A routine is a sequence of instructions to carry out a particular task. A'routine is a general tenn for a relatively small self-contained set of instructions. It may refer to:
1 A complete short program; or
2 A part of a program; or
3 A set of instructions used by other programs.
A subroutine is a routine which is used by one or more programs. A subroutine is not itself a complete program. It has to be called by other programs.
A program calls the subroutine and the subroutine's task is carried out. Control then returns to the correct place in the calling program, which continues from where it left off.
Often a computer system has a library of subroutines available for programmers to use.
A library subroutine is a subroutine which is available to all users of a computer system as part of the software.
A parameter or argument is a value passed between a program and a subroutine. Most languages (though not some versions of BASIC) allow a programmer to do either of the following:
1 Use one name for a parameter in a subroutine and another name for the same parameter when it appears in the calling program.
2 To have a variable in a subroutine and another. one in the calling program, both with the same name. This is made possible by having local variables.
A local variable is one whose name is restricted to one subroutine.
A global variable is one which has the same name throughout a program and all its routines.
Advantages of these arrangements are:
1 A subroutine can be used in more than one program without worrying
(a) Whether names in the subroutine and the program clash; or
(b) Whether the right names have been used for the parameters.
2 Subroutines from subroutine libraries can be used without a problem over names.
3 The same subroutine can be used to operate on two different variables in the same program.
Languages such as COMAL and Pascal (and some versions of BASIC) allow a type of subroutine called a procedure. Procedures are generally used in stepwise refinement of a program .
1 Procedures are executed using an EXEC statement.
2 A procedure is defined between a PROC and an END PROC.
3 Parameters are put in brackets after the name of the procedure.
4 All variables in a procedure are made local using CLOSED.
The following COMAL routine produces an introductory pattern for a game of noughts and crosses. It simply prints a pattern made of alternate blocks of o's and x's. In fact it repeats the following a times down the page:
00000000000000000000
00000000000000000000
00000000000000000000
00000000000000000000
xxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxx
100 //I ROUTINE TO PRINT ALTERNATELY BLOCKS OF o'S AND x'S //
200 FOR I : = 1 TO 3
210 EXEC BLOCK("o")
220 EXEC BLOCK("x")
230 NEXT I
299 //
300 PROC BLOCK(CHAR$) CLOSED 310 FOR I : = 1 TO 4
320 EXEC ROW(CHAR$)
330 NEXT I
340 END PROC BLOCK 399 //
400 PROC ROW (CH$) CLOSED 410 FOR I : = 1 TO 20
420 PRINT CH$;
430 NEXT I
440 PRINT
450 END PROC
1 Because the procedures BLOCK and ROW are both CLOSED the variables in them are local. In particular the changes of I in ROW and BLOCK do not affect the I in the main program.
2 The variable CHAR$ in BLOCK is a parameter which takes first the value "0" and then the value "x".
In high level programming a function is a sequence of instructions used by a program to supply a value in an expression.
There are two main types of function:
1 Built-in functions (or standard functions). These are functions which are always available to users of a particular language.
2 Programmer-defined (or user-defined) functions. These are functions written by the programmer.
1 Built-in functions.
(a) LEN
100 LET N1=LEN(A$)
LEN is a function giving the number of characters in a string. For example, if A$ is the string "HELLO", Nl will be assigned the value 5.
(b) INT
110 IF Num= INT(Num) THEN PRINT "Whole number"
INT gives the whole number below the value of the expression in the brackets. For instance LNT(2.6) is 2 and INT( -3.4) =-4
Num will only be equal to INT(Num) if Num is a whole number.
2 Programmer-defined functions.
The following function C would correct a value to the nearest whole number. It uses the built-in function INT mentioned above.
Definition of the function C: 100 DEF FNC(N)~INT(N+.5)
Use of the function C to correct the values of Hand W:
500 PRINT "Approximate height and weight are ": FNC(Hl, FNC(W)
In structured languages it is usual to write first the main structure of a program. The details are then filled in by defining functions and procedures. In other words these structures make stepwise refinement easier while doing top-down programming .
The following program is a COMAL version of the program discussed . It has been written top-down in three levels as suggested there:
1 The function of the program is expressed in line 100.
2 This function is refined into procedures in lines 110 to 130.
3 The procedures are written in detail in lines 200 to 430.
100 //PROGRAM : Input three numbers and print out their average.//
110 EXEC input_ Three..Numbers
120 EXEC Calculate., Total_And_Average
130 EXEC Print-Nvumbers-And- Their-Average
140 END
199 //
200 PROC input-Three-Numbers
210 PRINT "The Average of Three Numbers"
220 PRINT
230 INPUT "Please type your first number ": Num1
240 INPUT "Please type your second number ", Num2
250 INPUT "Please type your third number ": "Num3
260 END PROC Input-Three Numbers
299 //
300 PROC Calculate., Total-And-Average
310 Total: = Numl + Num2 + Num3
320 Average: = Total 3
330 END PROC Calculate- Total-And-Average
399 //
400 PROC Print-numbers-And-Average
410 PRINT "The Numbers are :- ";Num1,Num2,Num3
420 PRINT "Their average is :- ": Average
430 END PROC Print-Numbers-And-Average
Labels: Computer