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Schleifen in ECMAScript
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Im ECMAScript werden die Befehle (statement) in einer Reihenfolge vom oben nach hinten durchgeführt. Allerdings wenn Sie eine Menge der Befehle mehrmals durchführen möchten, können Sie die Schleife benutzen (loop).
ECMAScript bietet Sie 3 folgende Arte von der Schleife
- while loop
- for loop
- do .. while loop
Die Befehle (statement) können innerhalb der Schleife benutzt werden
- continue
- break
| Der Befehl | Die Bezeichnung |
|---|---|
| break | Die Schleife stoppen |
| continue | Die restlichen Schleife in dem Befehlsblock ignorieren und sofort die Bedingung vor der Wiederdurchführung des Befehlsblock überprüfen |
Die Syntax der Schleife while:
while (condition) {
// Do something here
// ....
}
Zum Beispiel:
while-loop-example.js
console.log("While loop example");
// Declare a variable, and assign value of 2.
let x = 2;
// Condition is x < 10
// If x < 10 is true then run block
while (x < 10) {
console.log("Value of x = ", x);
x = x + 3;
}
// This statment is out of while block.
console.log("Finish");
Output:
While loop example
Value of x = 2
Value of x = 5
Value of x = 8
Finish
Die Standard-Schleife for hat die folgende Syntax:
for(InitialValues; condition; updateValues )
{
// Statements
}
- InitialValues: die Wert für die betreffenden Variables in die Schleife initialisieren
- condition: Die Bedingungen definieren um das Command-Block durchzuführen
- updateValues: Die neue Wert für die Variables aktualisieren.
for-loop-example.js
console.log("For loop example");
for( let i = 0; i < 10; i= i + 3) {
console.log("i= "+ i);
}
Output:
For loop example
i= 0
i= 3
i= 6
i= 9
for-loop-example2.js
console.log("For loop example");
for(let i = 0, j = 0; i + j < 10; i = i+1, j = j+2) {
console.log("i = " + i +", j = " + j);
}
Output:
For loop example
i = 0, j = 0
i = 1, j = 2
i = 2, j = 4
i = 3, j = 6
Benutzen Sie die Schleife for damit Sie die Elemente des Arrays durchsuchen können
for-loop-example3.js
console.log("For loop example");
// Array
let names =["Tom","Jerry", "Donald"];
for (let i = 0; i < names.length; i++ ) {
console.log("Name = ", names[i]);
}
console.log("End of example");
Output:
For loop example
Name = Tom
Name = Jerry
Name = Donald
End of example
Die Schleife for ... in hilft Ihnen bei der Iteration der Property eines Objekt.
for (prop in object) {
// Do something
}
Zum Beispiel:
for-in-loop-example.js
// An object has 4 properties (name, age, gender,greeing)
var myObject = {
name : "John",
age: 25,
gender: "Male",
greeting : function() {
return "Hello";
}
};
for(myProp in myObject) {
console.log(myProp);
}
Output:
name
age
gender
greeting
Die Schleife for ... of hilft Ihnen bei der Iteration auf einer Collection , z.B Map, Set
Mehr sehen
Zum Beispiel:
for-of-example.js
// Create a Set from an Array
var fruits = new Set( ["Apple","Banana","Papaya"] );
for(let fruit of fruits) {
console.log(fruit);
}
Output:
Apple
Banana
Papaya
for-of-example2.js
// Create a Map object.
var myContacts = new Map();
myContacts.set("0100-1111", "Tom");
myContacts.set("0100-5555", "Jerry");
myContacts.set("0100-2222", "Donald");
for( let arr of myContacts) {
console.log(arr);
console.log(" - Phone: " + arr[0]);
console.log(" - Name: " + arr[1]);
}
Output:
[ '0100-1111', 'Tom' ]
- Phone: 0100-1111
- Name: Tom
[ '0100-5555', 'Jerry' ]
- Phone: 0100-5555
- Name: Jerry
[ '0100-2222', 'Donald' ]
- Phone: 0100-2222
- Name: Donald
The do-while loop is used to execute a section of program many times.The characteristics of the do-while is that the block of statement is always executed at least once. After each iteration, the program will check the condition, if the condition is still correct, the statement block will be executed again.
do {
// Do something
}
while (expression);
Zum Beispiel:
do-while-loop-example.js
let value = 3;
do {
console.log("Value = " + value);
value = value + 3;
} while (value < 10);
Output:
Value = 3
Value = 6
Value = 9
break ist ein Befehl, der in einem Befehlsblock einer Schleife liegen können. Das ist der Befehl zum bedingunglosen Stoppen der Schleife
loop-break-example.js
console.log("Break example");
// Declare a variable and assign value of 2.
let x = 2;
while (x < 15) {
console.log("----------------------");
console.log("x = ", x);
// If x = 5 then exit the loop.
if (x == 5) {
break;
}
// Increase value of x by 1
x = x + 1;
console.log("x after + 1 = ", x);
}
console.log("End of example");
Output:
Break example
----------------------
x = 2
x after + 1 = 3
----------------------
x = 3
x after + 1 = 4
----------------------
x = 4
x after + 1 = 5
----------------------
x = 5
End of example
continue ist ein Befehl, der in einer Schleife liegen können. Beim Treffen des Befehl continue ignoriert das Programm die anderen Befehle des hinten Block continue und beginnt eine neue Schleife
loop-continue-example.js
console.log("Continue example");
// Declare a variable and assign value of 2
x = 2
while (x < 7) {
console.log("----------------------")
console.log("x = ", x)
// % is used for calculating remainder
// If x is even, then ignore the command line below of continue
// and start new iteration.
if (x % 2 == 0) {
// Increase x by 1.
x = x + 1;
continue;
}
else {
// Increase x by 1.
x = x + 1
console.log("x after + 1 =", x)
}
}
console.log("End of example");
Output:
Continue example
----------------------
x = 2
----------------------
x = 3
x after + 1 = 4
----------------------
x = 4
----------------------
x = 5
x after + 1 = 6
----------------------
x = 6
End of example
The ECMAScript allows you to stick a label to a loop. It is like you name a loop, which is useful when you use multiple nested loops in a program.
- You can use break labelX statement; to break a loop is attached labelX.
- You can use continue labelX statement; to continue a loop is attached labelX.
Die Syntax
// for loop with Label.
label1: for( ... ) {
}
// while loop with Label.
label2: while ( ... ) {
}
// do-while loop with Label.
label3: do {
} while ( ... );
Example of using nested loops, labelled and labelled break statement.
labelled-loop-break-example.js
console.log("Labelled Loop Break example");
let i = 0;
label1: while (i < 5) {
console.log("----------------------\n");
console.log("i = " + i);
i++;
label2: for (let j = 0; j < 3; j++) {
console.log(" --> " + j);
if (j > 0) {
// Exit the loop with label1.
break label1;
}
}
}
console.log("Done!");
Output:
Labelled Loop Break example
----------------------
i = 0
--> 0
--> 1
Done!
Example of using nested labelled loops and labelled continue statement.
labelled-loop-continue-example.js
let i = 0;
label1: while (i < 5) {
console.log("outer i= " + i);
i++;
label2: for (let j = 0; j < 3; j++) {
if (j > 0) {
continue label2;
}
if (i > 1) {
continue label1;
}
console.log("inner i= " + i + ", j= " + j);
}
}
Output:
outer i= 0
inner i= 1, j= 0
outer i= 1
outer i= 2
outer i= 3
outer i= 4
*
