Paradigmas de Linguagens de Programação

Paradigma Imperativo [Passagem de parâmetros]Aula #4

(CopyLeft)2009 - Ismar Frango ismar@mackenzie.br

Passagem de parâmetros

Valor – C, Java...Referência – FORTRAN, C++, Pascal...Resultado (out) – Ada, C# Valor-resultado /*Copy-restore*/ (inout) – Ada, C#. Read-only – C/C++ (const)Macro - CNome – ALGOL...

“I write the music, produce it and the band plays within the parameters that I set.” Sting

Passagem por valor vs. referência

Valor• Caller passa o r-value do argumento para o sub-

programa– Reduz “aliasing” (dois nomes referenciando a mesma

localização de memória)• O sub-programa não pode mudar o r-value da variável

no escopo do caller– E quando usamos ponteiros?

– void swap(int *a, int *b) { … }

Referência• Caller passa o l-value do argumento para o sub-

programa.

Valor ou referência?

int h, i;void B(int* w) { int j, k; i = 2*(*w); *w = *w+1;} void A(int* x, int* y) { float i, j; B(&h);}int main() { int a, b; h = 5; a = 3; b = 2; A(&a, &b);}

int*

&a

Passagem por valor (C++)

21

void f(int *p) { *p = 5; p = NULL;}

int main() { int x=2; int *q = &x; f(q);} x = 5

q != NULL

Passagem por referência (C++)

22

void f(int A[]) { A[0] = 5;}

int main() { int B[10]; B[0] = 2; f(B); cout << B[0] << endl;}

5

Testando...

void Mystery( int & a, int & b, int c ){ a = b + c; b = 0; c = 0;}

void Print(){ int x = 0, y = 1, z = 2;

Mystery(x, y, z); cout << x << " " << y << " " << z; Mystery(x, y, z); cout << x << " " << y << " " << z << endl;}

A. 0 1 2 0 1 2 B. 3 0 0 3 0 0 C. 0 1 2 3 0 0D. 3 0 2 2 0 2E. 3 0 0 0 0 0

Testando...

#include <iostream>

void Sum(int a, int b, int & c){ a = b + c; b = a + c; c = a + b;}

int main(){ int x=2, y=3;

Sum(x, y, y); cout << x << " " << y << endl; return 0;}

A. Erro de compilação: y é passado por valor e ref. B. Erro de run-time : y é passado por valor e ref. C. 2 3D. 6 9E. 2 15

Associação posicional vs. Nomeada (Ada)

procedure POS_PARAM is procedure ADD_UP(X_VAL,Y_VAL,Z_VAL:INTEGER) is begin PUT(X_VAL+Y_VAL+Z_VAL); NEW_LINE; end ADD_UP;

begin

end POS_PARAM

ADD_UP(2,4,6); ADD_UP(X_VAL=>2, Y_VAL=>4, Z_VAL=>6); ADD_UP(Z_VAL=>6, X_VAL=>2, Y_VAL=>4); ADD_UP(2, Z_VAL=>6, Y_VAL=>4);

PPAPOSITIONAL PARAMETER

ASSOCIATION

NPANAMED

PARAMETER ASSOCIATION

Passagem por valor x referência (Pascal)

program P;var X, Y: Integer;

procedure Swap(A, B: Integer);var Temp:begin Temp := A; A := B; B := Temp;end;

begin readln(X, Y); Swap(X, Y); writeln(X, Y)end.

valor

Novas variáveis no ARI de

Swap

PFs VL

http://stwww.weizmann.ac.il/g-cs/benari/articles/varparm.pdf

Passagem por valor x referência (Pascal)

program P;var X, Y: Integer;

procedure Swap(var A, B: Integer);var Temp:begin Temp := A; A := B; B := Temp;end;

begin readln(X, Y); Swap(X, Y); writeln(X, Y)end.

ref

aliases VL

Passagem por valor (Java)

public class Teste{

private static void aloca(String x){ x="Hello";}

public static void main(String args[]){String b="World";aloca(b);System.out.println(b);}}

