Variables and Arithmetic Expressions.

Variables in C have the same meaning as variables in algebra.  That is, they represent some unknown, or variable, value.
x = a + b
Variables in C may be given representations containing multiple characters.  But there are rules for these representations.
Variable names (identifiers) in C
May only consist of letters, digits, and  underscores
May be as long as you like, but only the first 31 characters are significant
May not begin with a digit
May not be a C reserved word (keyword)

Reserved Words (Keywords) in C

auto     break        float     for                        sizeof                 static

case     char          goto        if

const                continue           int      long            struct                 switch

Default                 do                     register      return

double                else               short                 signed            typedef      union

enum                   extern            unsigned      void                     volatile      while

Naming Conventions

C programmers generally agree on the following conventions for naming variables.

Begin variable names with lowercase letters

Use meaningful identifiers

Separate “words” within identifiers with underscores or mixed upper and lower case.  

Be consistent!

C is case sensitive.

Use all uppercase for symbolic constants   (used in #define preprocessor directives).

Note:  symbolic constants are not variables, but make the program easier to read.

Examples:

    #define PI 3.14159

    #define AGE  52

What Does a Variable Have?

Every variable has:

a name (e.g., number of students),

an address (i.e., a location in memory, such as 123456),

a data type (e.g., int, float, char), and

a value (which may be undefined, also known as garbage). The value is also known as the contents of the variable — that is, the value is the contents of the variable’s memory location.

Compile Time and Run-time

Events that occur while a program is being compiled are said to happen at compile time.

Events that occur while a program is running are said to happen at runtime.

For example, the address of a variable is chosen at compile time, while its value often is determined  at runtime.

Declaring Variables

Before using a variable, you must give the compiler some information about the variable; i.e., you must declare it.

The declaration statement includes the data type of the variable.

Examples of variable declarations:

  int age ;

  float  area ;

When we declare a variable

Space is set aside in memory to hold a value of the specified data type

That space is associated with the variable name

That space is associated with a unique address

Unless we specify otherwise, the space has no known value.

Visualization of the declaration

int  age ;

Notes About Variables.

You must not use a variable until you somehow give it a value.

You can not assume that the variable will have a value before you give it one.

Some compilers do, others do not!  This is the source of many errors that are difficult to find.

Assume your compiler does not give it an initial value!

Variables may be given initial values, or initialized, when declared. 

Displaying Variables.

Variables hold values that we occasionally want to show the person using the program.

We have a function called printf( ) that will allow us to do that.

The function printf needs two pieces of information to display things.  

How to display it

What to display

printf( “%f\n”, diameter );

The name of the function is “printf”.

The %f is known as a placeholder: it holds the place of the value of the variable that we actually want to output.

Inside the parentheses are:

print specification, where we are going to  display:

a floating point value (“%f”)

We want to have the next thing started on a new line (“\n”).

We want to display the contents of the variable diameter.

Code	Format
%c	character
%d	signed integers
%i	signed integers
%e	scientific notation, with a lowercase "e"
%E	scientific notation, with a uppercase "E"
%f	floating point
%g	use %e or %f, whichever is shorter
%G	use %E or %f, whichever is shorter
%o	octal
%s	a string of characters
%u	unsigned integer
%x	unsigned hexadecimal, with lowercase letters
%X	unsigned hexadecimal, with uppercase letters
%p	a pointer
%n	the argument shall be a pointer to an integer into which is placed the number of characters written so far
%%	a '%' sign

scanf (“%f”, &diameter);

The scanf( ) function also needs two items:

The input specification “%f”.  (Never put a “\n” into the input specification.)

The address of where to store the information.  (We can input more than one item at a time if we wish, as long as we specify it correctly.)

Notice the “&” in front of the variable name.  It says to use the address of the variable to hold the information that the user enters.

Comments should explain why you are doing something, not what you are doing it.

Reading Multiple Variables with a Single scanf

C allows inputting multiple variables per scanf statement. At runtime, when the user types in the input values, they can separate the individual input values

by blank spaces, and/or

by tabs, and/or

by carriage returns (newlines).

Blank spaces, tabs and carriage returns, as a group, are known as white space.

Arithmetic Operators in C

Name       Operator   Example

Addition             +     num1 + num2

Subtraction      -     initial - spent

Multiplication    *    fathoms * 6

Division                /    sum / count

Modulus            %     m % n

If both operands of a division expression are integers, you will get an integer answer.   The fractional portion is thrown away.

Examples : 17  /  5  =  3

   4  /  3  =  1

  35  /  9  =  3

Division By Zero.

Division by zero is mathematically undefined.

If you allow division by zero in a program, it will cause a fatal error.  Your program will terminate execution and give an error message.

Non-fatal errors do not cause program termination, just produce incorrect results.

Arithmetic Operators Rules of Operator Precedence

Operator (s)              Precedence & Associativity

( )                      Evaluated first. If nested   (embedded), innermost first.  If              on same level, left to right.

*   /   %                  Evaluated second.  If there are          several, evaluated left to right.

    +   -              Evaluated third.  If there are     several, evaluated left to right.

      =      Evaluated last, right to left.   

What is a Constant?

In mathematics, a constant is a value that cannot change.

In programming, a constant is like a variable, except that its value cannot change.

There are two categories of constants:

literal constants, whose values are expressed literally;

named constants, which have names.

A literal constant is a constant whose value is specified literally:

int literal constants

(e.g., 2, 1, 7,98, -1)

float literal constants

(e.g., 2.4, 0.6, -14.7)

char literal constants

(e.g., ’B’, ’4’, ’?’)

Named Constants

A named constant is a constant that has a name.

A named constant is exactly like a variable, except that its value is set at compile time and CANNOT change at runtime.

A named constant is exactly like a literal constant, except that it HAS A NAME.

In a named constant declaration, we indicate that it’s a constant via the const attribute, and we MUST initialize it:

const float pi = 3.14;

When you embed numeric literal constants in the body of your program, you make it much harder to maintain and upgrade your program.