nov 2005 sdp-msc slide 1 section 10 functional programming in c#

Nov 2005 SDP-MSc Slide

1

Section 10

Functional Programming in C#


2

References

http://www-compsci.swan.ac.uk/~csetzer/articles/javafunctional.pdfJava as a Functional Programming Language - A.Setzer

http://www.cs.hmc.edu/claremont/keller/webBook/ch07/sec23.htmlHigher-Order Functions as Objects


3

Principles Reviewed

3

•No Side effects

•No variable assignment

•A Function has no effect other than calculating its value


4

Key Features- Functional Programs

4

•Higher Order Functions

•Lazy Evaluation

•Currying

•Lambda Expressions

•List Processing


5

Functional Programs

5

•Program consists only of Functions

•Main program is a function, receives input as parameters

•May in turn be defined as other functions


6

- No side effects

6

class Fun2{ public static int res; // global variable

public static void sum(int a, int b){ res= a+b;} // side effect

public static void Main(string[] args) { int x=Convert.ToInt32(args[0]); int y=Convert.ToInt32(args[1]); sum(x,y); Console.WriteLine("Sum of values:{0}", res);

}

}


7

Function result only depends on input parameters

Always returns the same result, whenever called

Order of function calls irrelevant:

f(x)=2*x + 1 => f(2)=5 always

Functional Programming -


8

No Side-effects - Summary

•No global variables

•No variable assignment

•Function just calculates a value based on input parameters

Next: Another counter example


9

class Fact1{

public static int factorial(int n){ int res=1;

for (int i=1;i<=n;i++) res*=i; // var assignment return res;}

public static void Main(string[] args) { int x=Convert.ToInt32(args[0]); Console.WriteLine("Factorial= {0}", factorial(x));

}

}


10


11

Recursion

Most Functional Programming Languages use recursion

- avoids variable assignment

- facilitates Lazy Evaluation


12

class Fact2{ public static int factorial(int n){

if (n==0) return 1; else return n * factorial(n-1); }

public static void Main(string[] args) { int x=Convert.ToInt32(args[0]);Console.WriteLine("Factorial= {0}", factorial(x)); }

}


13

Functional Programming

Two other key features

- List Processing

- Polymorphic Types


14

class Vec3{ public static int total(ArrayList vec){

int res=0; for(int i=0; i<vec.Count;i++) { res+= (int)vec[i]; } return res;}

public static void Main() { ArrayList v=new ArrayList(); v.Add(1); v.Add(2); v.Add(3); Console.WriteLine("Sum of Elements: {0}",

total(v));} }


15

Next Fuctional Version


16

class Vec4{public static int convert(Object o){ return ((Integer)o).intValue();}

public static int total(Vector vec,int first){ if (vec.isEmpty()) return 0; if (first==vec.size()-1) return convert(vec.elementAt(first)); else return convert(vec.elementAt(first)) + total(vec,first+1);

}


17

:public static void main(String[] args) { Vector v=new Vector(); v.addElement(new Integer(Integer.parseInt(args[0]))); v.addElement(new Integer(Integer.parseInt(args[1]))); v.addElement(new Integer(Integer.parseInt(args[2]))); System.out.println("Sum of Elements: "+ total(v,0));

}}


18

Now to search vector [3,4,5] for a value


19

Functional Programming -other key features

•Higher Order Functions•Lazy Evaluation•Currying•Lambda expressions (Nameless functions)

All of these depend on Higher Order Functions


20

Lazy Evaluation

Normal Evaluation: mult (fac 3) (fac 4)

Evaluate parameter first:

mult 6 24 => 144

Lazy Evaluation - only evaluate (fac 3) etc when needed


21

Lazy Evaluation

mult (fac 3) (fac 4)

becomes

(fac 3) * (fac 4) = 6 * 24 => 144

Lazy Evaluation - only evaluate (fac 3) etc when needed


22

Lazy Evaluation - C#

Using ‘if’ statements in C# to implement ‘Lazy’

: if (x <10) return increment(x); else return 10;} :


23

Lazy Evaluation - Java

Recursive Functions also facilitate Lazy Evaluation

Factorial…


24

Lazy Evaluation - Java

Ability to pass functions ‘unevaluated’ is centralto ‘Lazy Evaluation’