World

Passagem por valor (Java)

public class Teste{

private static String aloca(String x,String y){ x=x+y; return x;}

public static void main(String args[]){String a="Hello, ";String b="World";String c = aloca(a,b);

System.out.println(c==a);

String d="World";String e=new String("World");

System.out.println(b==d);System.out.println(b==e);

String f=e.intern();System.out.println(b==f);

}

}

falsetruefalsetrue

void Foo (StringBuilder x) { x = null; } ... StringBuilder y = new StringBuilder(); y.Append ("hello"); Foo (y); Console.WriteLine (y==null);

Passagem por valor (C#)

false

void Foo (StringBuilder x) { x.Append (" World"); } ... StringBuilder y = new StringBuilder(); y.Append (“Hello"); Foo (y); Console.WriteLine (y);

Hello World

String first = new String(“Hello”);String second = first;first += “World”;System.out.println(second);

Passagem por valor (Java)

Hello

void giveMeAString (Object y) { y = "This is a string"; } ...Object x = null; giveMeAString (x); System.out.println (x);

null

O que ocorre aqui?public class Teste2{public static void foo (String x) { x+=" World"; }

public static void main(String []a){String y = new String("Hello"); foo (y); System.out.println(y); }}

Hello

public class Teste3{public static void foo (StringBuilder x) { x.append(" World"); }

public static void main(String []a){StringBuilder y = new StringBuilder();y.append("Hello"); foo (y); System.out.println(y); }}

Hello World

Passagem e AlocaçãoJava vs. C vs. C#

class Elefante{ public int peso=1000; //:-( Thou shalt not do this!}

public class Teste{

public static void main(String args[]){Elefante dumbo = new Elefante();dumbo.peso=2000;Elefante dumbinho = dumbo;dumbinho.peso=500;System.out.println(dumbo.peso);}}

500

Java

Passagem e Alocação Java vs. C vs. C#

#include <stdio.h>typedef struct { int peso; }Elefante;

int main(){Elefante *dumbo = malloc (sizeof(Elefante));dumbo->peso=2000;Elefante *dumbinho = dumbo;dumbinho->peso=500;printf("%d",dumbo->peso);return 0;}

500

C

Passagem e Alocação Java vs. C vs. C#

C#include <stdio.h>typedef struct {int peso; }Elefante;

int main(){Elefante dumbo;dumbo.peso=2000;Elefante dumbinho = dumbo;dumbinho.peso=500;printf("%d",dumbo.peso);return 0;} 2000

Java vs. C vs. C#

using System;

public struct Elefante{ public int peso; }

public class testecs{public static void Main(string []a){Elefante dumbo = new Elefante(); dumbo.peso=2000; Elefante dumbinho = dumbo; dumbinho.peso=500; Console.WriteLine (dumbo.peso); }} 2000

C#

Java vs. C vs. C#

using System;

public class Elefante{ public int peso; }

public class testecs{public static void Main(string []a){Elefante dumbo = new Elefante(); dumbo.peso=2000; Elefante dumbinho = dumbo; dumbinho.peso=500; Console.WriteLine (dumbo.peso); }} 500

C#

void Foo (ref StringBuilder x) { x = null; } ...StringBuilder y = new StringBuilder(); y.Append ("hello"); Foo (ref y); Console.WriteLine (y==null);

Passagem por referência (C#)

using System;using System.Text;

public class testecsref{public static void Foo1 (ref Elefante x) { x.peso=1000 ;}

public static void Foo2 (Elefante x) { x.peso=0 ;}

public static void Main(string []a){Elefante dumbo=new Elefante(); Foo1 (ref dumbo); Console.WriteLine (dumbo.peso); Foo2 (dumbo); Console.WriteLine (dumbo.peso); }}

public class Elefante{ public int peso; }

public struct Elefante{ public int peso; }

10000

10001000

Qual a diferença entre passar um value object

por referência e um reference object por

valor?

true

Value objects x reference objects (C#)

void Foo (ref IntHolder x) { x.i=10; } ... IntHolder y = new IntHolder();y.i=5; Foo (ref y); Console.WriteLine (y.i);

