/* free text */

comments cannot be nested, but can spread across several lines. Comments are ignored by the compiler

	/* some
	free
	text
	*/

is ok, but

	/* some free text
	/* some more free text */
	*/

is illegal, and will generate a syntax error (/* cannot be used inside a comment!)

        // ignore text to end of line

Use comments liberally - it makes programs readable!

Formatted printing

printf("format"[, expression [, expression]]);

format can contain free text, escape characters, and arguments
Some escape characters are

\n - newline
\t - tab
\b - backspace
\\ - slash
\" - double quote
Arguments are placeholders for values inside the format, e.g., if you want to print the contents of a variable.
Arguments are composed of % followed by an optional print-format, followed by a data type

Some examples

%d - print as decimal integer
%6d - print as decimal integer, at least 6 characters wide
%f - print as floating point
%6f - print as floating point, at least 6 characters wide
%.2f - print as floating point, two character after decimal point
%6.2f - print as floating point, at least 6 wide
%ns - print as string, at least n wide
%c - print as single character

Exercises

Next Lecture...

Introducing C (2)