Lecture 9 - September 25, 2007

graded Core exercise was returned.

in the 2pm class, we looked at all of the Alice programs that students wrote

graded text book homework was returned

graded Second FORTRAN was returned and some elements were discussed.

• how to simulate an IF (<condition>) THEN <statements> ELSE <statement> END IF using GO TO statements was illustrated
• how to simulate a WHILE (<condition>) DO <statements> END DO using GO TO statements was illustrated

green card with Pascal sample program and other Pascal information was handed out and discussed.

• Attention was paid to the need for including (input, output) in the program header statement if any input is to come from the keyboard and any output is to go to the screen.
• Constants (e.g. const) are to be declared before types (e.g. type) are to be declared before variables (e.g. var) .
• Procedures and functions are to be included in the main program and must precede the begin statement of the main program and follow the var declarations of the main program..
• Variables declared in sub-programs (i.e. procedure and function ) are local to the sub-programs (i.e. are not known by the main program)
• In Pascal, variables declared in the main program are accessible to the sub-programs unless the sub-programs contain variables of the same name.
• while not (eof) is identical to while not (eof(input)) and is testing for the end of the data stream coming from the keyboard. The user can signal this on a PC by hitting <ctrl-z>. On a linux box, type <ctrl-d>.
• while not (eoln) is identical to while not (eoln(input)) and is testing for the end of the line of data coming from the keyboard. The user can signal this by hitting the enter key.
• the reserved words (on the back of the green card) are words that may not be used as identifiers in Pascal programs: not as a const, not as a var, not as a type, not as a function name, not as a procedure name, not as a program name.
• Disk files used in a program should always be closed before the program ends.
• A ^ (e.g. circumflex) can be typed for the up-arrow used on the green sheet in relation to pointer variables
• Although it is possible to declare a type to be a file of real or a file of integer or a file of <component-type> we will always use the pre-defined type textfile when we declare our file variables (e.g. var infile : textfile; ) and although it is possible to use get and put with textfiles, we will not do so.

We discussed what the output from FirstPascal should look like and what should go to the screen and what should go to a file.

We looked at parts of a Pascal program using files. See the code below. Come to class prepared to answer questions about the meaning of various lines of the code. You will be given a chance to ask questions before you are questioned.

program useFiles (input, output, infile, outfile);

(* type declarations precede variable declarations *)

(* the word type only appears once even if multiple types are declared *)

type

aRecord = record (* record (heterogeneous data structure declaration *)

name : String;

age : integer;

average : real;

end;

anArray = array [1..5] of aRecord; (* array (homogeneous data structure) *)

(* variable declarations follow type declarations *)

(* word VAR only appears once even if multiple variables declared *)

var (* omitting this gives strange error message: begin expected *)

infile, outfile : textfile;

infileName, outfileName : String;

singleRecord : aRecord;

i,j : integer;

myArray : anArray;

(* subprograms, if any -- and there should be in here *)

(* what follows is part of a main program starting with begin *)

begin

write (' please enter the name of your input file and hit return ');

readln ( infilename);

write (' please enter the name of your output file and hit return ');

readln (outfilename);

assign (infile, infileName);

assign (outfile, outfileName);

reset (infile);

rewrite (outfile);

(* more code would be here *)

close (infile);

close (outfile);

(* more code would be here *)

end.