9
Functions
Itisbettertohave100functionsoperateononedatastructure
than10functionson10datastructures.
A.Perlis
Copyright ProgrammingLanguages
2ndedition
TuckerandNoonan
Chapter 2006 The McGraw-Hill Companies, Inc.
Contents
9.1BasicTerminology
9.2FunctionCallandReturn
9.3Parameters
9.4ParameterPassingMechanisms
9.5ActivationRecords
9.6RecursiveFunctions
9.7RunTimeStack
Copyright 2006 The McGraw-Hill Companies, Inc.
9.1BasicTerminology
Value-returning functions:
known as non-void functions/methods in C/C++/Java
called from within an expression.
e.g., x=(b*bsqrt(4*a*c))/2*a
Non-value-returning functions:
known as procedures in Ada,
subroutines in Fortran,
void functions/methods in C/C++/Java
called from a separate statement.
Copyright 2006 The McGraw-Hill Companies, Inc.
9.2FunctionCallandReturn
Example C/C++
Program
Fig 9.1
inth,i;
voidB(intw){
intj,k;
i=2*w;
w=w+1;
}
voidA(intx,inty){
booli,j;
B(h);
}
intmain(){
inta,b;
h=5;a=3;b=2;
A(a,b);
Copyright 2006 The McGraw-Hill Companies, Inc.
9.3Parameters
Definitions
An argument is an expression that appears in a function
call.
A parameter is an identifier that appears in a function
declaration.
E.g., in Figure 9.1
The call A(a,b) has arguments a and b.
The function declaration A has parameters x and y.
Copyright 2006 The McGraw-Hill Companies, Inc.
ParameterArgumentMatching
Usually by number and by position.
I.e., any call to A must have two arguments, and they must
match the corresponding parameters types.
Exceptions:
Perl - parameters arent declared in a function header.
Instead, parameters are available in an array @_, and are
accessed using a subscript on this array.
Ada - arguments and parameters can be linked by name. E.g.,
the call A(y=>b,x=>a) is the same as A(a,b)
Copyright 2006 The McGraw-Hill Companies, Inc.
9.4ParameterPassingMechanisms
By value
By reference
By value-result
By result
By name
Copyright 2006 The McGraw-Hill Companies, Inc.
PassbyValue
Compute the value of the argument at the time of the call
and assign that value to the parameter.
E.g., in the call A(a,b) in Fig. 9.1, a and b are passed by
value. So the values of parameters x and y become 3 and 2,
respectively when the call begins.
So passing by value doesnt normally allow the called
function to modify an arguments value.
All arguments in C and Java are passed by value.
But references can be passed to allow argument values to be
modified. E.g., voidswap(int*a,int*b){}
Copyright 2006 The McGraw-Hill Companies, Inc.
PassbyReference
inth,i;
Compute the address of the
argument at the time of the
call and assign it to the
parameter.
Example
voidB(int*w){
Fig 9.3
booli,j;
Since h is passed by
reference, its value changes
during the call to B.
intj,k;
i=2*(*w);
*w=*w+1;
}
voidA(int*x,int*y){
B(&h);
}
intmain(){
inta,b;
h=5;a=3;b=2;
Copyright The 2006 McGraw-Hill Companies, Inc.
PassbyValueResultandResult
Pass by value at the time of the call and/or copy the
result back to the argument at the end of the call.
E.g., Adas inout parameter can be implemented as valueresult.
Value-result is often called copy-in-copy-out.
Reference and value-result are the same, except when
aliasing occurs. That is, when:
the same variable is both passed and globally referenced
from the called function, or
the same variable is passed for two different parameters.
Copyright 2006 The McGraw-Hill Companies, Inc.
PassbyName
Textually substitute the argument for every instance of
its corresponding parameter in the function body.
Originated with Algol 60 (Jensens device), but was
dropped by Algols successors -- Pascal, Ada, Modula.
Exemplifies late binding, since evaluation of the argument
is delayed until its occurrence in the function body is
actually executed.
Associated with lazy evaluation in functional languages
(see, e.g., Haskell discussion in Chapter 14).
Copyright 2006 The McGraw-Hill Companies, Inc.
9.5ActivationRecords
A block of information associated with each function
call, which includes:
parameters and local variables
Return address
Saved registers
Temporary variables
Return value
Static link - to the functions static parent
Dynamic link - to the activation record of the caller
Copyright 2006 The McGraw-Hill Companies, Inc.
9.6RecursiveFunctions
A function that can call itself, either directly or
indirectly, is a recursive function. E.g.,
intfactorial(intn){
if(n<2)
return1;
elsereturnn*factorial(n1);
}
Copyright 2006 The McGraw-Hill Companies, Inc.
self-call
9.7RunTimeStack
A stack of activation records.
Each new call pushes an activation record, and
each completing call pops the topmost one.
So, the topmost record is the most recent call, and
the stack has all active calls at any run-time moment.
For example, consider the call factorial(3).
This
places one activation record onto the stack and generates a
second call factorial(2). This call generates the call
factorial(1), so that the stack gains three activation records.
Copyright 2006 The McGraw-Hill Companies, Inc.
StackActivityfortheCallfactorial(3)
Fig.9.7
n
3
3
n
3
n
3
n
2
n
2
n
2
n
First call
n
1
Second call
Third call
returns 1
n
3
Second call
First call
returns 2*1=2 returns 3*2=6
Copyright 2006 The McGraw-Hill Companies, Inc.
StackActivityforPrograminFig.9.1
Fig.9.8(linksnotshown)
h
i
a
b
undef
undef
3
2
Activation of main
h
i
a
b
x
y
i
j
5
undef
3
2
3
2
undef
undef
main calls A
Copyright 2006 The McGraw-Hill Companies, Inc.
h
i
a
b
x
y
i
j
w
j
k
5
10
3
2
3
2
undef
undef
5
undef
undef
A calls B