HPAVC/HPAVC CD-ROM.iso/PASCTXT.ZIP/CHAP4.TXT

Text File | 1988-01-15 | 16KB | 389 lines

           CHAPTER 4 - The Pascal loops and control structures


             Every program we have examined to this point has been a
        simple  one pass through with no statements being  repeated.
        As in all other languages, Pascal has extensive capabilities
        to  do looping and conditional branching.  We will  look  at
        these now.

                               THE FOR LOOP

             We will start with what may be the easiest structure to
        understand, the "for" loop.  This is used to repeat a single
        Pascal  statement  any  number of  times  we  desire.   Load
        LOOPDEMO and we will discuss the loops presented there.

             The  first  example  is the simplest and  is  simply  a
        repeat  of  a Writeln 7 times.  We have three  new  reserved
        words,  "for", "to", and "do" which are used as shown.   Any
        simple  variable of type integer, byte, or char can be  used
        for  the  loop  index  and  it must  be  defined  in  a  var
        statement.   Following the "do" reserved word is any  single
        Pascal statement that will be repeated the specified  number
        of  times.  Note that the loop is an incrementing  loop  but
        substitution   of   "downto"  for  "to"  will  make   it   a
        decrementing  loop as is illustrated in the last example  in
        this  program.   It  should be pointed  out  that  the  loop
        control variable can only be incremented or decremented by 1
        each time through the loop in  Pascal.

                        A COMPOUND PASCAL STATEMENT

             The  second example contains our first compound  Pascal
        statement.  It was mentioned in Chapter 1 that the begin end
        pair of reserved words could be used to mark the limits of a
        compound  statement.   In this case,  the  single  statement
        starting  with  the  "begin"  at the  end  of  line  17  and
        extending through and including the end statement in line 21
        is  the  single Pascal statement that will  be  executed  10
        times.   A  second  variable Total has  been  introduced  to
        simply add another operation to the loop.  Any valid  Pascal
        operation  can  be performed within the  "begin  end"  pair,
        including  another  for loop, resulting in nested  loops  to
        whatever depth you desire.

             The  third  example shows how the  char  type  variable
        could be used in a for loop.  Pascal requires that the  loop
        variable, the starting point, and the ending point all be of
        the  same  type or it will generate an  error  message.   In
        addition,  it must be a variable of type integer,  byte,  or
        char.  The starting point and ending point can be  constants
        or expressions of arbitrary complexity.




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           CHAPTER 4 - The Pascal loops and control structures


             The fourth example is a decrementing loop as  mentioned
        earlier.  It uses the reserved word "downto".

                             THE IF STATEMENT

             Pascal has two conditional branching capabilities,  the
        "if"  and the "case".  We will look at one of them now,  the
        if  statement.  Load IFDEMO for an onscreen look at the  "if
        then"  pair  of reserved words.  Any condition that  can  be
        reduced  to  a boolean answer is put between the  "if  then"
        pair  of  words.  If the resulting  expression  resolves  to
        TRUE,   then  the  following  single  Pascal  statement   is
        executed,  and if it resolves to FALSE, then  the  following
        single  statement  is  skipped over.   Of  course,  you  can
        probably  guess  that the single statement can  be  replaced
        with a compound statement bracketed with a "begin end"  pair
        and you are correct.  Study example 1 and you will see  that
        the line will always be printed in this particular  fragment
        because  Three is equal to One + Two.  It is very  difficult
        to come up with a good example without combining some of the
        other control structures but we will do so in the next file.

             The  second example in lines 14 through 19, is  similar
        to  the first but has the single statement replaced  with  a
        compound statement and should be easy to understand.

             The  third example in lines 21 through 24,  contains  a
        new reserved word, "else".  When the if condition is  FALSE,
        the  single  statement  is skipped and  if  a  semicolon  is
        encountered, the if clause is totally complete.  If  instead
        of  a  semicolon, the reserved word "else"  is  encountered,
        then the single Pascal statement following else is executed.
        One  and  only one of the two statements  will  be  executed
        every  time the if statement is encountered in the  program.
        Examination  of  the third example should clear this  up  in
        your mind.

