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Re: Filter array of objects for property

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Ben Bacarisse wrote:

> Andrew Poulos <ap_prog@hotmail.com> writes:
>> If I have an array that looks like this
>>
>> var arr =
>> [
>>   {
>>     a: "foo_2",
>>     b: "trigger_1",
>>     c: "bar_3"
>>   },
>>   {
>>     a: "foo_3",
>>     b: "foo_4",
>>     c: "bar_4"
>>   },
>>   {
>>     a: "foo_6",
>>     b: "trigger_5",
>>     c: "bar_5"
>>   }
>> ...
>> ];
>>
>> and to get an array of all "b" values that start with "trigger_" and
>> end with a number I'm calling
>>
>> function getValues(theArr, pre) {
>>   var i = 0,
>>       fnd = [],
>>       tmp;
>>
>>   for (i = 0; i < theArr.length; i++) {
>>     tmp = theArr[i].b;
>>     if ( /trigger_\d+$/.test(tmp) ) {
>>       fnd.push(tmp);
>>     }
>>   }
>>
>>   return fnd;
>> }
>>
>> While this works I'd like the argument "pre" to be a string I pass to
>> the regexp so that I can test other values and not just "trigger_" but
>> I don't know how.
> 
> You can construct a RegExp from a string:
> 
>   RegExp(pre + '\\d+$').test(tmp)

First of all, the original regular expression does _not_ match strings that 
*start* with “trigger_”; it matches all those that *contain* “trigger_” 
followed by a sequence of decimal digits, because the “^” anchor is missing.

Second, *in general*, string values need to be escaped if they are used in 
regular expressions to match substrings literally.  For example, “.” needs 
to be escaped as ”\.” to match a literal “.” character; otherwise it would 
match any non-newline character.  My jsx.regexp.escape() and 
String.prototype.regExpEscape() methods provide this functionality: you can 
pass a string and it returns a pattern string that, when passed to the 
RegExp constructor, matches the original string. [1]

However, *in this case* it might not be necessary to escape the pattern 
string because the examples do not contain RegExp-special characters.

Third, it is, *in general*, not necessary to use regular expressions for 
both parts of the pattern, therefore not necessary to escape the variable 
part.  The test can consist of two parts: one that tests whether the string 
starts with a certain substring, and another that tests whether it ends with 
a sequence of decimal digits:

  if (tmp.indexOf(pre) == 0 && /\d+$/.test(tmp))
  {
    // …
  }

However, note that although the two-part test may suffice *here*, is *not 
equivalent* to the original one because it also would find strings of the 
form "foo_bar3" if pre == "foo_".


[1] <http://PointedEars.de/wsvn/JSX/trunk/regexp.js>
-- 
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Thread

Filter array of objects for property Andrew Poulos <ap_prog@hotmail.com> - 2016-10-13 09:44 +1100
  Re: Filter array of objects for property Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-13 00:27 +0100
    Re: Filter array of objects for property Andrew Poulos <ap_prog@hotmail.com> - 2016-10-13 12:57 +1100
    Re: Filter array of objects for property Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-10-13 11:21 +0100
      Re: Filter array of objects for property Bunny Bocarisse <ben.usenet@bsb.me.uk> - 2016-10-13 13:28 +0100

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