C++ Program Anatomy

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2 Anatomy of a Program 1

Anatomy of a Program!

1E3!


Objectives!

n  To understand the role of basic C++ program elements.!

n  To provide a skeleton from which to generate simple programs.!

n  To understand how to compile and run programs.!

!n  Read Chapter 2 of the textbook.!

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Test on Chapter 1!

n  What’s secondary storage?!n  Name two input devices.!n  Give an example of a machine language

instruction.!n  A byte is …?!!

When we RUN a program we are running

A.  An object file B.  A C++ file C.  An assembly

language file D.  A machine language

file


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Programming Environment!n  iMacs running Mac OS X!n  Smultron text editor!

n  To type in and edit your program files!n  Terminal utility!

n  Gives UNIX interface!n  “make myprog”!

n  Make compiles (and link) myprog.cpp program file!n  “./myprog”!

n  To run the compiled program myprog!


Enter/edit your program using Smultron (your version may look different) Save it with .cpp extension to your own filespace. Save often!

Smultron for Entering/Editing!

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Compile and Run …!


cd command allows you to change directory if needed. ls allows you to make sure your program is here make compiles the program. NB No file extension!!!

./ runs the compiled program

Enter data and hit return

Use the Terminal utility to enter Unix commands to your iMac.

This line tells us what make decided to do …

Make sure make succeeds …!


You probably forgot to save the edited program

Don’t use .cpp with make! Tell it what it is to produce.

You mistyped the filename or the file is not in the current directory

Watchout for these responses!!

This runs the old program, since none of our attempts to compile the new one succeeded!

This is what make should do – call the c++ compiler

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Errors: Things can go wrong…!


The program has syntax errors, on lines 12, 15, and 23, all because I didn’t declare n.

A new version of the program gives the wrong answer! This is a “bug”.

NB Remember you have to recompile (make) after each edit/save


A first program!n  On the next slide is a complete C++ program.!n  It looks scary but I don’t expect you to get it all

at once or even this month!!n  The program will ask a user for an integer and

will return the sum of the integers from 1 to that number.!

n  First think about what the program might need to do…!n  (This is where being in the lecture is invaluable!!)!

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C++ Program to Add Integers from 1 to an entered number!

Declare three integer variables, i, n, sum.!

Read in a value for n!

Output a message and/or a value.!

Do the computation!


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!

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A.  120 B.  5 C.  0 D.  None

of these

The value printed is:


Now compute factorial!n  Given the program on

your handout, and the fragment we just studied, write a program to compute factorial of n, n!.!

n  n! = 1*2*…*n or!n  n! = n * n-1 * … *1!


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Next!

n  The rest of this “topic” briefly!n  Outlines the standard gobbledy-gook that all

programs will have.!n  Introduces variables, data types and

assignment.!n  Explains “cin” and “cout”.!

n  The FOR statement is not covered till later.!


C++ Program skeleton!

Allow the program to use built-in facilities for Input and Output (IO)!

This bit changes from program to program

A comment - ignored!

main is a function that returns an integer (int)!

End of main definition!

main returns the !integer 0!

These bits are standard

gobbledy-gook

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Variables!n  Variables are places which store values.!n  i, n, sum are names of variables.!n  Valid variable names!

n  Start with a letter or _ (underscore)!n  Rest is letters, digits, or _ !

n  Case sensitive - rate, RATE, Rate all different.!n  Use meaningful names.!

n  total_pay, annual_rate, sum !n  Don’t use reserved keywords.!

n  E.g. int, for, while, …!


Variables and Data Types!n  The system needs to know what kinds of things

you will want to store in the variable.!n  Known as the Data type !n  int is the data type for integers!

n  e.g. 125, -34678!n  double is for decimal number (See next slide)!n  bool is the data type for true/false values!n  char is used to hold a letter/digit or other symbol!

n  e.g. ‘a’, ‘A’, ‘4’, ‘&’, ‘ ‘ (a space)

n  string is for sequences of characters (See later)!

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Double data type!n  We’ll use double for all non-integer values.!

n  float can be used for smaller less precise numbers!n  Uses scientific notation!


String data type!

n  A sequence of zero or more characters.!n  E.g. “Lucy”, “mary had a lamb”, “”!n  “” is the empty string.!n  This data type is not built-in like the ones

we’ve seen.!n  You have to include the string library at the

top of your program!n  #include <string>!

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Variable declaration!

n  Variables and their data type must be declared before use.!

This says allocate a (big) space for a floating point number, and call the space amountDue.


Giving Variables Values!n  Reading!

n  cin >> annual_rate;!n  Assignment (C++ uses “=“ for this!!!)!

n  sum = 0;!n  name = “Mary”;!

n  NB Initialisation!n  The initial value of a variable is arbitrary.!

n  But always something; never blank/empty.!n  You must always give it an initial value before use.!

n  E.g. counter = 0; found = false; cin >> name;!

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Assignment!

n  “=“ is the assignment operator.!n  counter = 0;!

n  Give (or assign) counter the value 0.!n  sum = sum + 4;!

n  Give sum the value got by adding 4 to the old value of sum.!

3 sum

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Input and Output!n  iostream contains definitions for cout and cin.!n  cout << “Hello world\n”;!

n  Send the message to the standard output device (screen)!

n  “\n” is a special sequence meaning “newline”.!n  cout << answer;!

n  Send the value of answer to the output device!n  cin >> number;!

n  Get/Read a value from the standard input device (keyboard).!

n  These examples assume that answer and number have been declared as variables.!

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Let’s try some variations!n  Simple problems :!

n  Read in an amount in pounds and output its equivalent in euros (£1 = 1.27).!

n  Read in two numbers, write out their sum, difference and product.!

n  Using a FOR loop:!n  Compute n! (i.e. the factorial of n).!n  Print out the integers from 1 to n.!n  Print n down to 1. (“i--” subtracts 1 from i).!n  Print the even numbers 2 to 50. (“i = i+2” adds 2 to i).!


Next …!

n  We can’t cover every detail in class.!n  Read Chapter 2 of the textbook.!n  You need to cover, by yourself!

n  Arithmetic operators and operator precedence!n  Type conversion!n  Details of input and output!

!

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C++ Program Anatomy