Javascript Filter

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The JavaScript () filter method creates an array of elements from an existing array. The filter () method takes a callback function as an argument. This function evaluates whether an item should be added to the new list from the existing one.

If you don’t know JavaScript , you might not have heard of Javascript functions filter () and reduce (). These functions can be useful when working with a list of objects, also called an array.

If you only want to get certain values ‚Äã‚Äãfrom an array, you should be able to use the filter () JavaScript function. In this tutorial, we will see, with reference to examples, how to use the filter () function.

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A JavaScript array stores zero or more elements. The elements can be numbers, other arrays, strings, or use some other type of data. They use numbered indexes starting with " 0 " to access specific items.

Arrays allow you to store multiple values ‚Äã‚Äãin the same variable. It can help you write better and cleaner because you don’t need to write multiple variables to store your values.

Here is an example of a JavaScript array:

JavaScript filters array

The JavaScript array filter () method creates a new array based on the contents of an existing array. The filter () method evaluates a callback function on each element of an existing array. Anything that satisfies the condition of the callback function is added to a new list.

Using the filter method, we are able to retrieve the values ‚Äã‚Äãthat meet certain criteria and add them to a new array.

JavaScript array filter syntax

The filter method uses the following syntax:

This code defines an array named" newArr "based on the contents of the array "oldArr". Our callback function returns a value to the new array if a condition is met. Examples of conditions:

You can use JavaScript comparison operators to create your conditions. Or you can use an if statement. Using an if statement allows you to control which of the multiple values ‚Äã‚Äãis added to a new list.

Examples of JavaScript filters < / h2>

Filter a list of numbers

Let’s start with a simple example. We have a shopping list at a cafe. We want to create a new list showing purchases over $ 10 .

Our shopping list looks like this:

We can use the filter () method to retrieve purchases of value greater than $ 10:

Our program checks if a purchase is worth more than $ 10. A purchase is added to our big_purchases list if it is worth more than $ 10. Otherwise, the purchase is omitted.

The value of the big_purchases JavaScript variable is:

The filter () method returns all the elements that it exceeds in the test we have defined. This test is "item> 10". If we wanted to access each purchase, we could refer to our original "purchases" list . Our filter () method returns an array of values ‚Äã‚Äãgreater than $ 10.

Filter an array of objects

We have an array that contains the names of everyone in a school:

And if we only want to recover students who are in the first year ? Filter () makes this easier. Let’s write a function that will return all the first graders in the "students" array:

Filter () accepts one parameter. This is a function that determines what action you take on your data.

The function () filter examines each object in the "students" array. filter () returns students whose class is equal to "First year". The operator "===" means equality.

The filter function will create a new array with these new values, accessible via the "firstGradeStudents" variable.



Conclusion

You can use the filter () array method to create a new table from the contents of an existing table. The index is in the name with the filter () method. filter () filters the elements of an existing array.

And that’s it ! You can now use reduce () in JavaScript with even more security. To learn more about how to code in JavaScript, read our article on the best JavaScript tutorials for beginners .

Javascript Filter filter: Questions

List comprehension vs. lambda + filter

5 answers

I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items.

My code looked like this:

my_list = [x for x in my_list if x.attribute == value]

But then I thought, wouldn"t it be better to write it like this?

my_list = filter(lambda x: x.attribute == value, my_list)

It"s more readable, and if needed for performance the lambda could be taken out to gain something.

Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way‚Ñ¢ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)?

957

Answer #1

It is strange how much beauty varies for different people. I find the list comprehension much clearer than filter+lambda, but use whichever you find easier.

There are two things that may slow down your use of filter.

The first is the function call overhead: as soon as you use a Python function (whether created by def or lambda) it is likely that filter will be slower than the list comprehension. It almost certainly is not enough to matter, and you shouldn"t think much about performance until you"ve timed your code and found it to be a bottleneck, but the difference will be there.

The other overhead that might apply is that the lambda is being forced to access a scoped variable (value). That is slower than accessing a local variable and in Python 2.x the list comprehension only accesses local variables. If you are using Python 3.x the list comprehension runs in a separate function so it will also be accessing value through a closure and this difference won"t apply.

The other option to consider is to use a generator instead of a list comprehension:

def filterbyvalue(seq, value):
   for el in seq:
       if el.attribute==value: yield el

Then in your main code (which is where readability really matters) you"ve replaced both list comprehension and filter with a hopefully meaningful function name.

957

Answer #2

This is a somewhat religious issue in Python. Even though Guido considered removing map, filter and reduce from Python 3, there was enough of a backlash that in the end only reduce was moved from built-ins to functools.reduce.

Personally I find list comprehensions easier to read. It is more explicit what is happening from the expression [i for i in list if i.attribute == value] as all the behaviour is on the surface not inside the filter function.

I would not worry too much about the performance difference between the two approaches as it is marginal. I would really only optimise this if it proved to be the bottleneck in your application which is unlikely.

