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:

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: 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: 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