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Add CStringArray::join() method #2668

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merged 1 commit into from
Sep 25, 2023
Merged

Add CStringArray::join() method #2668

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22 changes: 22 additions & 0 deletions Sming/Core/Data/CStringArray.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -168,3 +168,25 @@ unsigned CStringArray::count() const
}
return stringCount;
}

String CStringArray::join(const String& separator) const
{
if(isNull()) {
return nullptr;
}
unsigned len = length();
if(len == 0) {
return "";
}
len -= count(); // NUL separators
len += separator.length() * (stringCount - 1);
String s;
s.reserve(len);
for(auto it = begin(); it != end(); ++it) {
if(it.index() != 0) {
s += separator;
}
s += *it;
}
return s;
}
9 changes: 9 additions & 0 deletions Sming/Core/Data/CStringArray.h
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Expand Up @@ -246,6 +246,15 @@ class CStringArray : private String
*/
unsigned count() const;

/**
* @brief Get contents of array as delimited string
* @param separator What to join elements with
* @retval String
*
* e.g. CStringArray(F("a\0b\0c")).join() returns "a,b,c"
*/
String join(const String& separator = ",") const;
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@mikee47 Is there a reason for the separator parameter to be optional? In JavaScript, PHP and Python as far as I can say this parameter is mandatory.

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I hadn't considered PHP but default is same as javascript https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join.

Python would be ",".join(cs) which is an interesting construction - clearly there cannot be any 'default' in this case. However, to actually implement this efficiently for Sming would require that the array itself does the join. We could add this as a template method to String easily enough:

class String
{
  ...

  template <class Array> String join(const Array& array)
  {
    return array.join(*this);
  }

  ...
};

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I don't know of any C++ STL equivalent for this, though I see Boost has https://www.geeksforgeeks.org/boostalgorithmjoin-in-cpp-library/ https://www.boost.org/doc/libs/1_83_0/doc/html/boost/algorithm/join.html

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@mikee47 mikee47 Sep 25, 2023 •
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to actually implement this efficiently for Sming would require that the array itself does the join

The efficiency here is avoiding reallocations for the returned String as in the general case:

template <class Array> String join(const Array& array, const String& separator)
{
  String s;
  for(auto& e: array) {
    if(s) {
      s += separator;
    }
    s += e;
  }
  return s;
}

This should work for any Vector or other iterable objects with an implicit String() operator. Something that works with HashMaps would also be useful (returning delimited 'name=value' pairs). However the implementation as it stands does not perform any string escaping, for example:

CStringArray cs;
cs.add("first");
cs.add("second,plus,stuff");

Then cs.join() returns "first,second,plus,stuff" as requested. If "first,""second,plus,stuff""" is required that'll have to be done manually. This is same behaviour as python.


/**
* @name Iterator support (forward only)
* @{
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19 changes: 19 additions & 0 deletions tests/HostTests/modules/CStringArray.cpp
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Expand Up @@ -213,6 +213,25 @@ class CStringArrayTest : public TestGroup
REQUIRE(csa.back() == nullptr);
REQUIRE(!csa.popBack());
}

TEST_CASE("join")
{
CStringArray csa;
REQUIRE(!csa.join());

csa = "";
REQUIRE(csa.join() == "");

csa.add("a");
REQUIRE(csa.join() == "a");

csa.add("");
REQUIRE(csa.join() == "a,");

csa.add(F("test\0again"));
REQUIRE_EQ(csa.join("}+{"), "a}+{}+{test}+{again");
REQUIRE_EQ(csa.join(nullptr), "atestagain");
}
}
};

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