I am new to erd-model and concepts. Just wondering when can an attribute be derived? E.g when the attribute requires certain formula to compute?
In my case my attribute SectionsOffered is to capture the number of sections available for the course hence should my SectionsOffered be a simple attribute or a case of derived attribute since i can count the number of sections tied to the course?
Related
I am working on a project using the Html-Agility-Pack and I need to build a list of each link that has an href attribute as its first attribute. What XPath expression would be used for this?
Example (I would want to only select the first):
<a href="http://someurl.com"/>
<a id="someid" href="http://someurl.com"/>
No, don't do that.
You really don't want to select elements based upon the ordering of their attributes because attribute order is arbitrary in HTML and XML. Find another criteria to limit your selections:
attribute presence or attribute value
child element presence or string value
preceding element value, possibly a label
etc
You want to choose a criteria that's invariant across all instances of the HTML/XML documents you may encounter. Attribute order is not such a criteria.
In HTML we use id's and classes. we can choose any name for id.Can we choose any name for class also? Or is there any specification name for classes ?
From the HTML 5 specification:
ID:
The id attribute specifies its element's unique identifier (ID). The
value must be unique amongst all the IDs in the element's home subtree
and must contain at least one character. The value must not contain
any space characters.
There are no other restrictions on what form an ID can take; in
particular, IDs can consist of just digits, start with a digit, start
with an underscore, consist of just punctuation, etc.
Class:
The attribute, if specified, must have a value that is a set of
space-separated tokens representing the various classes that the
element belongs to.
The classes that an HTML element has assigned to it consists of all
the classes returned when the value of the class attribute is split on
spaces. (Duplicates are ignored.)
Also, there are style guides that define good and very used pattern for choose the values to use for id or classes, etc. I recommend you this one from W3Schools.
"The id attribute got more classy in HTML5" is written at some pages. If I use class attribute instead of id, does it conform to HTML5? Thanks for your help.
I believe you're talking about this article. Well, nothing has really changed — classes and ids are used the same way as in HTML4.
Except one thing: The HTML4 spec says the following
ID and NAME tokens must begin with a letter ([A-Za-z]) and may be
followed by any number of letters, digits ([0-9]), hyphens (“-“),
underscores (“_”), colons (“:”), and periods (“.”).
However, in the HTML5 spec, the requirement for ids is much less rigid
The value must be unique amongst all the IDs in the element’s home
subtree and must contain at least one character. The value must not
contain any space characters.
So HTML5 ids can accept more characters, which is what the article you're reffering to is talking about.
The attributes still have two different purposes:
class can contain multiple classes and multiple elements can have the same classes
id contains a single ID and that ID can only be used for one element.
The statement you quoted exists because some restrictions on how an ID must look like have been lifted in HTML5 - classes never had those restrictions.
The difference between ID and class is that the ID has to be unique at one page but the class can be used multiply times.
Both is valid HTML 5, you can validate your page here: http://validator.w3.org/
Nothing has really changed with HTML5 in that respect. IDs are still unique, and classes can be shared across elements. Not sure what the quote was referring to.
class is for generic purpose and id is for unique identification purpose. I mean, id uniquely identifies an element and class specifies a group elements to have the same type of behavior. I hope, it clears the prupose of class and id.
http://www.w3.org/TR/WCAG-TECHS/F17.html
I have some issues understanding the criteria of this and what makes a website fail BASED on the series of tests involved.
Check for id and accesskey values which are not unique within the document.
Check that attribute values that have an idref value have a corresponding id value.
For tables that use the axis attribute, check that all values listed in the axis attribute have a corresponding id value in a table header cell in the same table.
For client-side image maps, check that the value of the usemap attribute has a corresponding id value if the usemap attribute is not a URI.
If step #1, step #3 or step #4 is true or step #2 is false, then this failure condition applies and the content fails the success criterion.
These are a series of requirements that make the HTML you have written valid.
