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POWhen to use Hold / ReleaseHold in Mathematica?
 

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  1. POWhen to use Hold / ReleaseHold in Mathematica?
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    <p>Example and background ( note the usage of Hold, ReleaseHold ):</p> <p>The following code represents a static factory method to create a scenegraph object ( from an XML file ). The (output-)field is an instance of CScenegraph ( an OO-System class ).</p> <pre><code> new[imp_]:= Module[{ ret, type = "TG", record ={{0,0,0},"Root TG"} }, ret = MathNew[ "CScenegraph", 2, MathNew["CTransformationgroup",1,{type,record},0,0,0,0,Null]]; ret@setTree[ret]; ret@getRoot[]@setColref[ret]; csp = loadClass["CSphere"]; spheres = Cases[imp, XMLElement["sphere", _, __], Infinity]; codesp = Cases[spheres, XMLElement["sphere", {"point" -&gt; point_, "radius" -&gt; rad_, "hue" -&gt; hue_}, {}] -&gt; Hold[csp@new[ToExpression[point], ToExpression[rad], ToExpression[hue]]]]; ret@addAschild[ret@getRoot[],ReleaseHold[codesp]]; ret ]; </code></pre> <p>My question is about the following:</p> <pre><code>spheres = Cases[imp, XMLElement[\sphere\, _, __], Infinity]; codesp = Cases[spheres, XMLElement[\sphere\, {\point\ -&gt; point_, \radius\ -&gt; rad_, \"hue\" -&gt; hue_}, {}] -&gt; Hold[csp@new[ToExpression[point], ToExpression[rad], ToExpression[hue]]]]; ret@addAschild[ret@getRoot[],ReleaseHold[codesp]]; </code></pre> <p>where </p> <pre><code> addAschild </code></pre> <p>adds ( a list of ) geometries to a ( root ) transformationgroup and has the signature </p> <pre><code> addAsChild[parent MathObject, child MathObject], or addAsChild[parent MathObject, Children List{MathObject, ...}] </code></pre> <p>and the XML element representing a sphere looks as follows:</p> <pre><code> &lt;sphere point='{0., 1., 3.}' radius='1' hue='0.55' /&gt; </code></pre> <p>If I do NOT USE Hold[] , ReleaseHold[] I end up with objectdata like</p> <pre><code> {"GE", {"SP", {CScenegraph`point, CScenegraph`rad}}, {CScenegraph`hue}} </code></pre> <p>while I would have expected</p> <pre><code> {"GE", {"SP", {{4., 3., -4.}, 3.}}, {0.45}} </code></pre> <p>(The above code with Hold[], ReleaseHold[] yields the correct data.)</p> <h3>Questions</h3> <h3>1. Why is Hold necessary in this case? ( In fact, is it? Is there a way to code this without Hold[], ReleaseHold[]? ) ( I got it right by trial and error! Don't really understand why. )</h3> <h3>2. As a learning point: What is the prototypical example / case for the usage of Hold / ReleaseHold?</h3> <h2>EDIT:</h2> <p>Summary of Leonid's answer. Change this code</p> <pre><code> codesp = Cases[spheres, XMLElement["sphere", {"point" -&gt; point_, "radius" -&gt; rad_, "hue" -&gt; hue_}, {}] -&gt; Hold[csp@new[ToExpression[point], ToExpression[rad], ToExpression[hue]]]]; ret@addAschild[ret@getRoot[],ReleaseHold[codesp]]; </code></pre> <p>to:</p> <pre><code> codesp = Cases[spheres, XMLElement["sphere", {"point" -&gt; point_, "radius" -&gt; rad_, "hue" -&gt; hue_}, {}] :&gt; csp@new[ToExpression[point], ToExpression[rad], ToExpression[hue]]]; ret@addAschild[ret@getRoot[],codesp]; </code></pre>
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