{"id":102,"date":"2009-06-03T18:10:05","date_gmt":"2009-06-03T23:10:05","guid":{"rendered":"http:\/\/stoney.sb.org\/wordpress\/?p=102"},"modified":"2016-02-28T12:13:58","modified_gmt":"2016-02-28T17:13:58","slug":"defining-new-format-wrappers-in-mathematica","status":"publish","type":"post","link":"https:\/\/stoney.sb.org\/wordpress\/defining-new-format-wrappers-in-mathematica\/","title":{"rendered":"Defining new format wrappers in Mathematica"},"content":{"rendered":"

One problem I had with the expression-to-C converter<\/a> is that CForm converts negations of terms into a negation of parenthesized terms.<\/p>\n

<\/p>\n

For example:<\/p>\n

In[1]:= -a b \/\/ CForm<\/b><\/p>\n

Out[1]\/\/CForm= -(a*b)<\/p>\n

That looks ugly. I tried to define a new format for Times[-1, v], but this requires unprotecting Times, and led to an infinite recursion when I tried to use it. I finally found a suggestion on how to define whole new format wrappers on the comp.soft-sys.math.mathematica list, in a message posted by Carl Woll (a Wolfram employee). This is how it works:<\/p>\n

This assignment marks CCForm (my new formatter) as a format wrapper.<\/p>\n

In[2]:= Format[CCForm[expr_], CCForm] := expr<\/b><\/p>\n

Note that the Out[3] text shows the new formatter name.<\/p>\n

In[3]:= 2 + 3 \/\/ CCForm<\/b><\/p>\n

Out[3]\/\/CCForm= CCForm[5]<\/p>\n

It’s showing the CCForm wrapper, though, which we don’t want to look at, so we define a top-level format for CCForm that replaces it with its argument.<\/p>\n

In[4]:= Format[CCForm[expr_]] := expr<\/b><\/p>\n

Now, the output looks correct.<\/p>\n

In[5]:= 2 + 3 \/\/ CCForm<\/b><\/p>\n

Out[5]\/\/CCForm= 5<\/p>\n

The case I’m trying to fix isn’t helped by this. We need to make it produce the CForm.<\/p>\n

In[6]:= -a b \/\/ CCForm<\/b><\/p>\n

Out[6]\/\/CCForm= -a b<\/p>\n

In[7]:= Format[CCForm[expr_]] := CForm[expr]<\/b><\/p>\n

In[8]:= -a b \/\/ CCForm<\/b><\/p>\n

Out[8]\/\/CCForm= -(a*b)<\/p>\n

We don’t want the result to be evaluated, so we add a HoldForm.<\/p>\n

In[9]:= Format[CCForm[expr_]] := HoldForm[CForm[expr]]<\/b><\/p>\n

In[10]:= -a b \/\/ CCForm<\/b><\/p>\n

Out[10]\/\/CCForm= -(a*b)<\/p>\n

Now it’s in CForm, but we still have the parenthesis. I don’t understand just why this works, but making CCForm HoldAll fixes the problem.<\/p>\n

In[11]:= SetAttributes[CCForm, HoldAll]<\/b><\/p>\n

In[12]:= -a b \/\/ CCForm<\/b><\/p>\n

Out[12]\/\/CCForm= -a * b<\/p>\n

This also prevents evaluation of the expression being formatted.<\/p>\n

In[13]:= 2 + 3 \/\/ CCForm<\/b><\/p>\n

Out[13]\/\/CCForm= 2 + 3<\/p>\n

Note that CForm does what I want if it’s made HoldAll.<\/p>\n

In[14]:= -a b \/\/ CForm<\/b><\/p>\n

Out[14]\/\/CForm= -(a*b)<\/p>\n

In[15]:= SetAttributes[CForm, HoldAll]<\/b><\/p>\n

In[16]:= -a b \/\/ CForm<\/b><\/p>\n

Out[16]\/\/CForm= -a*b<\/p>\n

Although it’s not styled as nicely as the CCForm version, and could mess up existing code, so we won’t do that.<\/p>\n

In[17]:= ClearAttributes[CForm, HoldAll]<\/b><\/p>\n

Now I can use CCForm instead of CForm for the output formatting of my converter, and have nicer-looking results.<\/p>\n","protected":false},"excerpt":{"rendered":"

One problem I had with the expression-to-C converter is that CForm converts negations of terms into a negation of parenthesized terms.<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false},"version":2}},"categories":[13,20,14],"tags":[],"jetpack_publicize_connections":[],"yoast_head":"\nDefining new format wrappers in Mathematica - Stoney's Zone<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"http:\/\/stoney.sb.org\/wordpress\/defining-new-format-wrappers-in-mathematica\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Defining new format wrappers in Mathematica - 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