10

public struct IntHolder { public int i; }

Value object + passagem por

referência

void Foo (IntHolder x) { x.i=10; } ... IntHolder y = new IntHolder();y.i=5; Foo (y); Console.WriteLine (y.i);

5

public class IntHolder { public int i; }

Reference object + passagem por

valor

Parâmetros out (C#)

void Foo (out int x) { int a = x; // Can't read x here - it's considered unassigned

// Assignment - this must happen before the method can complete normally x = 10; // The value of x can now be read: int a = x; } ... // Declare a variable but don't assign a value to it int y; // Pass it in as an output parameter, even though its value is unassigned Foo (out y); // It's now assigned a value, so we can write it out: Console.WriteLine (y);

Parâmetros in, out, in out (Ada)

procedure Quadratic(A, B, C: in Float; Root1, Root2: out Float);procedure Sort(V: in out Vector);

with CS_IO; use CS_IO;

procedure OUT_MODE_EXAMPLE is VALUE_1, VALUE_2: float; MEAN: float;

procedure MEAN_VALUE (NUM_1, NUM_2: in float; NUM_3: out float) is begin NUM_3:= (NUM_1+NUM_2)/2.0; end MEAN_VALUE;

begin MEAN_VALUE(VALUE_1, VALUE_2, MEAN); put("The mean is "); put(MEAN);new_line; new_line;end OUT_MODE_EXAMPLE;

Parâmetros in, out, in out (Ada)

with CS_IO; use CS_IO;

procedure IN_OUT_MODE_EXAMPLE is VALUE_1, VALUE_2: float;

procedure MEAN_VALUE (NUM_1: in out float; NUM_2: in float) is begin NUM_1:= (NUM_1+NUM_2)/2.0; end MEAN_VALUE;

begin get(VALUE_1);get(VALUE_2); put("BEFORE MEAN VALUE: VALUE_1 = "); put(VALUE_1);put(", VALUE_2 = ");put(VALUE_2);new_line; MEAN_VALUE(VALUE_1, VALUE_2); put("The mean is "); put(VALUE_1);new_line; put("AFTER MEAN VALUE: VALUE_1 = "); put(VALUE_1);put(", VALUE_2 = ");put(VALUE_2);new_line;end IN_OUT_MODE_EXAMPLE;

Parameter arrays –”params” (C#), ... (Java)

void ShowNumbers (params int[] numbers) { foreach (int x in numbers) { Console.Write (x+" "); } Console.WriteLine(); }

... int[] x = {1, 2, 3}; ShowNumbers (x); ShowNumbers (4, 5);

static int sum (int ... numbers) { int total = 0; for (int i = 0; i < numbers.length; i++) total += numbers [i]; return total; }

C#

Java

Parâmetros variáveis (C++)

void myprintf( char *format, ... ){va_list vl;int i;va_start( vl, args );for( i = 0; args[i] != '\0'; ++i ){ union any_t { int i; float f; char c;} any;if( args[i]=='i' ){ any.i = va_arg( vl, int ); printf( "%i\n", any.i );}else if( args[i]=='f' ){ any.f = va_arg( vl, float ); printf( "%f\n", any.f );}else if( args[i]=='c' ){any.c = va_arg( vl, char );printf( "%c\n", any.c );}else{ throw SomeException; }va_end( vl );}

Parâmetros default (Ada, C++)with CS_IO; use CS_IO; procedure EXAMPLE is N1: float:= 4.2; N2: float:= 9.6; procedure MEAN_VALUE (X1: in out float; X2: in float := 6.4) is begin X1:= (X1+X2)/2.0; end MEAN_VALUE

begin MEAN_VALUE(N1); put(N1); new_line; MEAN_VALUE(N1, N2); put(N1); new_line; end EXAMPLE;

#include <iostream>using namespace std; int a = 1;int f(int a) { return a; }int g(int x = f(a)) { return x; } int h() { a = 2; { int a = 3; return g(); }} int main() { cout << h() << endl;}

2

O que é impresso aqui?

Parâmetros read-only (C++)

class IntList{public: const Print (ostream &o);};

void f(const IntList &L) { L.Print(cout); }

bool Compare(const vector <int> & A)// precondition: A is sorted{ int k;

for (k=1; k<A.size(); k++) { if (A[k-1] == A[k]) return true; } return false;}

E se aqui não fosse

const?