1 is straightforward. There can't be more than one element on your page with the same ID. If you have multiple elements with the same ID on your page then when you call the Javascript function
document.getElementById("idnamehere")
then you will have trouble selecting all of them. If you want multiple items to have the same style, then you should be using the class attribute rather than ID. The ID must be unique!
2 If you have given an element the idref attribute, then it must correspond to an existing element with the id you have specified in the idref attribute. For example, if you wanted to use the following idref:
<p idref="data"></p>
Then there must be an existing id somewhere in your document, that looks something like:
<span id="data"></span>
You can't reference an id that doesn't exist!
3 I have never used the axis attribute before, but what I understand from reading that document and a small amount of Googling; if you want use the axis attribute then every cell must have a corresponding axis attribute supplied in the table header of the column it is in. Someone else may be able to expand on this.
4 Again, I have never used an ImageMap, but the W3C document has categorized this set of rules under the general theme of non-unique identities and mismatched references, so I can only assume that it is similar to 2 whereby, the imagemap has a corresponding usemap, which is referenced by its ID (unless, it has been specified as a URI).
I think the gist of this document is to enforce the concept that there should always be a corresponding attribute for the elements that require them, and your element ID's should always be kept unique.
If you are trying to fix something on your website, then http://validator.w3.org/ can be a very handy resource for pinpointing errors on your page and describing them. Hope this helps!
What's the point of the name attribute on an HTML form? As far as I can tell, you can't read the form name on submission or do anything else with it. Does it serve a purpose?
In short, and probably oversimplifying a bit: It is used instead of id for browsers that don't understand document.getElementById.
These days it serves no real purpose. It is a legacy from the early days of the browser wars before the use of name to describe how to send control values when a form is submitted and id to identify an element within the page was settled.
From the specification:
The name attribute represents the form's name within the forms collection.
Once you assign a name to an element, you can refer to that element via document.name_of_element throughout your code. It doesn't work to tell when you've got multiple fields of the same name, but it does allow shortcuts like:
<form name="myform" ...>
document.myform.submit();
instead of
document.getElementsByName('myform')[0].submit();
Here's what MDN has to say about it:
name
The name of the form. In HTML 4, its use is deprecated (id should be used instead). It must be unique among the forms in a document and not just an empty string in HTML 5.
(from <form>, Attributes, name)
I find it slightly confusing that specifies that it must be unique, non-empty string in HTML 5 when it was deprecated in HTML 4. (I'd guess that requirement only applies if the name attribute is specified at all?). But I think it's safe to say that any purpose it once served has been superseded by the id attribute.
You can use the name attribute as an "extra information" attribute - similarly as with a hidden input - but this keeps the extra information tied into the form, which makes it just a little simpler to read/access.
name attribute is not completely redundant vis-à-vis id. As aforementioned, it useful with <forms>, but less known is that it can also be used with with any HTMLCollection, such as the children property of any DOM element.
HTMLCollection, in additional to be a array-like object, will have named properties commensurate with any named members (or the first occurrence in case of non-unique name). It is useful to retrieve specific named nodes.
For example, in the following example HTML:
<div id='person1'>
<span name='firstname'>John</span>
<span name='lastname'>Doe</span>
<span name='middlename'></span>
</div>
<div id='person2'>
<span name='firstname'>Jane</span>
<span name='lastname'>Doe</span>
<span name='middlename'></span>
</div>
by naming each child, one can quickly and efficiently retrieve a named element, such as lastname, as such:
document.getElementById('person1').children.namedItem('lastname')
...and if there is no risk of 'length' being the name of a member element, (being that length is a reserved property of HTMLCollection), a more terse notation may be used instead:
document.getElementById('person1').children.lastname
DOM Living Standard 2019 March 29
An HTMLCollection object is a collection of elements...
The namedItem(key) method, when invoked, must run these steps:
If key is the empty string, return null.
Return the first element in the collection for which at least one of the following is true:
it has an ID which is key;
it is in the HTML namespace and has a name attribute whose value is key;