I.e. Higher Order Functions


25

Higher Order Functions

A Function which takes another function as parameter

Function can also be returned as a result

I.e Functions treated as Objects

increment :: Int -> Intincrement n = (n+1)


26



map1 :: (a -> b) -> [a] -> [b]map1 f [] = []

map1 f (x:xs) = f x : map1 f xs

> map1 increment [1,2,3,4]

[2,3,4,5]


27


Two Methods in C#

(1)Using interface and polymorphism: pass an Object which refers to a method

(2) Using delegate type pass a reference to a function


28


In this section

(1)Using interface and polymorphism for HOF:

(2) Delegate type in general

(3) Delegate type for HOF


29

Higher Order Functions in C# using interface

using System;public interface OneArgFunction{ int apply(int x); }

class Increment :OneArgFunction{

public int apply(int x){ return x+1;}

}



30

public class TestHOF1{

public static void map(OneArgFunction f, int[] a) { for (int i=0;i<a.Length;i++){ a[i]=f.apply(a[i]);}

}

public static void Main(string[] args){ int[] arr={3,4,5,6,7}; map(new Increment(),arr); for (int i=0; i<5;i++){ Console.WriteLine("["+arr[i]+"]");} Console.Read();

}}


31

Write a function which : - takes a function & list of Ints as paramemets - applies function to first & last element - return result as sum of these two wlaues

>sum_fl increment [3,4,5,6] 11 i.e 4 + 7

sum_fl :: (Int -> Int) -> [Int] -> Intsum_fl f (x:xs) = f x + f (last xs)

Now the Java Version -->


32

using System;

public interface Operation{ int apply(int x); }

class Increment :Operation{ public int apply(int x){

return x+1;}}:


33

:public class TestHOF4{

public static int sum_fl(Operation f, int[] a) { return f.apply(a[0])+ f.apply(a[a.Length-1]);

}

public static void Main(string[] args){ int[] arr={3,4,5,6,7}; int res=sum_fl(new Increment() ,arr); Console.WriteLine("Sum of 1st & Last= {0}",res); Console.Read();

}}


34

Another Example

Add.op(2,3) -> 5

apply_op (Add.op, [3,4,5,6,7]) -> 25


35

using System;using System.Collections;

public interface Operation{ int op(int x, int y); } class Add : Operation{ public int op(int x,int y){

return x+y;}}:


36

:public class Ex1{

public static int apply_op(Operation f, int[] a) { int res=0; for (int i=0; i<a.Length;i++) {res = f.op(res,a[i]);} return res;

}


37

:public static void Main(string[] args){

int [] arr={3,4,5,6,7}; int res=apply_op(new Add(), arr); Console.WriteLine("Sum of elements= "+res); Console.Read();}}


38

Exercise 1

Large implemements Validate -- inner class testval 8 returns false testval 11 returns true - values >= 10

test_list testval [2, 12, 8, 17, 9] returns false

test_list testval [12, 13, 18, 17,19] returns true


39

Exercise 2

Bigger implements Validate-- inner class compare 8,9 returns false compare 9,8 returns true compare 9,9 returns false

test_list compare [12, 8, 17, 9] 9 returns false

test_list compare [12, 8, 17, 9] 7 returns true

Create 2 versions: (a) Bigger as an Inner Class (b) Anonymous (nameless Inner Class)


40

Filter Example

Small 8 returns trueSmall 11 returns false

myfilter small, [2, 12, 8, 17, 9], 0

returns [2, 8, 9]


public interface Validate{ bool testval(int n); }

class Small : Validate{ public bool testval(int n){

if (n<10) return true;

else return false;}

}

public class HOFsmall1{

public static void myFilter(Validate f, ArrayList v, int index) { if (v.Count==0) return;

if (index==v.Count) return;else {int el=(int)v[index];

if (f.testval(el)==false) { v.RemoveAt(index);

myFilter(f,v,index);} else

myFilter(f,v,index+1);}

} :

: public static void Main(string[] args){

ArrayList a=new ArrayList(); a.Add(2); a.Add(19); a.Add(9); a.Add(14); a.Add(9); a.Add(8);

myFilter(new Small(),a,0); for (int i=0; i<a.Count;i++){

int val=(int)a[i]; Console.WriteLine("["+val+"]");} Console.Read();