             Notice that the Pascal compiler is looking for either a
        semicolon  to  end the if, or the reserved  word  "else"  to
        continue  the  logic.  It is therefore not legal  to  use  a
        semicolon  immediately preceding the reserved  word  "else".
        You will get a compiler error if you do so.

                         THE IF-THEN-ELSE block

             Put on your thinking cap because the next principle  is
        difficult  to grasp at first but will suddenly clear up  and
        be  one  of  the most useful facts  of  Pascal  programming.
        Since  the entire "if then else" block of code is  itself  a
        single  Pascal  statement  by definition,  it  can  be  used
        anywhere that an executable statement is legal without begin


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        end separators.  This is shown in the fourth example of  the
        IFDEMO Pascal example program.  Lines 27 through 30 comprise
        a single Pascal statement, and lines 32 through 35  comprise
        another.  The if statement begun in line 26 therefore has  a
        single statement in each of its branches.

             The  "if then else" construct is one of the most  used,
        most useful, and therefore most important aspects of Pascal.
        For this reason you should become very familiar with it.

             Try  changing  some of the conditions  in  the  example
        program to see if you can get it to print when you expect it
        to for your own practice.  When you are ready, we will go on
        to a program with loops and conditional statements  combined
        and working together.

                          LOOPS AND IFS TOGETHER

             Load  LOOPIF  and  observe it for a  few  minutes.   It
        contains most of what you have studied so far and should  be
        understandable  to  you at this point.  It contains  a  loop
        (lines 7 & 17) with two if statements within it (lines 8 & 9
        and lines 10 through 16), and another loop (lines 11 through
        15) within one of the if statements.

             You  should  make careful note of the  formatting  used
        here.   The "begin" is at the end of the line  which  starts
        the control and the "end" is lined up under the control word
        such  that  it  is  very clear  which  control  word  it  is
        associated with.  You will develop your own clear method  of
        formatting your code in time but until then it is  suggested
        that you follow this example.

             An  easily  made error should be pointed  out  at  this
        time.  If an extraneous semicolon were put at the end of the
        if  statement  in line 8, the code following  the  statement
        would  always be executed because the "null" statement  (the
        nothing  statement  between the "then"  and  the  semicolon)
        would be the conditional statement.  The compiler would  not
        generate  an  error  and you would get no  warning.   Add  a
        semicolon at the end of line 8 to see the error.

                       FINALLY, A MEANINGFUL PROGRAM

             Load  TEMPCONV  and  study its  structure.  Notice  the
        header block that defines the program and gives a very brief
        explanation  of what the program does.  This program  should
        pose  no problem to you in understanding what it does  since
        it  is  so clearly documented.  Run it and you will  have  a
        list  of  Centigrade to Fahrenheit  temperature  conversions
        with a few added notes.


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             Load,  examine, and run DUMBCONV for a good example  of
        poor  variable  naming.   The structure of  the  program  is
        identical to the last program and when you run it, you  will
        see that it is identical in output, but compared to the last
        program,  it  is  difficult to understand what  it  does  by
        studying the listing.  This program, like the last should be
        easily  understood  by  you, so we will go on  to  our  next
        Pascal control structure.

                           THE REPEAT UNTIL LOOP

             The next two Pascal constructs are very similar because
        they are both indefinite loops (indefinite because they  are
        not executed a fixed number of times).  One of the loops  is
        evaluated  at the top and the other at the bottom.  It  will
        probably  be  easier  to start  with  the  "repeat"  "until"
        construct which is the loop that is evaluated at the bottom.

             Retrieve  the  file REPEATLP to see  an  example  of  a
        repeat  loop.   Two more reserved words  are  defined  here,
        namely  "repeat" and "until".  This rather simple  construct
        simply repeats all statements between the two reserved words
        until the boolean expression following the "until" is  found
        to  be  TRUE.  This is the only expression I  know  of  that
        operates  on  a  range of statements rather  than  a  single
        statement and begin end delimiters are not required.

