Variables

We have encountered two data types so far
- int as in
- ```
int main () ...
```
- strings as in
- ```
cout << "The square of 27 is";
```
Each of these can hold data of a well defined type.
int - integers, and float - real numbers, will be discussed in chapter 3
For now, we will look at string and char
A data type is a set of data values along with a set of operations on those types.

char Data Type

The compiler translates values in a data type into the correct binary representation for us.
The char type contains a single character
Most machines use ASCII American Standard Code for Information Exchange
- This associates a binary number with each character.
  
  Character Decimal Hex Binary
  
  A 65 41 0100 0001
  
  B 66 42 0100 0010
  
  C 67 43 0100 0011
  
  # 35 23 0010 0011
  
  i 105 69 0110 1001
  
  j 106 6A 0110 1010
- Charts everywhere, including Appendix E.
- man ascii (on most unix boxes)
- There are non-printable characters in this set (7 is a bell)

Character	Decimal	Hex	Binary
A	65	41	0100 0001
B	66	42	0100 0010
C	67	43	0100 0011
#	35	23	0010 0011
i	105	69	0110 1001
j	106	6A	0110 1010

string Defined Type

A string is a collection of characters.
- This is not a built in data type
- A set of characters begun and ended with quotes "
- Example
- ```
"a string"
"0 u 8 1 2?"
	 
```
- A string containing no characters is the null string ""
- It is from the standard library

Variables

To be able to program, we need to have a place to put things.
These are called variables
A variable is a location in memory, that can contain data.
The process of making a variable is called declaring the variable
The statement that makes a variable is called a variable declaration.

Format:


   DataType  Identifier;
   DataType  Identifier, Identifier, ... ;

Some examples:

       char    Initial;  
       char    response;
       string  first_name;
       string  middle_name, last_name;
       int     age;
       int     day, month, year;

Comments

programming is hard,
programs are hard to read
Comments or notes to the programmer in the code makes code easier to read.
Single comments begin with a // and end with a newline
Comments are unstructured.

    char   initial;   // The player's first initial
    char   response;  // responses to yes/no questions
    string first_name;   // the first name of the player

For now, give definitions for all of your variables.
You should also include comments at the beginning of your program


//   Program 1, a program that does something 
    A simple, one sentence description of the program
//   Programmer:  Dan Bennett
    Always include your name
//   Class:  CSCI 130 Section 3, Fall 2001
//
//   This program will:
//          - do this thing
//          - do this other thing
//          - do this last thing
    A more detailed analysis of the program.

Assignments

An assignment places a value into a variable.
This is done with an equal sign (=)

         
int    age;    // the person's age
char   mi;     // the person's middle initial
string first;  // the person's first name

age = 18;
mi = 'A';
name ="Bob";

A variable must be declared before it can be used in an assignment statement


    main () {
       a = 7;
    }

    Will result in 
    foo.C:5: `a' undeclared (first use this function)
    foo.C:5: (Each undeclared identifier is reported only once for each 

    From the compiler, assuming the program is called foo.C

Constants

In the above example, the values 18, A and Bob are constants
Integer constants are just written
character constants are surrounded by single quotes '
String constants are surrounded by double quotes "
Constants in the program as above are called literals
Another type of constant is one we will call a Named Constant
They are values that never change, but we want to give them a name.
Constants are usually declared in all upper case letters.
Declared as
```
const datatype identifier = value;
   
```
For Example:


const float PI=3.14159;   // the value of pi
const char BLANK = ' ';   // a blank character
const string END_COMMAND = "exit";  // the command to stop processing

We are not allowed to make any further assignment into a constant.

const float PI = 3.14159;

main () {

    PI = 9.0394;
}

Will result in 
foo.C: In function `int main ()':
foo.C:5: assignment of read-only variable `PI'

It is best to use named constants whenever a value has a meaning.

Capitalization for Identifiers

These are not rules, but matters of choice. I like:
variables
- All lower case letters : avar
- First Letter of Each Word : aVeryLongVariable
- Include _ : A_Very_Long_Variable
Constants:
- Usually All upper case: END_MARKER
- Occasionally lower case, but only in special cases: e, pi
Procedure names:
- as variables.

Tracing Programs

Simulate the computer as it operates
Write the module name
Write the name and leave a column space for each variable
Write a value under the variable name as new values are assigned to a variable.

     const float PI = 3.1415;
     const char BLANK = ' ';

     int main () {
         char ch1, ch2;
	 float  x,y,z;
       
         x = 2;
	 y = 5;
	 z = x;
	 ch1 = 'a';
	 ch2 = BLANK;
	 x = PI;

         return(0);
     }


     main:
        ch1     ch2      x       y    z
	----    ----    ----    ---- ---
	 a      ' '      2       5   2
	                 3.1415