}}


44

Another Filter Example

greater 8 9 returns falsegreater 9 8 returns truegreater 9 9 returns false

myfilter greater, [2, 12, 4, 3, 9], 3, 0

returns [12, 4, 9]


45


public interface Condition{ bool test(int x,int y); }

class Greater :Condition{

public bool test(int x,int y){ if (x>y) return true; else return false;

}}


46

public class HOFgreater1{

public static void myFilter(Condition f,ArrayList v, int val,int index) { if (v.Count==0) return; if (index==v.Count) return; else { int el=(int)v[index]; if (f.test(el,val)==false) { v.RemoveAt(index); myFilter(f,v,val,index);}

else myFilter(f,v,val,index+1); }

}


47

public static void Main(string[] args){ ArrayList a=new ArrayList(); a.Add(7); a.Add(19); a.Add(9); a.Add(6); a.Add(14); a.Add(24);

myFilter(new Greater(),a,8,0); for (int i=0; i<a.Count;i++){


}}


48

Modify previous Filter Example in 2 ways

equal 8 9 returns falseequal 9 9 returns true

filter_Out equal, [3, 12, 4, 3, 9], 3, 0

returns [12, 4, 9]

Exercise 3


49

The delegate type

Pointer to function


50

class GFrame4 :GFrame3 { protected Button b1=new Button();

public GFrame4():base(){

b1.Text= "Change Colour"; b1.SetBounds(100,180,90,23); b1.Click += new EventHandler(this.button1_Click);

Controls.Add(b1); } private void button1_Click(object sender, EventArgs e){ brush.Color=Color.Yellow; this.Refresh();}

}

public class Test92{public static void Main(string[] args){

Application.Run(new GFrame4());}}}


51

First a Basic Example


52


public delegate void Operation(); // type definition

public class Test{

public static void print(){Console.WriteLine("Hello");}

public static void Main(string[] args){Operation o=new Operation(print); // delegateo();Console.Read();

}}


53


54

public delegate void Operation();

public class Test{public static void print1(){

Console.WriteLine("Hello");}

public static void print2(){ Console.WriteLine("World");}

public static void Main(string[] args){Operation o=new Operation(print1);o+= new Operation(print2);o();Console.Read();

}}


55

Now to Pass Pointers to Functions as Parameters


56


public class Test{private int x=2;

public void incr(){x++;}public void decr(){x--;}public void print(){

Console.WriteLine("Value={0}",x);}public void Action(Operation o){

o();}

}


57

public class Test99{public static void Main(string[] args){ Test t=new Test(); Operation o1=new Operation(t.incr); Operation o2=new Operation(t.print); t.Action(o1); t.Action(o2); Console.Read();

}}


58


59

public class Test99{public static void Main(string[] args){ Test t=new Test(); Operation o1=new Operation(t.incr); o1 +=new Operation(t.print); t.Action(o1); Console.Read();

}}

Alternative Test Function


60


61


public class Test{private int x=2;

public void incr(){x++;}public void decr(){x--;}public void print(){

Console.WriteLine("Value={0}",x); }


62

:

public void step2(){ Operation o = new Operation(incr); o(); o();}

public void stepandprint(){ Operation o = new Operation(incr); o += new Operation(print);

o();}

}


63

public class Test99{public static void Main(string[] args){ Test t=new Test(); t.print(); t.step2(); t.stepandprint(); Console.Read();

}}


64


65

Higher Order Functions Using Delegates


66


A Function which takes another function as parameter

Function can also be returned as a result

I.e Functions treated as Objects



67



map1 :: (a -> b) -> [a] -> [b]map1 f [] = []

map1 f (x:xs) = f x : map1 f xs

> map1 increment [1,2,3,4]

[2,3,4,5]


68

Higher Order Functions in C# using delegates


public delegate int OneArgFunction(int x);


public static int apply(int x){ return x+1;}

:



69

:public static void map(OneArgFunction f, int[] a) { for (int i=0;i<a.Length;i++){ a[i]=f(a[i]);}

