C++ Static array

Question

I would like to create a custom lower / upper case function for wstrings.

Therefore I am using a map of integers.

Currently I am automatically creating a function from these maps:

(...)
else if (iCharCode==65)
{
    iRet=97;
}
else if (iCharCode==66)
{
    iRet=98;
}
else if (iCharCode==67)
{
    iRet=99;
}
else if (iCharCode==68)
{
    iRet=100;
}
else if (iCharCode==69)
{
    iRet=101;
}
else if (iCharCode==70)
{
    iRet=102;
}
else if (iCharCode==42818)
{
    iRet=42819;
}
(...)

However, the function is going to be pretty large if I turn my map into a if-statement like this.

I would therefore like a use a real map instead, but I don't want to load it at runtime. I would prefer having a static map, but I am not sure how to do that.

Can somebody share his thoughts?

That was just a coincidence. Most of the times it is not +32. — tmighty, Nov 19 '13 at 17:50
You may want to take a look at this question: http://stackoverflow.com/questions/11491/string-to-lower-upper-in-c — Dweeberly, Nov 19 '13 at 17:53
@Dweeberly Not working for non-ASCII chars, therefore I am doing my own function. — tmighty, Nov 19 '13 at 17:55
@Jimmy Can you delete your comment? You got so many upvotes that it is misleading. — tmighty, Nov 19 '13 at 17:56
A `std::map` would use far too much memory. One approach is to create an array for the easy values, for example, an array of 256 `int` case-mapped values for the first 256 values, and a function for the rest. In general, a function can be much more compact than the code here; for example, `if(65 <= iCharCode && iCharCode < 71) iRet = iCharCode + 32; else ...`. — Pete Becker, Nov 19 '13 at 18:05
ok - i'll delete it (not often I have to delete something because it gets so many upvotes ;-) have you considered using a unicode library http://userguide.icu-project.org/transforms/casemappings — Jimmy, Nov 19 '13 at 18:40
The link I posted contained info on unicode conversions. Are you trying to do codepage conversions? Transcoding is hard and it's generally saver to find a well tested library — Dweeberly, Nov 19 '13 at 19:01

score 2 · Accepted Answer · answered Nov 19 '13 at 17:56

2

Maybe the following code can help:

#include<iostream>
#include<map>

std::map<int, int> code_map = {
  {65, 97},
  {66, 98},
  {67, 99},
  {68, 100},
  {69, 101},
  {70, 102},  
};


int main() {
  for(const auto & pair : code_map) {
    std::cout<<pair.first<<" maps to "<<pair.second<<std::endl;
  }
  return 0;
}

Compiling with g++ example.cpp -std=c++11 -Wall -Wextra (OS X 10.7.4 GCC 4.8.1) yields:

$ ./a.out 
65 maps to 97
66 maps to 98
67 maps to 99
68 maps to 100
69 maps to 101
70 maps to 102

answered Nov 19 '13 at 17:56

Escualo

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But I think the map is created anew each time I initiate code_map, right? – tmighty Nov 19 '13 at 17:58
`code_map` is only constructed once -- when the program is started. – Sam Cristall Nov 19 '13 at 18:11
I am getting the error "Initialization using {...} is not valid for type map". – tmighty Nov 19 '13 at 18:14
@tmighty: That style of initialisation is only valid in C++11. If you're stuck in the past, you'll need to write a function to initialise it, or use something like Boost.Assignment. Alternatively, use a sorted array of `pair` rather than a map, with a binary search for lookup. That's perhaps more efficient, and can be initialised statically. – Mike Seymour Nov 19 '13 at 18:17
@MikeSeymour I have updated from VS2010 to VS2012 now, but the IDE tells me the same error. – tmighty Nov 19 '13 at 20:16
The exact error is: IntelliSense: Initialization with "{...}" invalid for object of type ""std::map, std::allocator>>"" – tmighty Nov 19 '13 at 21:09

score 0 · Answer 2 · answered Nov 19 '13 at 18:26

You could implement the map as a sorted array of pairs. This can be initialised statically. Look up a value using a binary search, e.g. std::lower_bound with a comparator comparing the first element of each pair.

As further optimisation, you might consider mapping ranges rather than individual characters, using an array of (range_begin, range_end, offset) triplets. This would reduce the whole ASCII alphabet to a single entry; but might be less effective for other alphabets.

C++ Static array

2 Answers2