Re: How to encode text into html format

James Kanze <>
Mon, 2 Jun 2008 05:50:12 -0700 (PDT)
On Jun 2, 11:55 am, Kai-Uwe Bux <> wrote:

James Kanze wrote:

On Jun 1, 11:01 pm, Kai-Uwe Bux <> wrote:

James Kanze wrote:

On Jun 1, 8:11 pm, Kai-Uwe Bux <> wrote:

Fred Yu wrote:

I want to encode input text into html format such as
replace "<" with "&lt", replace "&" with "&amp". Could
you give me some ideas? Thanks.

Containers: std::map< char, std::string >
Iterators: std::istream_iterator, std::ostream_iterator
Algorithms: std::transform

Agreed for the first (although it may be overkill---in this
particular case, I think I'd go with a simple switch).

No real need for the second; just use istream::get() and
ostream::put() (or operator<< in some cases).

As to the third: how? You're replacing a single character
with a sequence of characters, and transform does a one to
one (which in practice makes it of fairly limited
utility---although I've used it with a vector<string>,
ostream_iterator, and as string transformer class that I've
written, which works something like $(patsubst...) in GNU

I was thinking of something like this:

#include <iostream>
#include <iterator>
#include <map>
#include <algorithm>
#include <cassert>

struct encoder {

  std::map< char, std::string > the_map;

  encoder ( void ) {
    the_map[ 'a' ] = "a";
    // ...
    the_map[ '&' ] = "&amp";
    // ...

  std::string const & operator() ( char ch ) const {
    std::map< char, std::string >::const_iterator iter =
      the_map.find( ch );
    assert( iter != the_map.end() );
    return ( iter->second );

int main ( void ) {
  encoder the_encoder;
  std::transform( std::istreambuf_iterator<char>( std::cin ),
                  std::ostream_iterator<std::string>( std::cout, "" ),
                  the_encoder );

Which looks like a lot of overhead (including in terms of
programming) for very little gain. It might be worth it if you
create some sort of generic encoder, in order to reuse the idiom
in many different contexts, but for such a simple problem, it
just seems overkill for a onetime solution.

It's just what came to mind first. I tend to think of std::map
whenever there is an obvious table lookup.

I'll admit that I didn't think of this particular problem in
terms of table lookup, except to find the replacement string.
That's probably why my approach is so different. (Why I didn't
think of it in these terms is another question. I tend to use
table lookup a lot, even in cases where other people wouldn't.)

I like that because (a) it tends to have exactly one line for
each table entry, which can be formatted in such a way that it
is easy to read,

Or even better, can be generated mechanically. If I used this
solution, I'd probably start with something like:

    for ( int i = std::numeric_limits< char >::min() ;
            i <= std::numeric_limits< char >::max() ;
            ++ i ) {
        the_map[ i ] = std::string( i, 1 ) ;

and then reseat the special cases. (There are only three, after
all.) Or given my experience using C style arrays indexed by a
char (which goes back to before I'd even heard of C++), I might
just do that.

and (b) the logic of table lookup is completely decoupled from
the rest of the program. Of course, a simple function

  char const * encode ( char ch ) {
    switch ( ch ) {

could do the same.

As I said, I'd probably go with the switch. If I were going
to go to the effort of initializing the map completely, I'd
probably go with a char const*[UCHAR_MAX], rather than
std::map. Or a map with just the elements which don't use
an identity transformation.

Initializing the map completely is not a big deal at all. Just
change the constructor slightly:

    for ( char ch = std::numeric_limits<char>::min();
          ch < std::numeric_limits<char>::max();
          ++ ch ) {
      the_map[ ch ] = ch;
    the_map[ std::numeric_limits<char>::max() ] =
    // now for the special characters:
    the_map[ '&' ] = "&amp";

And I'd probably still write out the loop; somehow, the idea
of transforming each individual character into a string just
to output it bothers me.

a) Note that the operator() of the encoder returns a string
const &. So, this does not really create a string each time
just for output. It only involves a few levels of indirection
(something like char*** instead of char*).

I wasn't thinking so much in terms of performance, as I don't
know what. Logically, I was approaching the problem from the
idea: copy the characters, with some special handling for a few
specific characters. Which of course suggests the switch. Of
course, that's probably conditionned by the number of times such
has really been the case: implementing things like printf, etc.,
where the special handling is more than just a one to one

The more I think about it, the more I think you're right: it is
a simple mapping problem.

b) You can use

  map< char, char const * >

instead of map< char, string >. Transform will just look up
the char const * and write it, which is very much the same as
a hand coded loop. The price to pay is that the trick from
above for initializing all the characters that are just passed
through becomes more tricky.

But nothing that a simple AWK script can't handle:-).

c) Maybe you are thinking of a _real_ alternative:

#include <iostream>
#include <istream>
#include <ostream>

int main ( void ) {
  char ch;
  while ( std::cin.get( ch ) ) {
    switch ( ch ) {
    case '&' : { std::cout << "&amp"; break; }
    case '<' : { std::cout << "lt"; break; }
    // ...
    default : { std::cout << ch; break; }

That's what I was thinking of.

I have to admit that I don't like that. It mixes flow control
and table lookup to the effect that different types are piped
to std::cout (char for default and const char * for the other

Yes, but that's the way I first saw the problem. Special
handling for a few special characters, and not table driven code
translation. In this case, I'm probably wrong. I guess I've
just written too much code where it was a case of special

James Kanze (GABI Software)
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