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SUBPROGRAM IN PASCAL

Function

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Pascal Functions

In Pascal, a procedure takes inputs (parameters) but does not return a value (except by using reference parameters).

A Pascal function takes inputs (parameters) and returns a single value to the calling program.

The function name is used to indicate the value returned by the function.

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Pascal Function Cont’d

Second type o subprogram supported by Turbo Pascal .

We have already seen and used some of Pascal’s own functions e.g.

Upcase, Length, Sqr, Sqrt.

A Function may be viewed as a special type of procedure that returns exactly one result.

Functions are declared , called and can be used with variable and value

parameters.

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Pascal Function Cont’d

Functions differ from procedures in that :

• The return type of a function is included in the function heading, Return type can be any simple data type i.e. REAL, INTEGER, CHAR, BOOLEAN, STRING, an Enumerated type or a subrange type.

• Within the function body, some value must be assigned to the function name I.e. the return value.

• Unlike procedure calls, function calls do not appear as separate statements. They are treated as though they were values and so are used in assignment statement, input/output statements and general expressions.

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Template

FUNCTION function_name ( parameter list) : Data_ Type;

e.g. Function Cube (num :integer) :integer;

Begin

Cube := num * num * num;

end;

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Exampleprogram calcCube;function Cube( n : integer) : integer;begin

Cube := n * n * n;end;var

number, answer : integer;begin

writeln('Enter a number: ');readln(number);answer := Cube(number);writeln('The cube of ', number, ' is ', answer);

end.

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Recall Mathematical Functions

A function takes one or more inputs and produces a single output for each input or set of inputs.

Examples:A function to double the value of the input:

d(x) = 2x d(2) = 4d(17) = 34

A function to cube the value of the input:c(x) = x3 c(2) = 8c(5) = 125

A function to compute the volume of a cylinder from the radius and height:v(r, h) = r2h

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If X, Y, A and B are global variables of type integer, some typical calls might be :

1. X := cube(5);

2. Y := 3* Cube(2) - 10 ;

3. A:= 3, Writeln(cube(A));

4. If Cube(A) > B then….

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NOTE:

1. When declaring a function, you must always give the function a data type which indicates the type of the single result.

E.g. FUNCTION Sample : INTEGER;

or FUNCTION Sample (a:integer;b,c :real) : BOOLEAN;

2. The function code must include a statement that assigns the result to the function name. In this way the function acts as a ‘variable’ in the main program.

3 Use functions when you wish to calculate a single value that will be used in the main program otherwise use procedures.

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ExampleWrite a function that sums integer elements in an array and returns the

value of sum.Incorporate this function into a main program that sums the contents of two arrays. (assume the arrays contain values in the elements and are of equal length)

Given the global variables

Type

array_num = array [1..50] of integer;

VAR

array1, array2, : array_num;

Total : integer;

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Example (Cont’d)FUNCTION SumArray( A : Array_num): integer;

Var

index, sum : integer;

Begin

sum:= 0;

for index := 1 to 50 do

sum:= sum + A[index];

SumArray := sum;

End;

Begin { Main}

Total: = SumArray(array1) + SumArray( array2)

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Example (Cont’d)Write a function that indicates if a Name parameter is valid . A name is valid if it contains

uppercase characters.

Global variable , PersonName type string

FUNCTION ValidName ( Name : String) : BOOLEAN;

Var

index : integer;

begin

ValidName : = true;

For index := 1 to length(Name) do

If (Name[index] < ‘A’) OR (Name[index] > ‘Z’) then

ValidName := False;

end;{ function}

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Example (cont’d)

Begin

writeln(‘Enter name’);

Readln( PersonName);

If ValidName (PersonName) then

writeln(‘Ok’);

End.

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Standard Pascal does not provide a standard power or factorial function.

1. Power

the function should receive two integers X and N , (where N>= 0) and compute N

X

Use the approach of multiplying repeatedly by X as we know the number of iterations. The loop will count down to zero fron an initial value of N.

For each iteration of the loop X is multiplied by the product.

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FUNCTION Power ( X, N :integer) : Integer;

Var

Result :integer;

Begin

Result :=1;

while N > 0 do

Begin

result := result * X;

N := N-1;

End;

power : result;

end;

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2. Function to calculate the Factorial

e.g. 5 factorial 5x4x3x2x1

FUNCTION Factorial (X : integer) : integer;

Var

Result : integer;

Begin

Result := 1;

While X > 0 do

Begin

Result : Result * x;

X:= X -1;

end;

Factorial : result;

End;

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Boolean FunctionBoolean function can be useful when a branch or loop depends on

some complex condition.

Rather than code the condition directly into the IF or WHILE

statements, can call a Boolean function to form the controlling

expression.

