Unit 8 - Subprograms, Parameter Passing

Question 1

Roughly speaking, a subprogram is

Answer 1

a named program block that can be called from other program blocks.

Question 2

Java methods, Ada procedures and functions, Python, Lisp and Haskell functions are all examples of

Answer 2

subprograms

Question 3

What are PL related examples of subprograms

Answer 3

• Java methods
• Ada procedures and functions
• Python, Lisp and Haskell functions

Question 4

As a tool for abstraction, a subprogram should implement

Answer 4

a single high level action that has a clearly defined effect when executed.

Question 5

Abstraction is an

Answer 5

alternative view of a system or activity, in which some details are hidden so that a more important aspect of the system or activity stands out.

Question 6

In a description of a subprogram aimed at the users the following aspects should be made clear:

Answer 6

• Does the subprogram take some values as parameters? (How many and what types)
• Does the subprogram access some external variables (ie variables whose scope is
  not confined to the subprogram block, eg global variables)? (Does it read them, modify their values.)
• Does the subprogram read from or write to some input/output devices (eg the console, files or network)?
• Does the subprogram return some values? (How many and what types)

Question 7

Depending on the main effect, we classify subprograms as:

Answer 7

Procedures and Functions

Question 8

Procedures =

Answer 8

Subprograms whose main purpose is to change the values of some external memory or variables and/or work with IO devices.

Question 9

Functions =

Answer 9

Subprograms whose main purpose is to create and return a single (perhaps composite) value.

Question 10

A pure function

Answer 10

returns a single value that depends only on the values of the function’s parameter(s) and does not modify any global (or other external) variables nor influences input/output devices.

Question 10

A pure procedure perform effects to achieve a

Answer 10

single coherent goal and does not return any values.

Question 11

An expression is

Answer 11

anything that can be evaluated to produce a value

Question 12

A command

Answer 12

modifies the program state

Question 13

A program state consists of the

Answer 13

internal state, which includes the current values of all program variables with a current lifetime, and the external state which includes the state of all its input and output devices such as keyboard, monitor, network, printer.

Question 14

In Java, some examples of commands are:

Answer 14

assignment, creating a new object, incrementing a variable (i++).

Question 15

In a PL which maintains a clear distinction between
expressions and commands;

Answer 15

• evaluating an expression yields a value but does not modify program state and
• executing a command changes state and does not produce a value

Question 16

Defining a function can be viewed as

Answer 16

encapsulating an expression so that it can be replaced by a single meaningful name.

Question 17

A thread’s lifetime is usually defined by

Answer 17

executing a single subprogram.

Question 18

Defining a procedure can be viewed as

Answer 18

defining a new, more abstract and powerful command.

Eg input/output commands in Java are provided by the libraries
as methods (eg println), rather than primitive commands.

Question 19

In Java, the main thread

Answer 19

executes the main method.

Question 20

In Ada, the main thread

Answer 20

executes a top-level procedure defined in a single file whose name coincides with the name of that procedure.

Question 21

In Haskell, the main thread

Answer 21

evaluates and executes the main function.

Question 22

In Python and Lisp, it is a bit different: the main thread

Answer 22

reads in the given file and executes the commands in it sequentially. It is as if the whole file defined an unnamed “main” subprogram.

Question 23

A parameter bridges the calling context and the subprogram definition, allowing for

Answer 23

values to be passed either way between the two contexts.

Question 24

Parameter names, as declared in the subprogram body, are called

Answer 24

formal parameters while the expressions that are aligned with the formal parameters in a particular call are called actual parameters.

Question 25

The ways in which actual parameters and formal parameters are linked with one another divide into the following two main approaches:

Answer 25

* Copy parameter passing mechanisms: * Reference parameter passing mechanisms:

Question 26

Copy parameter passing mechanisms:

Answer 26

The formal parameter is a special local variable whose lifetime starts and ends with the call. Usually, the value computed by the actual parameter is copied at the start of the call to the formal parameter. Sometimes, when the call is finishing, the value of the formal parameter is copied to the actual parameter (in which case the actual parameter has to be a variable).

