Fonctions : appel

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Del curso dictado por École Polytechnique Fédérale de Lausanne

Initiation à la programmation (en Java)

217 calificaciones

École Polytechnique Fédérale de Lausanne

Initiation à la programmation (en Java)

217 calificaciones

Ce cours initie aux bases de la programmation en utilisant le langage Java : variables, boucles, fonctions, ... Il ne présuppose pas de connaissance préalable. Les aspects plus avancés (programmation orientée objet) sont donnés dans un cours suivant, «Introduction à la programmation orientée objet (en Java)». Il s'appuie sur de nombreux éléments pédagogiques : vidéos sous-titrées, quizz dans et hors vidéos, exercices, devoirs notés automatiquement, notes de cours.

De la lección

Fonctions / Méthodes

Cette semaine aborde un sujet fondamental en programmation : les « fonctions » qui permettent de beaucoup mieux structurer les programmes et d'éviter d'avoir à récrire plusieurs fois la même portion de code.

Fonctions : introduction12:00

Fonctions : appel8:17

Fonctions : passage des arguments16:12

Fonctions : entêtes3:37

Fonctions : définitions16:42

Fonctions : méthodologie5:40

Fonctions : surcharge5:19

Conoce a los instructores

Jean-Cédric Chappelier
Dr.
School of Computer and Communication Sciences
Jamila Sam
Dr
School of Computer and Communication Sciences

In a previous episode, we introduced the notion of functions, of methods,

and their use in programming.

Today, we will focus on what happens

when we use, when we call a method.

Basically, at this stage, we know that the concept of a method is

associated to three aspects: the header, the definition and the call.

And today, we will look in detail into what happens

at the call.

Let's go back to our example on calculating means.

Here, we have a little main program which asks

the user to input two grades.

These two grades are input by the user via the keyboard,

and the mean of these two grades is displayed.

The calculation of the mean is done here by calling a method,

the "moyenne" method. [TN: "moyenne" means "mean"]

And we will now study the mechanisms involved in such a call.

Here, we have a real example of a method call, where the method called

is "moyenne", which we call by passing two arguments to it,

in the form of expressions which will of course correspond

respectively to the two parameters of the method.

Note that we usually name "parameters" the data which

the method needs to work as described in the header of the method,

and we name "arguments" the actual data passed to the method

at the moment it is called.

So the goal here is to examine what happens at the moment a call

like this is made.

The first step that is executed actually consists in evaluating

the different arguments and in Java, this evaluation will systematically be done

left-to-right.

So here, the first argument is evaluated.

The expression is evaluated, which will return a value of 2.3.

Second expression.

This will produce a value of 4.25.

Then, second step, a link will have to be established

between these arguments and the parameters of the method.

This is what happens during the second step.

So, during the second step, the parameter x will be matched with the result

of the first expression. x will be worth 2.3.

And y will be matched with the evaluation of the second

expression: 4.25.

We are thus in the situation where the parameters x and y have

actual values with which the method will now be able

to work and run.

Third step: the body of the method is executed.

Here, the body consists in a single instruction which is the return,

the return statement right here.

So here, the body doesn't actually execute anything in particular other

than the return statement.

Fourth step,

the expression which will serve as a return value will now

be evaluated.

So basically, (x+y)/2 will be calculated,

producing the following result.

And this value is now returned by the method to make it

available to the program that called it.

So actually,

this whole method call is replaced

by the result returned by the method.

So last step,

z takes a value of 3.275.

We have thus seen the five steps that usually occur

when we call a method.

First step: the expressions which are passed as arguments

to the method are evaluated.

Second, the values of these expressions are assigned to the parameters

of the method.

The method then has access to all the information that

it needs to work, and its body can be executed.

Fourth step: the expression which follows the first return statement encountered

during execution is evaluated and the result of the evaluation of this

expression is then returned as the result of the call.

These five steps occur in the most general case; there are situations

where the process is somewhat simplified.

We know that in the most general case a method needs

parameters, input data, and produces a result as output.

However, there are some situations where the method does not need any

input data. In such a case, of course, steps one and two,

which consist in evaluating the expressions passed as arguments and assigning

them to the parameters, no longer apply (for a method without arguments).

Then, there are situations where there is no output data, where the method

returns no result. And in this case, the steps which consist in

evaluating the expression following the return statement and returning this result,

i.e. steps 4 and 5, do not occur.

A method can easily call another method. This is the case in this

little example where you have a "afficheScore" (TN: means "displayScore") method which

takes as parameters a player, identified by an integer, a number of points won

by the player and the time he spent playing, and which will display

the player's score.

Here, it so happens that the score is calculated according to the number

of points and the time spent, and the calculation is done by a method.

The method is defined elsewhere

and takes as parameter the number of points and the game time.

You now know the different steps which are executed

when a method is called in a program, and you now can see

a little diagram which summarizes these various steps.

Here, we have a method "f" called by a method "g".

The match between the parameters of the method and the calling arguments

is done at the moment of the method call.

So here, when we call the method, the argument is copied into the parameter

of the method which uses this to fulfill its job.

In the most general case, the method calculated a return value

and this return value is transmitted to the calling method which will be able

to use it to carry out its instructions.

You will no doubt have noticed all along this episode that

there is a certain jargon, a terminology associated to a method call.

Let's now summarize this jargon.

You will have understood that when I speak of "calling a method", I mean

that I use the method; here, I am using the method f to calculate

a result that I then assign to a variable x.

When I speak of "passing a value" as argument to a method, I mean

that this value is simply copied into a parameter of the method.

And finally, when I say that a method "returns" the value of y, for example,

I mean that the expression of the function call will simply be

replaced by the returned value.

So if we take a real example now, suppose that we have a

method "f" whose last instruction is this one

Suppose that this method is called like so, and that at the moment

of the call, the value of y is 5. To say that the method

returns the value of y -- that is, 5 -- is the same as to say that we simply

replace this call by the return value, i.e. 5.

And so, I can equally say "cos(0) returns cosine of zero",

or "cos(0) returns 1", for example.

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