1 Anatomy of a Program 4

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1 Objectives Anatomy of a Program 1E3 To understand the role of basic C++ program elements. To provide a skeleton from which to generate simple programs. 1 Anatomy of a Program 1 1 Anatomy of a Program 2 Textbook & Webpage Problem Solving with C++: The object of programming, Walter Savitch, Fourth Edition, Addison Wesley We ll cover Chapters 1-5, 7, 10 and 13 Will also be used in 2E3 Strongly recommend that you buy it. Call number N93 (2nd edition) /BAI/JF_Subjects/1E3/ 1 Anatomy of a Program 3 1 Anatomy of a Program 4 1

2 Programming Language We ll use C++ C++ is an object-oriented language We ll focus on the procedural aspects. C++ is the OO extension of C C is a high-level language but allows lowlevel control of machines. C is the language used by UNIX programmers. Environment imacs running Mac OS X Panther SubEthaEdit text editor To type in and edit your program files Terminal utility Gives UNIX interface make myprog.cpp The make tool compiles (and links) programs./myprog To run the compiled program myprog 1 Anatomy of a Program 5 1 Anatomy of a Program 6 A first program On the next slide is a complete C++ program. It looks scary but I don t expect you to get it all at once or even this month! The program will ask a user for an integer and will return the sum of the integers from 1 to that number. First think about what the program might need to do (This is where being in the lecture is invaluable!!) C++ Program to Add Integers from 1 to an entered number Declare three integer variables, i, n, sum. Output a message and/or a value. Read in a value for n Do the computation 1 Anatomy of a Program 7 1 Anatomy of a Program 8 2

3 The computation Initialise sum For each value of i, from 1 up to and including n adding 1 to i each time do this: Add the current i to sum Next The rest of this topic briefly Outlines the standard gobbledy-gook that all programs will have. Introduces variables, data types and assignment. Explains cin and cout. The FOR statement is not covered till Topic 4. 1 Anatomy of a Program 9 1 Anatomy of a Program 10 C++ Program skeleton This bit changes from program to program A comment - ignored Allow the program to use built-in facilities for Input and Output (IO) main is a function that returns an integer (int) These bits are standard gobbledy-gook main returns the integer 0 End of main definition 1 Anatomy of a Program 11 Variables Variables are places which store values. i, n, sum are names of variables. Valid variable names Start with a letter or _ (underscore) Rest is letters, digits, or _ Case sensitive - rate, RATE, Rate all different. Use meaningful names. total_pay, annual_rate, sum Don t use reserved keywords. E.g. int, for, while, 1 Anatomy of a Program 12 3

4 Variables and Data Types The system needs to know what kinds of things you will want to store in the variable. Known as the Data type int is the data type for integers double is for decimal numbers Why double? bool is the data type for true/false values Variables and their data type must be declared before use. Reading Giving Variables Values cin >> annual_rate; Assignment (C++ uses = for this!!!) sum = 0; sum = sum + next_number; The new value of sum is the old value of sum plus the value of next_number. Initialisation The initial value of a variable is arbitrary. But always something; never blank/empty. You must always initialise before use. 1 Anatomy of a Program 13 1 Anatomy of a Program 14 Input and Output Confused? iostream contains definitions for cout and cin. cout << Hello world\n ; Send the message to the standard output device (screen) \n is a special sequence meaning newline. cout << answer; Send the value of answer to the output device cin >> number; Get/Read a value from the standard input device (keyboard). These examples assume that answer and number Don t panic. Confusion has more to do with how you feel about something than about a lack of data on the subject. Focus on the big picture. Focus on what you have grasped. Feel relaxed and happy with that. Gradually fit the harder bits in to the picture you are building up. It doesn t have to be all clear today. have been declared 1 Anatomy as variables. of a Program 15 1 Anatomy of a Program 16 4

5 Let s try some variations Using a FOR loop: Compute n! (i.e. the factorial of n). Print out the integers from 1 to n. Print n down to 1. ( i-- subtracts 1 from i). Print the even numbers 2 to 50. ( i = i+2 adds 2 to i). Simpler problems (no FOR loop needed): Read in an amount in pounds and output its equivalent in euros ( 1 = 1.27). Read in two numbers, write out their sum, difference and product. Now We can t cover every detail in class. Read Chapter 1 of the textbook. Then study sections 2.1 and Anatomy of a Program 17 1 Anatomy of a Program 18 5