{"id":2177,"date":"2023-12-14T10:14:50","date_gmt":"2023-12-14T09:14:50","guid":{"rendered":"https:\/\/oopm.org\/?page_id=2177"},"modified":"2024-12-20T09:07:35","modified_gmt":"2024-12-20T08:07:35","slug":"51-2-a-text-filter","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=2177","title":{"rendered":"15.2 A text filter"},"content":{"rendered":"
\"image_pdf\"<\/a>\"image_print\"<\/a><\/div>\n

Here we show an example in the domain of text processing. On the Macintosh, one may set an option that implies that two blanks in a text is replaced by a dot ('.'<\/code>) and a blank. The coroutine doubleSpaceToDot<\/code> below scans through a text string and makes these replacements.<\/p>\n\n\n\n

charFilter: obj<\/strong> \n   nextChar: var<\/strong> char\n   input: var<\/strong> String \n   i: var<\/strong> integer \n   next -> c: var<\/strong> char:\n      i := i + 1\n      c := input.get[i]\n   send(ch: var<\/strong> char):\n      nextChar := ch\n   doubleSpaceToDot: obj<\/strong> \n      ch: var<\/strong> char\n      doubleSpaceToDot.suspend\n      cycle\n         ch := next\n         if (ch = ' ') :then\n            ch := next\n            if (ch = ' ') :then\n               send('.')\n               doubleSpaceToDot.suspend\n               send(' ')\n               doubleSpaceToDot.suspend\n            :else\n               send(' ')\n               doubleSpaceToDot.Suspend\n               send(ch)\n               doubleSpaceToDot.Suspend\n         :else\n            send(ch)\n            doubleSpaceToDot.Suspend\n   input := \"Hello world  What a nice day!\\n\"\n   loop: do<\/strong>\n      cycle \n         doubleSpaceToDot.call\n         put(nextChar)\n         if (nextChar = ascii.newline) :then\n            leave(loop)<\/code><\/pre>\n\n\n\n
The central element of this program is the coroutine object doubleSpaceToDot<\/code>. When the object is generated, it starts by executing a suspend<\/code>. <\/p>\n\n\n\n
The object charFilter<\/code> has a local String input<\/code>, which holds the text string to be filtered. For the purpose of the example, we assign it the String<\/code> \"Hello world  What a nice day!\\n\"<\/code>. <\/p>\n\n\n\n
The charFilter<\/code> then starts executing a cycle and the first statement in the cycle<\/code> is a call of doubleSpaceToDot<\/code>. This implies that doubleSpaceToDot<\/code> is resumed and executes its cycle<\/code>-statement.<\/p>\n\n\n\n
It assigns the next char in input to ch<\/code> by executing ch := next<\/code>.<\/p>\n\n\n\n
If ch = ' '<\/code>,  it reads the next char and checks if it is also a blank. If blank it sends a dot ('.'<\/code>) and suspends executions, and when resumed it sends a blank. If the next char is not a blank, it sends a blank one at a time and suspends in between.<\/p>\n\n\n\n
The else<\/code>-part of the outermost if<\/code>-statement, is executed If ch is not a blank and here it just sends ch and suspends execution.<\/p>\n\n\n\n
A simple version using a method<\/h3>\n\n\n\n
A coroutine object may invoke methods and a method may suspend the coroutine object. In the next example we use this to make a simpler version of doubleSpaceToDot<\/code>. Here the send<\/code>-method is made a local method of doubleSpaceToDot<\/code> and send<\/code> executes the suspend<\/code>, suspending the enclosing doubleSpaceToDot<\/code> coroutine:<\/p>\n\n\n\n
   doubleSpaceToDot: obj<\/strong> \n     ch: var<\/strong> char\n     send(ch: var<\/strong> char):\n        c := ch\n        doubleSpaceToDot.suspend\t\n     doubleSpaceToDot.suspend\n     cycle\n        ch := next\n        if (ch = ' ') :then\n           ch := next\n           if (ch = ' ') :then\n              send('.')\n              send(' ')\n           :else\n              send(' ')\n              send(ch)\n        :else\n           send(ch)<\/code><\/pre>\n\n\n\n
When send<\/code> executes a suspend<\/code>, we have a situation where doubleSpaceToDot<\/code> has invoked send<\/code> after having read the 'o'<\/code> in \"Hello\"<\/code> and thus i = 5<\/code>. In the figure below, the left-side (A) shows the situation before suspend and the middle part (B) shows the situation after suspend.<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
The figure shows the activation stack. (A) shows that charFilter<\/code> has called doubleSpaceToDot<\/code>, which has called send<\/code>. Send<\/code> is currently the active object executing statements. Note that the elements <\/mark>of the activation stack may be objects as well as method objects. Here the bottom element is an object and the other elements are method objects. <\/p>\n\n\n\n
When send<\/code> executes suspend<\/code>, the situation is shown at (B). CharFilter.suspend<\/code> has the effect that control is returned to after charFilter<\/code> has called doubleSpaceToDot<\/code>. The arrow from doubleSpaceToDot<\/code> illustrates that it is suspended and that send<\/code> is at the top of its activation stack.<\/p>\n\n\n\n
When charFilter<\/code> makes another call to doubleSpaceToDot<\/code>, the situation is as in the (C)-part, but with ch = ' '<\/code> and i = 6<\/code>. As can be seen, doubleSpaceToDot<\/code> now again points to the calling charFilter<\/code>-object.<\/p>\n
\n\t\t<\/a>\n\t\t<\/div>","protected":false},"excerpt":{"rendered":"
Here we show an example in the domain of text processing. On the Macintosh, one may set an option that implies that two blanks in a text is replaced by a dot (‘.’) and a blank. The coroutine doubleSpaceToDot below scans through a text string and makes these replacements. The central element of this program […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1961,"menu_order":2,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-2177","page","type-page","status-publish","hentry"],"mb":[],"mfb_rest_fields":["title","gutenberg_elementor_mode"],"_links":{"self":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/2177","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2177"}],"version-history":[{"count":31,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/2177\/revisions"}],"predecessor-version":[{"id":10685,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/2177\/revisions\/10685"}],"up":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/1961"}],"wp:attachment":[{"href":"https:\/\/oopm.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2177"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}