             A word of caution is in order here.  Since the loop  is
        executed  until some condition becomes TRUE, it is  possible
        that  the  condition will never be TRUE and  the  loop  will
        never terminate.  It is up to you, the programmer, to insure
        that the loop will eventually terminate.

             Compile and run REPEATLP to observe the output.

                              THE WHILE LOOP

             The  file  WHILELP contains an example of  another  new
        construct,  the  "while" loop.  This uses the  "while"  "do"
        reserved words and will execute one Pascal statement (or one
        compound statement bounded with begin and end)  continuously
        until  the boolean expression between the two words  becomes
        FALSE.

             This  loop  is also indeterminate and could,  like  the
        repeat  until loop, never terminate.  You  should  therefore
        exercise care in using it.

             There are two basic differences in the last two  loops.
        The repeat until loop is evaluated at the bottom of the loop


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        and  must therefore always go through the loop at least  one
        time.  The while loop is evaluated at the top and may not go
        through even once.  This gives you flexibility when choosing
        the loop to do the job at hand.

             Compile,  run, and examine the output from the  example
        program WHILELP.

                            THE CASE STATEMENT

             The   final  control  structure  introduces  one   more
        reserved  word, "case".  The case construct actually  should
        be included with the if statement since it is a  conditional
        execution statement, but I chose to save it for last because
        it is rather unusual and will probably be used less than the
        others we have discussed in this chapter.

             The  case  statement  is used to  select  one  of  many
        possible  simple Pascal statements to execute based  on  the
        value  of  a simple variable.  Load the  file  CASEDEMO  and
        observe the program for an example of a case statement.  The
        variable between the "case" and "of" reserved words in  line
        9  is  the variable used to make the  selection.   Following
        that, the various selections are listed as a possible  value
        or  range, followed by a colon, a single  Pascal  statement,
        and  a semicolon for each selector.  Following the  list  of
        selections, an "else" can be added to cover the  possibility
        that none of the selections were executed.  Finally, an  end
        statement  is  used to terminate the case  construct.   Note
        that this is one of the few places in Pascal that an end  is
        used without a corresponding begin.

             The  example file uses Count for a variable and  prints
        the numbers one through five in text form, and declares that
        numbers  outside this range are not in the  allowable  list.
        The  program should be self explanatory beyond  that  point.
        Be sure to compile and run this example program.

             Load and display the sample program BIGCASE for another
        example of a case statement with a few more added  features.
        This  program uses the identical structure as  the  previous
        program  but in line 11 a range is used as the  selector  so
        that if the value of Count is 7, 8, or 9 this selection will
        be  made.   In line 12, three different listed  values  will
        cause  selection  of  this part of  the  code.   Of  greater
        importance  are the compound statements used in some of  the
        selections.  If the variable Count has the value of 2, 4, or
        6, a compound statement will be executed and if the value is
        3,  a  for  loop  is executed.  If the value  is  1,  an  if
        statement is executed which will cause a compound  statement
        to  be executed.  In this case the if statement will  always


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        be  executed  because  TRUE will always  be  true,  but  any
        Boolean expression could be used in the expression.

             Be sure to compile and run this program, then study the
        output until you understand the result thoroughly.

             This brings us to the end of chapter 4 and you now have
        enough information to write essentially any program  desired
        in  Pascal.   You  would  find that you  would  have  a  few
        difficulties  if  you attempted to try to write a  very  big
        program  without  the  topics  coming up  in  the  next  few
        chapters.   The  additional topics will greatly add  to  the
        flexibility  of Pascal and will greatly ease programming  in
        it.


                           PROGRAMMING EXERCISES

        1.  Write a program that lists the numbers from 1 to 12  and
            writes a special message beside the number  representing
            your month of birth.

        2.  Write  a program that lists all of the numbers from 1 to
            12 except for the numbers 2 and 9.





























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