Software

Software is the part of the computer you can't touch; it's the ideas of programmers codified into machine language, represented by electrical impulses, that tell the computer what to do (via the CPU). (There are other parts of the computer you shouldn't touch, like the power supply and the inside of the monitor, but software is something you can't touch because, in a sense, software doesn't really exist in the real world.) Programming, the topic of this course, is the process of writing programs (software).

There are two kinds of software: application and system software. Application software does what people use the computer for, e.g., word processors, spreadsheet programs, video games, etc. System software exists for the sake of the computer, from the operating system that organizes and allocates resources to a program to list the files in your account.

The operating system is a big piece of system software that is loaded into main memory when the computer first comes up, and stays there until you turn off the computer. It manages the activities of the computer, allocating and controlling access to resources (such as memory, CPU time, storage, the printer, etc.) On the main computers we use, the operating system is a version of Linux, which is an operating system designed to be very similar to the Unix operating system. Other operating systems are BSD Unix, Solaris, MacOS, VMS, and Windows NT.

Computer Languages

In the beginning, programmers simply programmed in machine language. This can be quite tedious, so computer scientists invented computer languages that could be used instead of machine language. A program would be written in this high-level language, then automatically translated to machine language by another program.

The first computer languages were simply human-readable forms of machine language called assembly languages. For instance, if the machine language instruction for "add two numbers" was 100101010, the assembly language version might be "ADD". A program called an assembler (hence the name "assembly language") would translate the program into machine language. Assembly language is still used today for specialized applications.

Higher level languages were developed that used mathematical and human language notation in place of machine instructions. Two of the first were FORTRAN, for scientific programming, and LISP, for list processing (used mostly for artificial intelligence). A FORTRAN program looks like this:
	program	number
		integer	i

		do 10 i=1,10
			print*, i
 10 		continue
		stop
	end
This simple program prints the numbers from 1 through 10.

The same program in LISP:
(defun number (n)
	(cond ((equal n 10) n)
	      (t (print n) (number (+ n 1)))))
(number 10)


The same program in C, the language we're using for this class:
#include <stdio.h>
#include <stdlib.h>

int main () {
	int	i;

	for (i=1; i<=10; i++) printf ("%d\n", i);
	exit (0);
}
A text file containing a program written in assembly or a high level language is called a source file. It can be translated by a compiler into a machine language file, called an object file, that must then be linked with some other object files, and can then be executed. On the main computers, the compiler and linker are accessed through the cc command; cc stands for "C Compiler."

Those "other object files" are libraries of standard routines used to access input and output devices and do certain common computations.



The C Language

C is the computer language we will be using in this course. It is about 20 years old. The original purpose of C was to be the systems programming language of the Unix operating system, which is written almost entirely in C with a little assembly language. C has evolved over the years into a robust language used for a wide variety of applications. It is succeeded by two new object oriented languages, C++ and Objective C. It is also the language upon which Java is based. We will be using ANSI C, the "official" version of C codified by the American National Standards Institute.

A source program written in C is a text file containing words and punctuation conforming to a proper syntax or grammar, as well as certain semantic rules governing the meaning of the program. The syntactic rules are somewhat loose; for instance, a C program can be all on one line, or can have each word on a separate line; C doesn't care much where you begin a new line (however, there is a conventional style most people use to make their programs readable).

Here is a simple C program. It prints Hello, World! on the screen:
#include <stdio.h>

int main () {
	printf ("Hello, World!\n");
	exit (0);
}

Elements of a C program:

Let's look at some more simple C programs. This program prints the square of 123:
#include <stdio.h>

int main () {
	float	num;

	num = 123;
	printf  ("%f squared is equal to %lf\n", num, num * num);
        return 0;
}
output:
123.000000 squared is equal to 15129.000000
When doing aerobic exercise, your heart rate should be between 65% and 80% of your maximum heart rate, which is determined by subtracting your age from 220. The following program does these calculations, asking you for your age and giving you your aerobic heart rate range.
#include <stdio.h>

int main () {
	int	age;
	float	mhr, r1, r2;

	printf ("How old are you? ");
	scanf ("%d", &age);
	mhr = 220.0 - age;
	r1 = 0.65 * mhr;
	r2 = 0.80 * mhr;
	printf  ("Your heart rate should be"
		" between %0.2f and %0.2f during aerobic exercise\n", r1, r2);

        return 0;
}
input: 20
output:
Your heart rate should be between 130.00 and 160.00 during aerobic exercise
This program prompts for and reads an angle in degrees, converts the angle to radians, then prints the result on the screen.
#include <stdio.h>

int main () {
        float   degrees, radians;

        printf ("Enter the angle in degrees: ");
        scanf ("%f", &degrees);
        radians = (degrees / 360.0) * 2 * 3.14159;
        printf ("%f degrees = %f radians.\n", degrees, radians);
        return 0;
}
This program prompts for a letter, then prints the ASCII code for that letter, in both uppercase and lowercase.
#include <stdio.h>
#include <ctype.h>

int main () {
	char	in_char, up, lo;
	int	code;

	printf ("Enter a letter: ");
	scanf ("%c", &in_char);
	up = toupper (in_char);
	lo = tolower (in_char);
	printf ("lowercase is %c, uppercase is %c\n", lo, up);
	code = (int) in_char;
	printf ("ASCII code is %d\n", code);
	return 0;
}
input: f
output:
Enter a letter: f
lowercase is f, uppercase is F
ASCII code is 102
This program converts the input decimal number to hexadecimal, printing the result.
#include <stdio.h>

int main () {
	int	a;

	printf ("Enter a number: ");
	scanf ("%d", &a);
	printf ("%d in hexadecimal is %x\n", a, a);
}
output:
Enter a number: 31
31 in hexadecimal is 1f

Make sure for next time that you have read through Chapter 2 of the book, and begin reading Chapter 3.