Macros

#define MAXLINE 100

char line[MAXLINE]; ... getline(line, MAXLINE);

#define A 2 #define B 3 #define C A + B

int x = C * 2;

O que acontece

aqui?

Por que não tem ponto-e-

vírgula?

Pass-by-name (ALGOL)

procedure double(x); real x; begin x := x * 2 end;

double(C[j])

C[j] := C[j] * 2.

n

i

iix1

*][

real procedure Sum(j, lo, hi, Ej); value lo, hi; integer j, lo, hi; real Ej;begin real S; S := 0; for j := lo step 1 until hi do S := S + Ej; Sum := Send;

Sum(i, 1, n, x[i]*i)

Como simular isso em C++

ou Java?

Pass-by-name (ALGOL)

procedure swap (a, b);integer a, b, temp;begin temp := a; a := b; b:= tempend;

swap(x, y)

temp := x; x := y; y := temp

swap(i, x[i])

temp := i;i := x[i]; x[i] := temp Antes

i = 2

x[2] = 5

Depoisi =

5

x[2] = 5

x[5] = 2

+Exercícios...

/* If *y is zero and x > 0 : set *k to biggest integer.* If *y is zero and x <=0 : leave *k unchanged.* If *y is not zero :* set *k to be x divided by *y and increment *y by 1.*/#define INT_MAX 1000void check_divide (int *k, int x, int *y) {if (*y = 0)if (x > 0)*k = INT_MAX;else*k = x / *y /* *y cannot be 0 */;*y++;}

int main( ){ int a=2; check_divide(&a, a, &a); cout<<a;}

0

C++

/* If *y is zero and x > 0 : set *k to biggest integer.* If *y is zero and x <=0 : leave *k unchanged.* If *y is not zero :* set *k to be x divided by *y and increment *y by 1.*/#define INT_MAX 1000void check_divide (int *k, int x, int *y) {if (*y == 0) {if (x > 0)*k = INT_MAX;}else{*k = x / *y /* *y cannot be 0 */;(*y)++;}}

int main( ){ int a=2; check_divide(&a, a, &a); cout<<a;}

3

C++

#define TAM 10void change (int v[ ], int &ntx){ for (;ntx>0;ntx--) v[ntx]=ntx---1;}

int main(){ int k[TAM],i=0,; for(;;) { if (i==TAM) break; k[i++]= i*2;} change(k,i); for(;;) { if (i>TAM) break; cout<<k[i++]<<endl;}}

02163

105

147

189

C++

using namespace System; double average( ... array<Int32>^ arr ) { int i = arr->GetLength(0); double answer = 0.0; for (int j = 0 ; j < i ; j++) answer += arr[j]; return answer / i; }

int main() { Console::WriteLine("{0}", average( 1, 2, 3, 6 )); }

Visual C++ (.NET)

3

void g (int &a) { a++; }void h (int &a) { a*=2; }void k (int &a) { a*=10; }void f( int &number, void (*function)(int&) ){ (*function )( number );}

int main(){ int select, count=0; for (select = 1; select<=3 ;select++) { switch( select ) { case 1: f( count, g ); break; case 2: f( count, h ); break; case 3: f( count, k ); default: break; } } cout << count;}

20

C++

Pascalprocedure swap(arg1, arg2: integer); var temp : integer; begin temp := arg1; arg1 := arg2; arg2 := temp; end;

...{ in some other procedure/function/program }var var1, var2 : integer;begin var1 := 1; var2 := 2; swap(var1, var2);end;

Var1=1Var2=2

public class TestY{public static void differentArray(float[] x) { x = new float[100]; x[0] = 26.9f; }

public static void main(String a[]){ float[ ] xx = new float[100]; xx[0] = 55.8f; differentArray(xx); System.out.println("xx[0] = " + xx[0]);}}

Java

55.8

procedure sub1(x: int; y: int); begin x := 1; y := 2; x := 2; y := 3; end;

sub1(i, a[i]);

Algol

a={2,3}