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| <h1 class="content-header header-section1" id="dgui_datamodel_basics" itemprop="headline">Basics</h1> |
| </div></div><div class="page-menu"> |
| <div class="page-menu-title">Page Contents</div> |
| <ul><li><a class="page-menu-link" href="#topic.value" data-menu-target="topic.value">What is a value?</a></li><li><a class="page-menu-link" href="#autoid_13" data-menu-target="autoid_13">What is type?</a></li><li><a class="page-menu-link" href="#autoid_14" data-menu-target="autoid_14">The data-model is a hash</a></li></ul> </div> <div class="callout note"> |
| <strong class="callout-label">Note:</strong> |
| |
| <p>It is assumed that you have already read the <a href="dgui_quickstart.html">Getting Started</a> chapter.</p> |
| </div> |
| <p>Understanding the concept of values and types is crucial for the |
| understanding of data-models. However, the concept of values and types |
| is not confined to data-models, as you will see.</p> |
| |
| |
| |
| |
| <h2 class="content-header header-section2" id="topic.value">What is a value?</h2> |
| |
| |
| |
| |
| <p><span class="marked-for-programmers">Real programmers can safely skip |
| this section.</span></p> |
| |
| <p>Examples of <em>values</em> as you know the term |
| from the everyday math are 16, 0.5, and so on, i.e. numbers. In the |
| case of computer languages the value term has a wider meaning, as a |
| value needn't be a number. For example, take this data-model:</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-data-model"><a name="example.stdDataModel"></a>(root) |
| | |
| +- user = "Big Joe" |
| | |
| +- today = Jul 6, 2007 |
| | |
| +- todayHoliday = false |
| | |
| +- lotteryNumbers |
| | | |
| | +- (1st) = 20 |
| | | |
| | +- (2st) = 14 |
| | | |
| | +- (3rd) = 42 |
| | | |
| | +- (4th) = 8 |
| | | |
| | +- (5th) = 15 |
| | |
| +- cargo |
| | |
| +- name = "coal" |
| | |
| +- weight = 40</pre></div> |
| |
| <p>We say that the <em>value</em> of the the |
| <code class="inline-code">user</code> variable is "Big Joe" (a string), the |
| <em>value</em> of <code class="inline-code">today</code> is Jul 6, |
| 2007 (a date), the <em>value</em> of |
| <code class="inline-code">todayHoliday</code> is false (a boolean, ie. a yes/no |
| thing). The <em>value</em> of |
| <code class="inline-code">lotteryNumbers</code> is the sequence that contains 20, |
| 14, 42, 8, 15. Surely <code class="inline-code">lotteryNumbers</code> is multiple |
| values in the sense that it <em>contains</em> multiple |
| values (for example, the 2nd item in it is a the |
| <em>value</em> 14), but still, |
| <code class="inline-code">lotteryNumbers</code> itself is a single value. It's |
| like a box that contains many other items; the whole box can be seen |
| as a single item. Last not least we also have the |
| <em>value</em> of <code class="inline-code">cargo</code>, which is a |
| hash (a box-like thing again).So, a value is something that can be |
| stored in a variable (e.g., in <code class="inline-code">user</code> or |
| <code class="inline-code">cargo</code> or <code class="inline-code">cargo.name</code>). But a |
| value need not be stored in a variable to be called a value, for |
| example we have the value 100 here:</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-template"><#if cargo.weight < <strong>100</strong>>Light cargo</#if></pre></div> |
| |
| <p>Or the temporaly result of a calculations are also called |
| values, like 20 and 120 when this template is executed (it will |
| print 120):</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-template">${cargo.weight / 2 + 100}</pre></div> |
| |
| <p>Explanation for this last: As the result of dividing the two |
| values, 40 (the weight of the cargo) and 2, a new value 20 is |
| created. Then 100 is addted to it, so the value 120 is created. Then |
| 120 is printed |
| (<code class="inline-code">${<em class="code-color">...</em>}</code>), and the |
| template execution goes on and all these values gone.</p> |
| |
| <p>Certainly now you feel what the value term means.</p> |
| |
| |
| |
| |
| |
| <h2 class="content-header header-section2" id="autoid_13">What is type?</h2> |
| |
| |
| <p>Values have an important aspect, their type. For example the |
| type of the value of the <code class="inline-code">user</code> variable is string, |
| and the type of the value of the <code class="inline-code">lotteryNumbers</code> |
| variable is sequence. The type of a value is important because it |
| determines to a large extent how and where you can use the value. |
| Like <code class="inline-code">${user / 2}</code> is an error, but |
| <code class="inline-code">${cargo.weight / 2}</code> works and prints 20, since |
| division only does make sense for a number, but not for a string. |
| Or, using dot like in <code class="inline-code">cargo.name</code> does make sense |
| only if <code class="inline-code">cargo</code> is a hash. Or, you can list with |
| <code class="inline-code"><#list <em class="code-color">...</em>></code> |
| sequences only. Or, the condition of <code class="inline-code"><#if |
| ...></code> must be a boolean. And so on.</p> |
| |
| <div class="callout note"> |
| <strong class="callout-label">Note:</strong> |
| |
| <p>A little terminology... Saying "a boolean" or "a boolean |
| value" or "a value of type boolean" are all the same.</p> |
| </div> |
| |
| |
| <p><a name="topic.multitype"></a>A value can have multiple types at the same time, |
| although it's rarely utilized. For example in the data-model below |
| <code class="inline-code">mouse</code> is both a string and a hash:</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-data-model">(root) |
| | |
| +- mouse = "Yerri" |
| | |
| +- age = 12 |
| | |
| +- color = "brown"</pre></div> |
| |
| <p>If you merge this template with the above data-model:</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-template">${mouse} <#-- uses mouse as a string --> |
| ${mouse.age} <#-- uses mouse as a hash --> |
| ${mouse.color} <#-- uses mouse as a hash --></pre></div> |
| |
| <p>the output will be:</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-output">Yerri |
| 12 |
| brown</pre></div> |
| |
| |
| |
| |
| |
| <h2 class="content-header header-section2" id="autoid_14">The data-model is a hash</h2> |
| |
| |
| <p>Looking at the various data-model examples you may already |
| realized: the thing marked as "(root)" is just a value of type hash. |
| When you write something like <code class="inline-code">user</code>, that means |
| that you want the "user" variable stored in the root hash. Like if |
| you were writing <code class="inline-code">root.user</code>, except that there is |
| no variable called "root" so that wouldn't work.</p> |
| |
| <p>Some may get confused by the fact that our example data-model, |
| that is, the root hash, contains further hashes and sequences |
| (<code class="inline-code">lotteryNumbers</code> and <code class="inline-code">cargo</code>). |
| There is nothing special in that. A hash contains other variables, |
| and those variables have a value, which can be a string, a number, |
| etc., and of course it can be a hash or sequence as well. Because, |
| as it was explained earlier, a sequence or a hash is just a value, |
| like a string or a number is.</p> |
| <div class="bottom-pagers-wrapper"><div class="pagers bottom"><a class="paging-arrow previous" href="dgui_datamodel.html"><span>Previous</span></a><a class="paging-arrow next" href="dgui_datamodel_types.html"><span>Next</span></a></div></div></div></div> </div> |
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