What does a computer program look like: a general overview

Computer programming: solving a problem with a computer

• Goal of computer programming:

• We write a computer program to solve a problem or perform a task

Computer programming: solving a problem with a computer (cont.)

• In order to solve a problem, we need:

The basic fact of life is that without information, we cannot complete the task !!!

• An algorithm (detailed steps that solve the problem)

• Information !!!

Computer programming: solving a problem with a computer (cont.)

• Example: compute the sum of 2 numbers

Without the value of the 2 numbers, we cannot perform the addition operation !!!

• Algorithm: perform an addition

• Information: the value of the 2 numbers

Computer program = an algorithm + (lots of) information

• A computer program consists of:

• computer instructions that tells a computer precisely what to do to accomplish the job

• (Lots of) information to guide the algorithm to do the job

Computer program = an algorithm + (lots of) information (cont.)

• This fact is expressed in the title of a classical Computer Science book:

Computer program = an algorithm + (lots of) information (cont.)

• The title of this classical Computer Science book reflects the fact that to solve a problem (write a program) with a computer, you need:

• An algorithm, and

• Information (that is stored in data structure)

Statements: expressing the steps in an algorithm

• Statements

• Statement = an instruction written in a (High level) Programming Language

• A statement tells a computer in very precise terms what exactly it must do

Statements: expressing the steps in an algorithm (cont.)

• Example of statements

• Store the value 26 in the memory cell named "A"

• Add the number A to the number B

• If the number A ≥ 0 then do this otherwise do that

Variable: storing the information

• Variables

• A variable = a identifiable memory cell stores information that is necessary to solve a problem

• A variable behaves like a piece of paper that is identified by a unique name where we can record (a small amount of) information

Example:

Variable: storing the information (cont.)

• Data structure:

• Example

• A data structure = a collection of variables that store related information

• One variable stores the name of a person

• Another variable stores the phone number of a same person

• These 2 variables together would form a data structure

Data types

• There are different types of data:

• Numerical data, such as: age, salary, and so on.

• Textual data (or alpha-numerical data) such as books, news paper articles, webpages, and so on.

• Etc.

Data types (cont.)

• A programming language always provides a number of built-in data types to get your started

Common built-in data types:

The built-in data types are called primitive data types

• Single precision floating point numbers

• Double precision floating point numbers

• Integer numbers

• A character type

• A logical data type

Data types (cont.)

• In addition, a modern programming language allows the programmer to define his/her own data types

These are called user-defined types.

Variable definition

• In modern programming languages, we must "define" every variable in the computer program

• Effect of a variable definition:

• Each variable definition must use a unique name to identify the variable.

• A variable definition will tell the computer to create the variable that will be identified by that (unique) name

This is like telling the computer to:

1. get a new sheet of paper and

2. tag the new sheet of paper with the name of the variable

Starting point of the execution of a computer program

• Every computer program written in a high level programming language has a starting point of execution

• Each programming language has its own convention of where a program will start execution

Starting point of the execution of a computer program (cont.)

• Example starting point in some popular programming languages:

Programming language Starting point of a program Java main()C/C++ main()Python First line in the program

Execution of a computer program

• How a computer program is executed:

• The execution will start at the statement at the starting point of the execution

(In Java, this will be the first statement in main())

• After a statement is executed, the next statement executed depends on the flow of control

Execution of a computer program

• Flow of control:

• Normal flow of control is:

• There are control statements in a programming language that can change the normal flow of control

• After executing a statement, the next statement that will be executed is the statement following the last executed statement

A taste of "flow of control" (program execution): the Euler algorithm in Java

• Recall the Euclid Algorithm:

We saw the execution of the Euclid algorithm using 2 pieces of paper: http://192.168.1.3/~cheung/teaching/web/170/Syllabus/01/alg.html#exec

As long as one of the number is not 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) = B

A taste of "flow of control": the Euler algorithm in Java (cont.)

• We have also seen a Java program that implements the Euclid algorithm:

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);

}

}

A taste of "flow of control": the Euler algorithm in Java (cont.)

• When the Java program is run on a computer, the computer will be doing pretty much the same thing as the execution on 2 pieces of paper that we saw here: http://192.168.1.3/~cheung/teaching/web/170/Syllabus/01/alg.html#exec

A taste of "flow of control": the Euler algorithm in Java (cont.)

• I will illustrate how a computer executes the Euclid algorithm with a series of diagram:

• The program execution of a Java program starts at beginning of main(...)

A taste of "flow of control": the Euler algorithm in Java (cont.)

• Executing the line "int A;" tells the computer to create a new variable name A

This is like getting a new sheet of paper and tag it with the name A

• Result:

• The flow of control goes to the next line

A taste of "flow of control": the Euler algorithm in Java (cont.)

• Executing the line "int B;" tells the computer to create a new variable name B

This is like getting a new sheet of paper and tag it with the name B

• Result:

• The flow of control goes to the next line

A taste of "flow of control": the Euler algorithm in Java (cont.)

• Executing the line "A=28;" tells the computer to put the number 28 in the variable A

• This is like writing "28" on the sheet of paper tagged with the name A

• Result:

A taste of "flow of control": the Euler algorithm in Java (cont.)

• Executing the line "B=36;" tells the computer to put the number 36 in the variable B

This is like writing "36" on the sheet of paper tagged with the name B

• Result:

A taste of "flow of control": the Euler algorithm in Java (cont.)

• Executing the line "while (A≠0 && B≠0)" tells the computer only continue inside the statements inside {...} if the condition is true.

The symbol && means "and".

• Since 28 (A) ≠ 0 and 36 (B) ≠ 0, the condition is true and the execution continues inside {...}

• Result:

A taste of "flow of control": the Euler algorithm in Java (cont.)

• Executing the line "if (A≥B)" tells the computer to: continue inside the first {...} if the condition is true and continue inside the second {...} if the condition is false

Since 28 (A) < 36 (B), the condition is false and the execution continues inside the second {...} (after the word else)

• Result:

A taste of "flow of control": the Euler algorithm in Java (cont.)

• Executing the line "B=B−A;" tells the computer to put the difference (36−28) in the variable B

This is like writing the difference (36−28) on the sheet of paper tagged with the name B

• Result:

A taste of "flow of control": the Euler algorithm in Java (cont.)

• When the execution reaches the line "}", the flow of control will go back to up to the while line and execute the steps all over again !!!

In other words:

A taste of "flow of control": the Euler algorithm in Java (cont.)

• The computer executing the Euclid algorithm written in the Java programming language is doing the exact same steps as what a human would do using 2 pieces of paper.

• A computer program is nothing more than a precise sequence of steps (that a human would use) to solve a problem that is expressed in a programming language

Conclusion:

A programming language is a language

• It is important to realize that a programming language is like any of the human languages

• Important facts about programming languages:

1. A programming language has syntax rules that govern the correctness of the "sentences"

2. The compiler (translator) is extremely inflexible on the syntax rules:

• If the compiler finds any syntax error in a program, it will not compile (translate) the entire program

A programming language is a language (cont.)

• Important advice:

(Don't worry, the syntax rules are very simple; still, you need to remember them well !)

• Remember the syntax rules of the programming language carefully