Re: std::string find_first_of and replace

From:

Chris Vine <chris@cvine--nospam--.freeserve.co.uk>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 10 Jul 2014 08:31:14 CST

Message-ID:

<20140710110416.7aabb9bb@bother.homenet>

On Wed, 9 Jul 2014 18:47:33 CST
Christopher Pisz <nospam@notanaddress.com> wrote:

My code below is not behaving as expected. I want to transform
"11/21/13" into "11-21-13"
My loop never exits and the first time through pos = 1, when I would
think it should be 2. Then after the first replace the debugger says
my string is

""1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\
x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1\x1/22/2001"

and I don't know where the heck 1x is coming from.

Any hints?

#include <iostream>
#include <string>

//--------------------------------------------------------------------------
------------------------

bool Transform_DatetimeString(const std::string & fromXml,
std::string & toSql)
{
    toSql = fromXml;

    std::string::size_type pos = std::string::npos;

    while( pos = toSql.find_first_of('/') != std::string::npos )
    {
        toSql = toSql.replace(pos, 1, '-', 1);
    }

    return true;
}

//--------------------------------------------------------------------------
------------------------

int main()
{
    const std::string test = "11/21/13";

    std::string output;
    if( !Transform_DatetimeString(test, output) )
    {
        return 1;
    }

    std::cout << output << std::endl;

    return 0;
}

You need additional parenthesis in your while test:

  while( ( pos = toSql.find_first_of('/') ) != std::string::npos )

and the replacement should be:

  toSql = toSql.replace(pos, 1, 1, '-');

However, in this example it will be more efficient not to use
std::string::replace and just have as the body of the while loop:

  toSql[pos] = '-';

but, I realise that this is just a test case and you may in real life
want to insert more than one character, hence your choice. Also, if the
string could be lengthy you really need to cache the result by reusing
'pos' and pass it as the second argument of std::string::find_first_of:

  std::string::size_type pos = 0;
  while( ( pos = toSql.find_first_of('/', pos) ) != std::string::npos )
  {
    toSql[pos] = '-';
    ++pos;
  }

For search of a single character rather than a set of characters I
would also be inclined to use std::string::find rather than
std::string::find_first_of. You could also turn this 'initialisation +
while loop with increment' into a for loop if you wanted.

Chris

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