procedure definition (or declaration - some make a distinction, we generally won t), e.g., void foo(int x) {... }

Transcription

1 Parameters Terminology: procedure invocation, e.g., (in different language notations) foo(a*8) x:=goo(19,a) call hoo() actual parameters, e.g., a*8 is an actual parameter of the above inv ocation of foo 19and a are actual parameters of the above inv ocation of goo procedure definition (or declaration - some make a distinction, we generally won t), e.g., void foo(int x) { procedure goo(y, z); integer y, z;begin end procedure hoo() begin end formal parameters, e.g., xis foo s formal parameter yand z are goo s formal parameters ECS140A Programming Languages 08-1

2 procedure return: transfer control and possibly a result back to invoker. usual return statements, e.g., return; return 81; in some languages (e.g., Algol), assign return value to procedure name. then returning to invoker is via just return or reaching the end of the body of the procedure. the procedure name can be used as a variable within the body of the procedure. E.g., integer procedure fact(n); integer n; begin fact := fact*i; end ECS140A Programming Languages 08-2

3 Parameter Passing call by value aka copy-in value is copied in changes to formal not reflected back to actual requires copy, even oflarge arrays call by reference pointer to value is passed in changes to formal reflected back to actual immediately does not require copy ofarray, only pointer to it call by value-result aka copy-in/copy-out like call by value on way in onexit, copy back values into same location from which they were obtained recompute location order of evaluation left right (usual) right left not defined changes to formal reflected back to actual after proc executes call by name aparameter evaluation method ineffect, textually substitute formal for actual. ECS140A Programming Languages 08-3

4 example: int a[1:2], i, j; proc P(c,d,e) e:=e+1 d:=200 write( c,d,e,i ) end a[1] := 5; a[2] := 8 i:=1 P( a[i], a[2], i ) write( a[1], a[2], i ) method value reference output value-result copy-back to same value-result copy-back 5 5or200 2 recompute name ECS140A Programming Languages 08-4

5 More on pass by name note connection of call by name to, e.g., C s macros: #define max(x,y) (x)>(y)?(x):(y) a=max(b++,c); /* DANGEROUS! */ call by name = call by reference if no array, noexpressions ECS140A Programming Languages 08-5

6 when is it useful? Jensen s device #procedure to implement sigma notation # careful: must pass i, o.w. lose effect to a[i] x:=sigma(i,1,n,a[i]) real procedure sigma(k,lb,ub,v) value lb,ub; # (default is name.) #critical that k,v be passed by name integer k,lb,ub; real v; begin sigma := 0; for k := lb to ub do sigma := sigma+v; end invoke via: above, orsigma(i,1,n,a[i]*b[i]) evaluation as needed. avoid evaluation of arguments #from text proc cand(p,q) boolean p,q; if p then cand := q else cand := false consider evaluation of cand( x 0, 3/x > 12 ) inac-like language, i.e., call by value call by name (LISP has mechanisms to avoid evaluation: quote, fexpr/expr.) ECS140A Programming Languages 08-6

7 when is it dangerous? proc swap(x,y) integer t; t:=x;x:=y;y:=t swap(i,j) OK swap(j,i) OK swap(a[i],i) OK swap(i,a[i]) not OK let i be 1, A[1] be 14 effect is i is 14, A[14] is 1, A[1] is 14. #noway to define swap using name only that works for all actuals. how implemented? thunk invisible, parameterless, function that returns an address of a value. function built up as part of call. inv oked within called proc whenever address needed. obviously more expensive than other methods (could use macro expansion / text substitution, i.e., procedure inlining, except that can lead to large code size and doesn t work for recursion) ECS140A Programming Languages 08-7

8 different kinds of parameter passing used in same language Algol C can pass by value, or name (default) pass by value, pass address for reference, arrays by reference (because ) C+ + like C,but also allows pass by reference Java Python Pascal Ada Fortran pass by value (note: references are values) user specified, value (default), or var (reference) user specified, in (default), or out, or in-out out can be implemented either as copy-out or reference programs where it makes a difference are defined as errors reference constant problem in old versions!!!! why Ada does it the way it does implementation detail. simple parameters by copy, but big arrays by pointer. distributed processing. Pascal and var parameters actual must be a variable some other languages (FORTRAN, Algol, Ada, but not C++) with reference parameters allow anarbitrary expression, f(3) or g(h(x)+2) for such, they create a "dummy" argument ECS140A Programming Languages 08-8

9 aliasing problem with call by reference or value-result two formals bound to same actual, e.g., boolean proc p(x,y) x:=y+1 p:=x=y end a:=10 write( p(a,a) ); write(a) value false 10 reference true 11 value-result false 10/11 use non-local and formal bound to non-local within proc saw effect in earlier example ECS140A Programming Languages 08-9

10 Reference parameters in C++ #include <iostream.h> // call by value void foo(int x){ x+=10; cout << "x is " << x << endl; main(){ int y = 4; int *p = &y; cout << "y is " << y << endl; foo(y); cout << "y is " << y << endl; foo(*p); cout << "y is " << y << endl; // foo(p); illegal. why? yis4 xis14 yis4 xis14 yis4 #include <iostream.h> // call by reference void foo(int & x){ x+=10; cout << "x is " << x << endl; main(){ int y = 4; int *p = &y; cout << "y is " << y << endl; foo(y); cout << "y is " << y << endl; foo(*p); cout << "y is " << y << endl; // foo(p); illegal. why? yis4 xis14 yis14 xis24 yis24 ECS140A Programming Languages 08-10

11 Procedures as first-class citizens beable to have procedure-valued variables and parameters usefulness: example from dynamic scoping: find sum of f(i), i := 1 to 10 instead of using dynamic scope, pass in function to use. (typical example here is sorting or integration routines.) avoids duplication of code and makes clearer what is going on. UNIX: find file read routine, e.g., for redirection from tty or from disk file ECS140A Programming Languages 08-11

12 how done #C int (*compare)() compare = some_func; /* or passed as parameter */ if ( (*compare)(x,y) ) /* early C required explicit dereference */ if ( compare(x,y) ) /* ANSI C allows this too */ #notype-checking #Pascal proc x( function f:real; y;real) := f() #doesn t type check f s parameter! #can t have variables of type function #Java doesn t have pointers of any kind, so seems can t doit, but #ISO Pascal proc x( function f(a,b:real):real; y;real) := f() #now type check f s parameter. #still can t have variables of type function #Modula 2 #can have variables too #Icon bar := write; bar("hi") foo := "writes"; foo("hi") ("write" "s")("hi") l:=[write,writes] l[2]("hello") #LISP will see later ECS140A Programming Languages 08-12

13 More complete C example: sort by normal string comparison or sort by case insensitive string comparison write body of sort once, but include comparison function as parameter main() { char *d [] sort (d, dsize, strcmp); /* use C s standard string comparison function */ char *e [] sort (e, esize, caseinsensitivestrcmp); void sort(char *s [], int n, int (*compare)(char *, char *)) { int i; for (i = 0; i < n-1; i+ +) { int j; for (j = i+1; j < n; j+ +) { if (compare(s[i], s[j]) > 0) { swap(s, i, j); /* swaps i-th and j-th elements of s */ int caseinsensitivestrcmp(char *s1, char *s2) { while (tolower(*s1) == tolower(*s2)) { if (*s1 == \0 ) return 0; s1+ +; s2+ +; return tolower(*s1) - tolower(*s2); ECS140A Programming Languages 08-13