Programming a computer

What does programming a computer mean ?
- Programming a computer:
- Since a computer can only execute machine instructions (encoded in binary numbers), this means:
  This is a painfully complicated process
  But fortunately, we have developed tools that help make this process less painful

Instructing a computer to execute an algorithm

A computer is the ideal machine to execute computational algorithms because:

The computer can perform arithmetic operations
It can also perform an operation only when some condition is satisfied (using the conditional branch instruction)

Problem:

We (a human) has to tell (communicate) the computer what steps to do in machine language

Humans are not good in machine languages

We have developed specialized languages (based on the English language) to instruct/command a computer
These specialized languages are called:

Types of languages used in computer programming

Machine language

The machine language (or instruction code) consists of (binary) numbers that encode instructions for the computer
Every computer (CPU) has its own machine language
(I.e., the instruction code 1 can encode a different instruction for different CPUs)
Instruction encoding was discussed in this webnote: click here

What a machine language program looks like:

10110101 10101111 01010110 10110111 ...

Assembler language or low level programming language
What an assembler language program looks like:
Each assembler instruction is a mnemonic that corresponds to a unique machine instruction

High level programming language

A high level programming language consists of (English-like) "people" language to simplify the writing computer algorithms
A high level programming language allows the programmer to write sentences in this language which can be easily translated into machine instructions
The sentences written in a high level programming language are called:

What a high level language program looks like:

main() { if ( x > y ) // One sentence (statement) in a high level prog. lang. { max = x; } else { max = y; } ..... }

One statement in a high level programming language is usually translated into multiple machine instruction that achieve the effect specified by the statement

Some well-known programming languages

Well-known programming languages:

Name	Application area
Fortran	Scientific application (FORmula TRANslator)
Cobol	Business application (COmmon Business Oriented Language)
C	System application (successor of a language called "B")
C++	System application (successor of the language "C")
Java	General purpose (a simplification of "C++")
C#	General purpose (another simplification of "C++")
Perl	Scripting language with many string processing capabilities
Python	Scripting language using indentation as block denotation.

A taste of computer algorithm

Recall the Euclid's Algorithm: (to find the GCD)

As long as neither number is zero (0) do { if ( number on A ≥ number on B ) replace the number on A by the value (A - B) otherwise replace the number on B by the value (B - A) } The Greatest Common Divisor (GCD) of the numbers A and B is the non-zero number on one of the papers

Here is the Euclid Algorithm written in the Java programming language:

public class Euclid { public static void main(String args[]) { int A; // Memory cell named "A" int B; // Memory cell named "B" // These memory cells are like the 2 pieces of paper // we used above. They can store and recall a value A = 28; // Write "28" on the piece of paper named "A" B = 36; // Write "36" on the piece of paper named "B" // ================================ // This is the Euclid Algorithm: // ================================ while ( A != 0 && B != 0 ) { if ( A >= B ) A = A - B; // Replace the number on A by (A-B) else B = B - A; // Replace the number on B by (B-A) } System.out.println("GCD = " + B); } }

Note:

Don't worry if you do not understand this program.
All I want to do is to show you is the fact that a computer can be instructed by a specialized (programming) language to do things.
This is what you will learn later in this course !!!

Example Program: (Demo above code)
- Prog file: click here
How to use this program:

Computer Programs

Computer Program:

Computer program = a computer algorithm that is expressed (written) in some programming language
Most programming languages are "English-like"
They use words from the English language

The representation gap: algorithm in a programming language and in computer instructions

Notice the following representation gap:

An algorithm expressed in a programming language is "English-like":

Example:

main() { if ( x > y ) // One sentence (statement) in a high level prog. lang. { max = x; } else { max = y; } ..... }

The algorithm consists of instructions that the computer must perform to accomplish a given task/solve a given problem

However: the instructions that a computer executes are (binary) numbers (that encode the instruction):
Example:
These numbers that encode computer instructions are not "English-like" !!!

Houston, we have a problem:

Solution:

Someone (or something) must translate the algorithm expressed in a programming language into (binary) numbers that encode the same instructions as the original algorithm

Bridging the translation gap: compiler

We bridge the representation gap using 2 techniques:

We design the programming language in such a way that the sentences in the programming language can be easily understood

We write a (pretty complex) computer program to do the translation from:

Algorithm written in a program language ⇒ same algorithm in machine instruction (numbers)

The (pretty complex) computer program that does this translation is called:

The compilation (= translation) process

You write an algorithm in some programming language
E.g.: a very simple program in the C programming language
Before the computer can execute the instructions in a C program, we must first compile (translate) it into machine operations using a C translator (compiler):
cc is the command to run the C compiler (translator)
The second word prog.c is the name of the file that contains the C program (that is being translated)
The machine operations that result from the translation process is stored in the file:
Now we can execute the program in machine instruction by using the command:

The following figure summarizes the translation process:

Note:

A C compiler is a program that is executed by a computer
Therefore, a C compiler consists of machine instructions !!
(Computers only execute machine instructions !)

Example Program: (Demo above procedure)

C Prog file: click here

How to do the demo:

Right click on the link and save the file prog,c in a scratch directory
Open a command line window and go to the scratch directory
Use this command to compile: (ignore some warning messages - I want to keep the program simple)
Use this command to execute the file with computer instructions:

Writing computer programs: trust your compiler !
- When you write a computer program, you will write it in a high level programming language and trust the compiler (translator) to do the translation
- It is virtually impossible to write a large computer program in machine language (binary numbers)
- Therefore:
- The high level programming language used in this course is: