CSCI 240 Lecture Notes -- Part 1

A program = data + instructions

Data:

• there are several data types (numbers, characters, etc.)
• each individual data item must be declared and named
• each individual data item must have a value before use
• initial values come from
  • program instructions
  • user input
  • disk files
• program instructions can alter these values
• original or newly computed values can go to
  • screen
  • printer
  • disk

Instructions:

• for data input (from keyboard, disk)
• for data output (to screen, printer, disk)
• computation of new values
• program control (decisions, repetition)
• modularization (putting a sequence of instructions into a package called a function)

Sample C++ program

#include <iostream>       // a.

using namespace std;      // b.

int main()                // c.
  {
  int num1, num2;         // d.

  num1 = 10;              // e.
  num2 = 15;

  num1 = num1 + num2;     // f.

  cout << "Sum is: " << num1;  // g.

  return 0;              // h.
  }

Notes:

• a. so we can use cout for output
• b. so we can use the shortcut name cout instead of the full name std.cout
• c. program name is always main with int (which stands for integer) before it
• d. num1 and num2 are the data items (which we often call variables)
• e. they get initial values from instructions via assignment (the = is the assignment command)
• f. then the value in num1 is changed by adding num1 and num2 and putting the result back into num1
• g. the result is displayed via cout: "Sum is: 25"
• h. return an integer: main "promised" to provide an integer "answer". In this program (and usually) the "answer" doesn't matter.  But we have to keep the promise, so we just return 0.  We will discuss this further later.
• on any line, typing // turns the rest of the line into a comment. A comment is not "seen" by the computer - it is not part of the program.  It is used by human readers of the program to explain something in the program.
• there are no program control or modularization statements
• this program has no input 
• notice the semicolons - where they are and aren't

Program Error Types

1. Compile error - invalid C++ statements; so your code cannot be translated to machine language.  Examples:

1. integer x, y;
2. num1 = num1 + num2

In example 1 you must use int, not integer.
In example 2 the ; at the end of the line is missing

2. Link error: one of the functions used in the program is not found when it's time to run the program.  In the program above, there are no such functions, but we could encounter this problem later.

3. Run-time error - program does not run to completion.  Example:

• divide by zero: set x = 10 and y = 0 and then try to compute x/y

4. Logic error - program runs to completion, but the results are wrong.  Example:

• you really wanted x - y but coded x + y
• in calculating an average, you use the wrong number to divide

Fixing 3 and 4 is called debugging.

You must learn to understand that the computer did what you said, not necessarily what you meant.

The C++ Language - Introduction

Note: C++ is a superset of the C programming language.  Any C code is also C++ code, but some C++ code is not C.  Much of what you learn in this course could also be used in a C program.  There are lots of new features and capabilities in C++.  We will learn some of them (but far from all of them) in this course.

The C++ Language is made up of

• keywords/reserved words (if, while, int, etc.)
• symbols: { } =  |  <=  ! [ ]  *  &  (and more )
• programmer-defined names for variables and functions

These programmer-defined names:

• 1 - 31 chars long; use letters, digits, _ (underscore)
• start with a letter or _
• are case-sensitive: Num is different than num
• should be meaningful: studentCount is better than s or  sc

C++ Data Types

Each data item has a type and a name chosen by the programmer.
The type determines the range of possible values it can hold as well as the operations that can be used on it.  For example, you can add a number to a numeric data type, but you cannot add a number to someone's name.   (What would "Joe" + 1 mean?)

Three commonly-used data types

1. int (integer numbers)

• no decimal point, comma, or $
• leading + - allowed
• range (on our system): about plus or minus 2 billion
• int constants are written as: 1234  -3   43

To make a variable that your program can use, you must declare it.

Declaration Examples:

int x;		// gives type and name; no value yet
  
int x, y;	// declares 2 ints; no values yet
 
int population = 16000;	  // declares & sets init value

So you could picture the results of the last two lines as:

Note: It is critically important that variables have values before they are used in a program.  In the examples above, x and y have no values even though they have been declared.  More accurately, they do have values but those values could be anything between -2 billion and +2 billion.  Before you use x or y you would have to give them values (perhaps as shown in the first program example.)

2 and 3. float and double (floating point or real numbers).  These data types are also commonly referred to as real variables (following mathematics usage) when it is not important which one we mean and we don't want to always have to say "float or double".

• has decimal point
• leading + - allowed
• no comma, $
• range (float)  plus or minus 10 to 38th  (limited to ~ 6 significant digits)
• range (double) plus or minus 10 to 308th (limited to ~12 significant digits)
• values are rarely exact
• floating point constants are written as 3.08  -32.0  0.15  9.3e7   (9.3 times 10 to the 7th power)

Declaration Examples:

float  pi = 3.1416;   //declares, names, sets init value
 
double x = 3.5,	      //note comma here
       y = 1245.6543; //can use > 6 digits

OR

double x   = 3.5;
double y   = 1245.6543;
 
float  big = 5.28e3;  // exponential notation: 5280
 

4. char (single characters)

• can hold just one character
• char constants are written with single quotes: 'a'

Declaration examples:

char ch;

char choice = 'q';

5. string (character "strings" or sequences)

• can hold 0 to many characters
• string constants are written with double quotes:  "Hello, world"

Declaration Examples:

string s;
 
string MyName = "Jim";
 

Arithmetic Operations and Examples

The arithmetic operators are:

• +  addition
• -  subtraction or unary negation (-5)
• *  multiplication
• /  division (see special notes on division below)
• %  integer remainder of integer division

Note: there is no exponentiation operator.

     3  R1		
   _______      4 is divisor
 4| 13		13 is dividend
    12		3 is quotient
     1		1 is remainder
 

   13/4 --> 3	// int quotient
   13%4 --> 1	// int remainder

So: 13.0 / 4 --> 3.25
    13 / 4.0 --> 3.25

Arithmetic Expressions are formed by operands and operators.  Operands are the values used, operators are the things done to the numbers.  Parts of arithmetic expressions are often grouped by () for clarity or to affect the meaning of the expression:

// declare:
int x, y;
double realnum;
 
// initialize
x       = 11;
y       = 2;
realnum = 2.0;
 

Note that the expressions above by themselves are not valid C++ statements - they would be part of a statement.  But each expression (if it is in a valid statement) has a value.

Also - the spaces written between operands and operators are not required by C++, but do improve legibility.

More complex expressions are evaluated by C++ using Rules of Precedence (like algebra)

1. sub-expressions in ()
2. unary -
3. * and / - left to right
4. + and - - left to right

In the examples below, the red numbers show what operation is done as C++ evaluates and expression:

So: 3 + 5 * 4 / 2 =
3 + 20 / 2 =
3 + 10 =
13

But () can be used to change this (or to make expression more readable)

(3 + 5) * 4 / 2 =
8 * 4 / 2 =
32 / 2 =
16

Example 1:

Suppose you accumulate 5 int test scores in sum.
And suppose that sum is 104.

Then to write an expression that is the average, you could write:

sum/5

and the value of that expression is 20 (a bit wrong - we lost the decimal fraction .8)

But if you write:

sum/5.0

Then the value of the expression is 20.8 (correct)

Example 2:

Suppose you don't know how many numbers there are when you write the program. So you can't use 5 or 5.0 because there might be 21 or 3 or 97 numbers. You could use a variable to hold the count of numbers:  declare int count and write some instructions that will give it the proper value before you want to calculate the average.  We'll see how to do this soon.

You can't write "Count.0" to force it to be real (like we wrote 5.0 above).  C++ simply does not allow this.

You must use a type cast - to tell C++ that it should temporarily use the variable as if it were some other data type. To do this, put the name of the data type you want in parenthesis before the variable name.  It will not change the variable type or value permanently, but it will force C++ to temporarily treat the variable as a different type:

(double) sum/count     // or
sum /(double)count

(It is true that we could avoid the need to type cast here by declaring count to be double instead of int.  But there are other situations where you will either want or need to do this.)

You cannot do just any type cast:  (char) sum would not make sense and would not compile.

Assignment Statement/Operation

The symbol "=" is a command.

It means: "evaluate the expression on the right and store this value in the variable on the left" 

Memorize that sentence.

In general:

someVariable = some expression;

Examples:

x = y * 3;

x = x + y;

Notice that there is always exactly one variable on the left to receive the value of the expression on the left.

This is NOT like algebra; it is NOT an equation.

Algebra: x = 1   (T or F for a given x)	
C++:     x = 1;  // store 1 into x
 
Algebra: x = x + 1  (false for all x)
C++:     x = x + 1; //find x+1; result in x

More Assignment Examples

//-- declare vars
double dAns;
int iAns;
int i, j;
double x, y;
 
//-- assign values via numeric literals
i = 5;
j = 8;
x = 4.0;
y = 1.5;
 
//-- assign values from complex expressions
i = i + 2;	// 7

dAns = x / y;	// 2.666..
iAns = j/2*2;	// 8/2*2 = 4*2 = 8
iAns = 8/5;	// 1
iAns = 8 % 5;	// 3
iAns = 5 / 8;	// 0
iAns = 5 % 8;	// 5
i = x/y;	// 2 

The last needs an explanation.  the value of the expression x/y is 2.666.. but i is an int variable and so cannot hold the .666 part.  C++ simply drops it (truncates it) and so only the whole number part of the value is stored into i.

Notes:
1. All variables in expressions on the right must have defined values or you get random results.  This is an example of what I meant before when I said variables must have values before they are used.

int x, y;	// declared; no init values
x = y + 1;	// ??? in x

 
int x;		// declared; no init value
int y = 2;	// declared; given a value
x = y + 1;	// x has value 3.  
		// It is Ok if x has no value before this

Inadvertant use of uninitialized variables is one of the most common causes of program errors.  Learn to be very careful about this.

2. You can do multiple assignments on one line:

x = y = z = 3;  //all get value 3

3. Naming Variables: use meaningful names that explain themselves to reader of the program. (You, me, the maintainer of the program after you go to your next job...)

No	Yes
s	score
cs	class_size	ClassSize	classSize
cav	class_average	ClassAvg	classAvg
num	student_count	NumStudents	numStudents
stnum	student_id	StudentID	studentID

Input

We will use the standard C++ library input and output features. 

For now we will use cin for input.  cin's task is to accept user input and store it into variables.

Note that

• the C++ language itself has no defined I/O functions or commands; just a standard library that can accomplish I/O.  We could use other libraries, including those from the C language or those developed by others.
• most often when we want user input, we will need to do two things:
  1. tell the user what to enter and then
  2. get the value entered.

So typically we will do something like this:

int i;                                 // a.
...
cout << "Enter the temperature: ";     // b.
cin >> i;                              // c.

Notes:

• a. we declare an int to hold the value to be typed in
• b. we tell ( prompt) the user to enter something
• c. we get the value that is entered by the user
• when a cout occurs in a program, the characters are displayed immediately on the screen
• when cin occurs in a program, the program waits for user to type in a value and press <Enter>.
• the << and >> are meant to suggest the direction the data is going:
  • for cout, the prompt is going from the string constant through cout to the screen
  • for cin the data entered is coming in from the keyboard, through cin, to the variable i
• for numeric input with cin, if the user types a non-number like "w" or "23e" then results are unpredictable.  (For a double or float, "2e5" is ok - it's the same as 200000)
• the variable receiving the value from cin must be declared (of course) but need not have an initial value. Any previous value in that variable is lost.
• details about formatting numeric output (number of decimal places, etc.) will be covered below.
• cin "understands" the different data types we have covered.  So you can do cin with int, float, double, char, and string.  But you must ensure that the data item getting the value is compatible with what the user is asked to enter.  You can't store "jim" into an int, for example.  You can store the value 4 into a double, however.

Output

We will use cout for output.  cout's task is to display information on the screen.

As shown above, the simplest form of cout is:

cout << "Hello";

You can output (for display) several values, including the values calculated and stored into variables, by stringing them together connected by "<<":

cout << "The average is: " << avg;     // avg is a variable

cout << "The temperature is: " 
     << temp 
     << " and the rainfall today was: " 
     << rainAmt;

Note that I have put each item to display on a separate line for clarity.  This is both legal and is usually desirable if you have more than 2 values.

There are a couple of special techniques to position cursor before or after you display characters, using an escape sequence. (A \ followed by a char. For example, \n is the newline escape sequence.)  Note that this is a backslash.

cout << "\nhello";    //moves to next line, shows hello
cout << "hello\n";    //shows hello, then goes to new line
cout << "hel\nlo";    //shows hel, then lo on next line

Note: to display a single \, you must code two: \\

cout << "the \\ char is special"; 

This shows: "the \ char is special" 

Also, there is an alternate way to move the cursor to the beginning of a new line: the special value "endl":

cout << endl << "Some stuff" << endl;

cout << "\nSome stuff\n";

Both will first move the cursor to a new line, then display "Some stuff" and then move the cursor to the next line.

Note that if you don't include endl or \n, all your output will be on one line:

cout << "The average is: " << avg;       // avg is a variable

cout << "The temperature is: " 
     << temp 
     << " and the rainfall today was: " 
     << rainAmt;

This will display something like this:

The average is 77.5The temperature is: 78 and the rainfall today was: 1.32362