How to Write a C++ Program? A step-by-step procedure

Transcription

1 How to Write a C++ Program? A step-by-step procedure

2 Main Elements of C++ Program (in order) Declaration of appropriate header files or preprocessor directives It tells the computer about where to look for the commands you use in your program. Declaration of constants (if any) This is not compulsory. Only needed if program involves constants. Body of program consisting of: Main program header Begin main program variables C++ statements and/or subprograms, also called functions End main program

3 Step 1: Program purpose This is compulsory to address the objective of the program. The first two columns of all lines describing the program purpose must start with // Start your program with: // Purpose: This program calculates // and displays the

4 Step 2: Declare preprocessor directives It is compulsory to include preprocessor directives in your program. Preprocessor directives involves declaration of header files, which are files with the.h extensions. A compulsory header file is iostream.h. The declaration is: #include <iostream.h> This header files MUST always be used in your program. Hence, if you do not know how to start your program, just remember to start it with this line as the first line in your program. Other directives - Will learn as we go along.

5 Step 3: Declare constants (optional) Constants are declared after declaration of directives. It is optional. Only needed if constants are used in your program. Declaration of constants come with initialization of their values. E.g.: To declare pi as a constant #define pi 3.14; pi is the name given to represent the constant. The name given to a constant can vary (up to the programmer), but make sure it is meaningful is the constant value.

6 Step 4: Begin program body A main program begins with the following lines: void main() { void is the program datatype. It means nothing will be returned. If the program will return an integer value, then the program needs to be declared as int main() main() means it is a main program. This is compulsory.

7 Step 5a: Declare variables A name is needed to represent a data or value of a variable. It is called a variable name. Variable names must be meaningful, E.g. height, number_1, number2, total Rules for variable names: Cannot be the same as C++ keywords/commands Cannot start with number, nor a symbol. Cannot have special symbols within it. If a variable name must be two words, the words cannot be separated by a space. Use underscore ( _ ) instead.

8 Step 5b: Include variable datatype Remember? Variables represent data of different types, e.g. integer numbers, fraction numbers, text string, array So, must specify the type of data with variable declaration. Common datatypes: Integer = int Floating point = float, double Character = char Condition = boolean E.g. A variable representing a student s year of study: int A variable representing the result of a division: float or double A variable representing a character: char A variable representing a condition of TRUE or FALSE: boolean

9 Step 5: Example of Variable declaration Problem: To calculate the division of two integer numbers. 3 variables are needed: 1. To store first number (let s call it number1) 2. To store second number (let s call it number2) 3. To store the result of division (let s call it res_div) So, the variable declaration required is, int number1, number2; float res_div; // why float datatype? Variable declaration is NEEDED BEFORE the variables are used in the program

10 Step 6: Write other statements This is then followed by other C++ statements relevant to solve the given problem. A completed main program is ended with a closing curly brace: }

11 Step 7: Add comment lines Why need comments? To help others understand the program To assist further modification How to write comments? Comments in C++ program starts with /* and end with */ Or, a comment can start with // with no end of comment symbol

12 How does a complete C++ program look like? // Purpose: This program gets input from users and display it on the screen. #include <iostream.h> // header file declaration #define pi 3.14; // constant declaration int main() { // begin program int value; // declaration of integer variable float weight; // declaration of float variable cout << "The constant value of pi is set to: " << pi << \n ; cout << "Enter your weight in kg: "; // comment asking user for weight value cin >> weight; cout << "Enter an integer value: "; // comment asking user for weight value cin >> value; cout << "You have entered " << weight << kg as your weight and ; cout << value << integer value ; return 0; } // end program