Javascript: Arrays and For Loop Basics
Question
I'm new to javascript and have been trying to teach myself the basics. I do have some experience with C++.
I came across this example in the source I'm using for studying and the for loop looks strange to me:
<html>
<head>
<script type="text/javascript">
<!--
function ReadCookie()
{
var allcookies = document.cookie;
alert("All Cookies : " + allcookies );
// Get all the cookies pairs in an array
cookiearray = allcookies.split(';');
// Now take key value pair out of this array
for(var i=0; i<cookiearray.length; i++){
name = cookiearray[i].split('=')[0];
value = cookiearray[i].split('=')[1];
alert("Key is : " + name + " and Value is : " + value);
}
}
//-->
</script>
</head>
<body>
<form name="myform" action="">
<input type="button" value="Get Cookie" onclick="ReadCookie()"/>
</form>
</body>
</html>
Would someone mind explaining why there is a [0] and [1] near the end of these statements?
name = cookiearray[i].split('=')[0]; value = cookiearray[i].split('=')[1];
6 answers
solution1
3 ACCPTED 2013-12-17 20:08:59
A clearer way to write this statement is this:
var parts = cookiearray[i].split('='),
name = parts[0],
value = parts[1];
solution2
2 2013-12-17 20:07:49
That doesn't have anything to do with the for-loop itself. Split returns an array of tokens; the string is split up at the given delimiter. You're simply accessing the first and second tokens in this array.
solution3
1 2013-12-17 20:09:53
String.split creates an array using the assigned delimiter (in this case '='). The [0] and [1] are selecting the first and second elements of the array respectively (Javascript array element indexes start at zero).
solution4
1 2013-12-17 20:10:39
Those are used to access the items in the arrays that you create.
It's more clear what they do, and also better for performance, if you put the array in a variable and access that. That way you don't have to create two identical arrays:
var cookie = cookiearray[i].split('=');
var name = cookie[0];
var value = cookie[1];
solution5
0 2013-12-17 20:07:09
该代码将每个cookie对key=value拆分为key（ [0] ）和value（ [1] ）。
solution6
0 2013-12-17 20:27:32
A for loop is an iterator. You can sort of think of it like a counter, or stepping progression. If you want to do something x number of times, then a for loop is your best friend. The issue, when you are first learning, is figuring out how to use them effectively.
I'll give you a relevant and nonrelevant (simplified) example:
Nonrelevant:
// Increase the value of x and print the new value to the console 20 times.
var x = 6;
for(var i = 0; i < 20; i++) {
x++;
console.log("Value of x: " + x);
}
If you take a close look it's really rather logical. You define an iteration variable i , tell it when to stop ( i < 20 ...so stop if i = 20), how to progress with each iteration ( i++ or i + 1).
So the code will enter the loop when i = 0 ...it will add 1 to the value of x (x++), resulting in x = 7. It does the same for each iteration, so when i = 1 , x (7) + 1 = 8, so on and so forth until i = 20, then the code breaks out of the loop and continues on it's merry little way. When it breaks we have just added 1 to x 20 times. Since x used to equal 6, it now equals 26.
Relevant:
// Given array of length 10, print each element to the console
for (var i = 0; i < cookiearray.length; i++){
console.log(cookiearray[i]);
}
So here the for loop is the same, it simply iterates until i equals the length (number of elements, in this case) of the array (so 10 times). When i = 0 , our code prints the element of the array at cookiearray[0] ...when i = 1 it prints cookiearray[1] ...so on and so forth for the entirety of the array.
What may confuse you in the example is the split function. Split returns an array. So this line:
name = cookiearray[i].split('=')[0];
...actually means assign the first element of the new array created from splitting the cookiearray's element at position i .
Let's assume that cookiearray[0] = "Hello=World" . When i = 0 , our code splits the string "Hello=World" into a new array where the element at the 0 position is "Hello", it then assigns this to the local variable name. So name = "Hello".
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.