}

public static void Main(string[] args){int[] arr={3,4,5,6,7};map(new OneArgFunction(apply),arr);for (int i=0; i<5;i++){ Console.WriteLine("["+arr[i]+"]");}Console.Read();

}}


70


71

Write a function which : - takes a function & list of Ints as paramemets - applies function to first & last element - return result as sum of these two wlaues

>sum_fl increment [3,4,5,6] 11 i.e 4 + 7

sum_fl :: (Int -> Int) -> [Int] -> Intsum_fl f (x:xs) = f x + f (last xs)

Now the Java Version -->


72

public delegate int Operation(int x);


public static int apply(int x){ return x+1;}


73

:public static int sum_fl(Operation f, int[] a)

{ return f(a[0])+ f(a[a.Length-1]);}

public static void Main(string[] args){ int[] arr={3,4,5,6,7}; Operation o=new Operation(apply); int res=sum_fl(o ,arr); Console.WriteLine("Sum of 1st & Last= {0}",res); Console.Read();

}}


74


75

Another Example

Add.op(2,3) -> 5

apply_op (Add.op, [3,4,5,6,7]) -> 25


76


public delegate int Operation(int x, int y);


public static int op(int x, int y){ return x+y;}

:


77

: public static int apply_op(Operation f, int[] a) { int res=0; for (int i=0; i<a.Length;i++) {res = f(res,a[i]);} return res;

} public static void Main(string[] args){

int[] arr={3,4,5,6,7}; Operation o=new Operation(op); int res=apply_op(o, arr); Console.WriteLine("Sum of elements= "+res); Console.Read();

}}


78


79

Exercise 1

Large implemements Validate – delegate type testval 8 returns false testval 11 returns true - values >= 10

test_list testval [2, 12, 8, 17, 9] returns false

test_list testval [12, 13, 18, 17,19] returns true


80

Exercise 2

Bigger implements Validate– delegate type compare 8,9 returns false compare 9,8 returns true compare 9,9 returns false

test_list compare [12, 8, 17, 9] 9 returns false

test_list compare [12, 8, 17, 9] 7 returns true


81

Filter Example

Small 8 returns trueSmall 11 returns false

myfilter small, [2, 12, 8, 17, 9], 0

returns [2, 8, 9]


82


public delegate bool Validate(int n);

public class TestHOF1{ public static bool testval(int n){ if (n<10) return true;

else return false;}


83

public static void myFilter(Validate f, ArrayList v, int index) { if (v.Count==0) return;

if (index==v.Count) return; else {int el=(int)v[index];

if (f(el)==false) { v.RemoveAt(index);

myFilter(f,v,index);} else

myFilter(f,v,index+1);}} :


84

public static void Main(string[] args){ ArrayList a=new ArrayList();a.Add(2);a.Add(19);a.Add(9);a.Add(14);a.Add(9);a.Add(8);Validate v=new Validate(testval);

myFilter(v,a,0); for (int i=0; i<a.Count;i++){


}}


85


86

Another Filter Example

greater 8 9 returns falsegreater 9 8 returns truegreater 9 9 returns false

myfilter greater, [2, 12, 4, 3, 9], 3, 0

returns [12, 4, 9]


87

public delegate bool Condition(int x, int y);


public static bool test(int x,int y){ if (x>y) return true; else return false;

}


88

public static void myFilter(Condition f,ArrayList v, int val,int index) { if (v.Count==0) return; if (index==v.Count) return; else { int el=(int)v[index]; if (f(el,val)==false) { v.RemoveAt(index); myFilter(f,v,val,index);}

else myFilter(f,v,val,index+1); }

}


89

public static void Main(string[] args){ ArrayList a=new ArrayList();

a.Add(7);a.Add(19);a.Add(9);a.Add(6);a.Add(14);a.Add(24); Condition c=new Condition(test);

myFilter(c,a,8,0); for (int i=0; i<a.Count;i++){


}}


90


91

Modify previous Filter Example in 2 ways

equal 8 9 returns falseequal 9 9 returns true

filter_Out equal, [3, 12, 4, 3, 9], 3, 0

returns [12, 4, 9]

Exercise 3

nov 2005 sdp-msc slide 1 section 10 functional programming in c#

Documents