E.g. If we had a program that works with triangles, the program reads 3

angles . Before performing any calculations on these angles we want to

check if they form a triangle ( sum the angles and confirm that they

add up to 180).

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FUNCION Triangle ( Angle1, Angle2, Angle3 : Real) :Boolean;

Begin

If Angle1 + Angle2 + Angle3 -180 = 0 then

triangle := true;

end;

Begin

Readln( AngleA, angleB, angleC);

If Triangle(( AngleA, angleB, angleC) then

writeln (‘the 3 angles form a triangle’);

else writeln (‘ the 3 angles do not form a triangle);

End.

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When to use a functionThere aren’t any formal rules determining when to use a PROCEDURE

and when to use a FUNCTION but here are some guidelines

1. If the subprogram must return more than one value or modify the actual parameters, don’t use a FUNCTION

2. If the subprogram must perform input/output, do not use a FUNCTION.

3. If there is only one value returned from the subprogram and it is boolean use a function

4. If there is only one value returned and that value is used immediately in an expression(statement) use a function.

5. When in doubt use a procedure.

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Multiple Subprogram

• When a program contains more than one subprogram, they can be called from the main program in any order.

• One subprogram may call another as long as the subprogram being called appears before the subprogram that calls it.

• The same identifier names can be used within different subprograms- as they are contained in separate blocks.

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Exercises• Tuliskan fungsi untuk mengkonversikan harga karakter angka (0..9)

menjadi harga numerik sesuai dengan karakter yang tertulis. Contoh: ‘8’8, ‘4’4

• Tuliskan fungsi IsAnA yang mentest apakah sebuah karakter yang diberikan kepadanya adalah sebuah huruf ‘A’. Harga yang dihasilkan adalah benar jika huruf itu ‘A’,salah jika huruf itu bukan ‘A’. Contoh:– IsAnA(‘A’) true– IsAnA(‘X’) false

• Tuliskanlah fungsi, yang jika diberikan sebuah angka Cm yang menyatakan panjang dalam cm, akan menghasilkan pasangan harga <x1,x2> sesuai dengan rumus ukuran metris (1 m = 100 cm) sehingga x1*100+x2 = cm. Contoh:– F(100) = <1,0>– F(355) = <3,55>

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Exercises

• Tuliskan fungsi IsPrima(), yang mentest apakah sebuah bilangan integer yang diberikan merupakan bilangan prima atau bukan. Contoh: IsPrima(2)true, IsPrima(15)false.

• Tuliskan fungsi IsEven(), yang mentest apakah sebuah bilangan integer yang diberikan merupakan bilangan genap atau bukan. Contoh: IsEven(43)false, IsEven(32)true.

• Tuliskan fungsi IsOdd(), yang mentest apakah sebuah bilangan integer yang diberikan merupakan bilangan ganjil atau bukan. Contoh: IsOdd(24)false, IsOdd(43)true.

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Exercises• Tuliskanlah sebuah fungsi MAX2, yang menerima

masukan dua buah bilangan integer dan menghasilkan sebuah bilangan integer yaitu salah satu diantara nilai dua buah bilangan tersebut yang terbesar.

• Kemudian, dengan memakai fungsi MAX2, tuliskan sebuah fungsi lain MAX3 yang menghasilkan nilai terbesar dari tiga buah bilangan integer. Contoh:

– MAX2(1,2)2

– MAX2(10,2)10

– MAX3(1,2,3) adalah MAX2(MAX2(1,2),3)3

– MAX3(10,2,3) adalah MAX3(MAX2(10,2),3)10

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ExercisesDidefinisikan tipe terstruktur untuk mewakili hari seperti dalam kalender

yang kita pakai sehari-hari:type nama: DATE<tanggal,bulan,tahun>

Tuliskan algoritma untuk:• Membaca sebuah tanggal dan sebuah kode bahasa penulisan

(1=Inggris, 2=Indonesia, 3=Perancis),• Menuliskan tanggal keesokan harinya sesuai dengan kode bahasa.

– Inggris, DATE ditulis dalam: bulan,’/’,tanggal,’/’,tahun– Indonesia, DATE ditulis dalam: tanggal,’-’,bulan,’-’,tahun– Perancis, DATE ditulis dalam: tanggal,’/’,bulan,’/’,tahun

• Proses menghitung hari esok dilakukan oleh sebuah fungsi NextDay yang menerima masukan sebuah tanggal dan menghasilkan tanggal keesokan harinya. Contoh:– NextDay(<13,4,1990>,1)4/14/1990– NextDay(<30,1,1990>,2)31-1-1990– NextDay(<31,12,1990>,1)1/1/1991

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Referensi

• Liem Inggriani, Diktat kuliah IF223 Algoritma dan Pemrograman, Jurusan Teknik Informatika ITB, 1999

• DT/266/1, Programming Language, Departement of Computer Science.