Question 27

Reference parameter passing mechanisms:

Answer 27

The actual parameter expression has to be a single variable. The actual parameter variable becomes the formal parameter during the call. Thus the parameter’s value is stored in the calling context and no copying of values occurs. Any access within the subprogram to the parameter results in reading from or writing to the actual parameter variable

Question 28

Copy parameter passing mechanisms

Answer 28

Copy-in parameter (also known as value parameter) Copy-out parameter (also known as result parameter) Copy-in-copy-out parameter (also known as value-result parameter)

Question 29

Copy-in parameter (also known as value parameter):

Answer 29

When the subprogram is called, the parameter (ie a local variable) is assigned the value given by the actual parameter. The parameter can then be used like any other local variable – read as well as updated. Any updates of the parameter value inside the subprogram have no effect outside the subprogram.

Question 30

Copy-out parameter (also known as result parameter):

Answer 30

When the subprogram is called, the parameter (ie a local variable) is not initialised. It must be assigned a value during the execution of the subprogram. When the subprogram finishes, the actual parameter (which in this case must be a variable or another assignable entity) is assigned the final value of the formal parameter. A copy-out parameter transfers a value in the opposite direction than a copy-in parameter. Also the transfer happens at the end of the subprogram call rather than in the beginning.

Question 31

Copy-in-copy-out parameter (also known as value-result parameter)

Answer 31

A combination of Copy-In and Copy-Out. The actual parameter must be a variable or another assignable entity (a memory reference). The formal parameter is initialised by an assignment from the actual parameter and when the call is finished, the actual parameter is assigned the final value of the formal parameter.

Question 32

Reference parameter mechanism Advantages and disadvantages

Answer 32

In this approach a formal parameter does not have its own memory cell but shares one with the actual parameter, which must be a variable The advantage of this approach is that there is no need for copying any values around. The disadvantage is that the value is aliased between the two contexts, which can lead to unintuitive behaviour. I.e., when the value is modified in the subprogram, it is modified also in the calling context.

Question 33

To alleviate this problem, it is possible to forbid making any changes to a parameter that uses the reference passing mechanism. Such parameters are called

Answer 33

constant parameters

Question 34

Parameter passing in Java. Java uses

Answer 34

only copy-in (value) parameters

Question 35

Parameter aliasing. In addition to the problem of aliasing between the actual and formal parameters, reference passing causes a more serious problem of aliasing

Answer 35

between multiple formal parameters. If the compiler chooses to use a reference passing mechanism for arrays, then the call Add_Vectors(A,B,C,C) which is supposed to add the three vectors A, B, C and store the result in C will instead return the sum of A and B multiplied by two because the parameters V3 and V_Sum become aliased, ie two names for the same variable

Question 36

An exception is an event

Answer 36

that is not expected to occur under normal circumstances.

Question 37

The occurrence of an exception has an important effect on the execution of a subprogram

Answer 37

The thread changes its behaviour and instead of continuing in the usual manner, it searches for a handler.

Question 38

If there is no suitable exception handler in the current subprogram, the subprogram

Answer 38

terminates, usually without any option of coming back to it.

Question 39

When an exception propagates up from a call

Answer 39

the call is abandoned and no assignment of return values takes place.

Question 40

One level higher, where the call was made to the current subprogram, the same excep- tion is thrown/raised and the thread looks for a handler in that context. Whenever this happens, we say that the exception has

Answer 40

propagated from the subprogram call.

Question 41

The same exception behaviour is simulated in C by having the subprogram

Answer 41

return a special status value.

Question 42

A function has a side effect if it

Answer 42

reads or modifies non-local variables or input/output devices. Anything in the behaviour of a function which cannot be described in terms of the return value and how the return value depends on the parameter values is a side effect.

Question 43

Pure functions are functions that are free from

Answer 43

side effects.

Question 44

Haskell does not permit the definition of any subprogram that is not a

Answer 44

pure function. Currently it is the only PL in widespread use with this property

Question 45

An expression is referentially transparent if

Answer 45

wherever it appears in a program, it can be replaced by any equivalent expression and this will not change the meaning of the program under any circumstances.

Question 46

Functions that take a function as one of its parameters are called

Answer 46

higher-order functions.