Also since the BDFL wanted filter gone from the language then surely that automatically makes list comprehensions more Pythonic ;-)

How do I do a not equal in Django queryset filtering?

5 answers

MikeN By MikeN

In Django model QuerySets, I see that there is a __gt and __lt for comparative values, but is there a __ne or != (not equals)? I want to filter out using a not equals. For example, for

Model:
    bool a;
    int x;

I want to do

results = Model.objects.exclude(a=True, x!=5)

The != is not correct syntax. I also tried __ne.

I ended up using:

results = Model.objects.exclude(a=True, x__lt=5).exclude(a=True, x__gt=5)
784

Answer #1

You can use Q objects for this. They can be negated with the ~ operator and combined much like normal Python expressions:

from myapp.models import Entry
from django.db.models import Q

Entry.objects.filter(~Q(id=3))

will return all entries except the one(s) with 3 as their ID:

[<Entry: Entry object>, <Entry: Entry object>, <Entry: Entry object>, ...]

Javascript Filter JavaScript: Questions

JSON datetime between Python and JavaScript

4 answers

kevin By kevin

I want to send a datetime.datetime object in serialized form from Python using JSON and de-serialize in JavaScript using JSON. What is the best way to do this?

403

Answer #1

You can add the "default" parameter to json.dumps to handle this:

date_handler = lambda obj: (
    obj.isoformat()
    if isinstance(obj, (datetime.datetime, datetime.date))
    else None
)
json.dumps(datetime.datetime.now(), default=date_handler)
""2010-04-20T20:08:21.634121""

Which is ISO 8601 format.

A more comprehensive default handler function:

def handler(obj):
    if hasattr(obj, "isoformat"):
        return obj.isoformat()
    elif isinstance(obj, ...):
        return ...
    else:
        raise TypeError, "Object of type %s with value of %s is not JSON serializable" % (type(obj), repr(obj))

Update: Added output of type as well as value.
Update: Also handle date

What blocks Ruby, Python to get Javascript V8 speed?

4 answers

Are there any Ruby / Python features that are blocking implementation of optimizations (e.g. inline caching) V8 engine has?

Python is co-developed by Google guys so it shouldn"t be blocked by software patents.

Or this is rather matter of resources put into the V8 project by Google.

260

Answer #1

What blocks Ruby, Python to get Javascript V8 speed?

Nothing.

Well, okay: money. (And time, people, resources, but if you have money, you can buy those.)

V8 has a team of brilliant, highly-specialized, highly-experienced (and thus highly-paid) engineers working on it, that have decades of experience (I"m talking individually – collectively it"s more like centuries) in creating high-performance execution engines for dynamic OO languages. They are basically the same people who also created the Sun HotSpot JVM (among many others).

Lars Bak, the lead developer, has been literally working on VMs for 25 years (and all of those VMs have lead up to V8), which is basically his entire (professional) life. Some of the people writing Ruby VMs aren"t even 25 years old.

Are there any Ruby / Python features that are blocking implementation of optimizations (e.g. inline caching) V8 engine has?

Given that at least IronRuby, JRuby, MagLev, MacRuby and Rubinius have either monomorphic (IronRuby) or polymorphic inline caching, the answer is obviously no.

Modern Ruby implementations already do a great deal of optimizations. For example, for certain operations, Rubinius"s Hash class is faster than YARV"s. Now, this doesn"t sound terribly exciting until you realize that Rubinius"s Hash class is implemented in 100% pure Ruby, while YARV"s is implemented in 100% hand-optimized C.

So, at least in some cases, Rubinius can generate better code than GCC!

Or this is rather matter of resources put into the V8 project by Google.

Yes. Not just Google. The lineage of V8"s source code is 25 years old now. The people who are working on V8 also created the Self VM (to this day one of the fastest dynamic OO language execution engines ever created), the Animorphic Smalltalk VM (to this day one of the fastest Smalltalk execution engines ever created), the HotSpot JVM (the fastest JVM ever created, probably the fastest VM period) and OOVM (one of the most efficient Smalltalk VMs ever created).

In fact, Lars Bak, the lead developer of V8, worked on every single one of those, plus a few others.

Django Template Variables and Javascript

4 answers

When I render a page using the Django template renderer, I can pass in a dictionary variable containing various values to manipulate them in the page using {{ myVar }}.

Is there a way to access the same variable in Javascript (perhaps using the DOM, I don"t know how Django makes the variables accessible)? I want to be able to lookup details using an AJAX lookup based on the values contained in the variables passed in.

256

Answer #1

The {{variable}} is substituted directly into the HTML. Do a view source; it isn"t a "variable" or anything like it. It"s just rendered text.

Having said that, you can put this kind of substitution into your JavaScript.

<script type="text/javascript"> 
   var a = "{{someDjangoVariable}}";
</script>

This gives you "dynamic" javascript.

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