LBS290                WHAT IS A PROGRAM - Lecture Notes

What is a program?
-  Executed instructions/series of steps - program instructions are done
   in a specific order.
-  Variables which are manipulated by the instructions

What is an instruction?
-  A specific step with an action to be taken
-  A command
-  An order
-  A demand
i.e. "Go to the grocery store" is an example instruction.

  Go to the grocery store
  Buy bread
  Buy milk
  Pay the Cashier
  Go Home

This is a program - it has a series of steps each of which has an
action to be taken.  It is "executed" in the order of the steps as
exactly as they are written down. No later step starts until the
preceeding step finishes.

What is a variable?
-  A variable is a "cubbyhole" or a place you can put things and
   retrieve them back later.
-  A variable has a label or name

The following is a "program" with three variables - the variables are
chalkboards - only ONE number can be on the board at a time - when a
new value is placed on a board the previous one is erased.

  You have 3 chalkboards labelled "A" "B" and "C"
  Write 12 on board A
  Write 7 on board B
  Write 6 on board B
  Write 5 on board C
  Look at board B, add 5 to the number and store the sum on board C
  Look at board A, look at board C, add the numbers and store the result
    on board B
  Tell me the contents of board B

Enough about chalkboards - What is a FORTRAN program?

This chalkboard example is the same as the following FORTRAN program:

      REAL A,B,C
      A = 12
      B = 7
      B = 6
      C = 5
      C = B + 5
      B = A + C
      PRINT *,B
      END

In this FORTRAN program there are three kinds of statements.  The
first type is a variable declaration statement.  These come at the
beginning of a program and determine the names of the variables the
program will use.  REAL A,B,C is an example of this type of statement.

The next type of statement is the ASSIGNMENT statement - it is
important to note that the FORTRAN assignment statement

      x = y

is quite different from the Algebraic expression

      x = y

In algebra this means "by definition, for the rest of this problem x
always has the same value as y"

If this is a FORTRAN assignment statement it means the following,
"look up the contents of the cubbyhole named Y.  Find cubbyhole X and
wipe out the contents of X and store the value you found in Y in X."

It is important to note that Y is unchanged by this operation - you
can look at the contents of a variable as much as you like without
changing the contents of Y.

Also the following is legal in algebra but NOT legal in FORTRAN:

      x + 7 = y + 12

The left hand side of the assignment MUST be a variable.  The right
hand side can be a single variable, constant value, or expression
using variables and/or constants.  For example

      X = 2
      Y = A
      Z = X + 2
      A = (X + 12) * (Y / 990)

It is important to be aware that the right hand side (or expression)
is completely evaluated BEFORE the resulting value is placed in the
variable named in the left hand side.  Consider the following code:

      X = 23
      Y = 12
      X = X + 7

What ends up in X?  30? or does it just keep going up forever, adding
7 to itself?  The answer is 30 - the left hand side is evaluated
completely using the OLD value of X.  This expression evaluates to 30.
The 30 is then stored into X wiping out the 23 but it doesn't matter
because the expression has already been calculated.

The third type of FORTRAN statement is called an "INPUT-OUTPUT"
statement.  The PRINT statement in our example looks up the contents
of a variable and displays it on the screen for the user.

Without print statements we could do all this nifty calculation but
never know what the final numbers were.

In addition to the PRINT statement which shows us the contents of
a variable, there is a statement called READ which allows us to enter a
number which will be stored in a variable.

One very unique feature of the READ statement is that while the READ
statement is executing waiting for the human user to type input, the
program is completely stopped waiting for the READ to complete.

An example program using the READ statement is as follows:

      REAL AGE,NEW
      NEW = 2
      READ *,AGE
      AGE = AGE + NEW
      PRINT *,AGE
      END

When this program executes 2 will be stored in NEW.  The the program
will stop and wait for you to type a number.  You type in 21.  The
program then stores 21 in the AGE variable and execution continues.
AGE is added to NEW and stored back into AGE.   Then AGE is printed
out and then the program terminates.

Note that because of the read statement, the program can work with
different data each time it runs.  Without READ statements, computers
would only be glorified hand calculators.

One additional concept is that of printing human readable messages.
The syntax for this is:

      PRINT *,'Human Readable Message Blah Blah Blah'

We can make the previous program run a little nicer by addiing a few
messages.

      REAL AGE,NEW
      NEW = 2
      PRINT *,'Enter your current age '
      READ *,AGE
      AGE = AGE + NEW
      PRINT *,'Soon you will be ',AGE
      END

It makes a program nicer when you are reading data to have a PRINT
immediately before the read which tells the user what data is being
read.  This is called the "prompt" because the user is being
"prompted" for a value to enter.

When this program stats up, the following will appear and the program
will wait for the data:

Enter your current age

Once the number has been entered the program continue.

For example the lbs290-prog which you run to turn in your programming
homework probably has some code like:

      PRINT *,'Enter the program number -'
      READ *,PROGNO

Now real programs have many more instructions than these programs but
these basic concepts do not change even in large programs.