std::string comparison

Question

I need to check whether an std:string begins with "xyz". How do I do it without searching through the whole string or creating temporary strings with substr().

Thanks.

Answer 1

I feel I'm not fully understanding your question. It looks as though it should be trivial:

s[0]=='x' && s[1]=='y' && s[2]=='z'

This only looks at (at most) the first three characters. The generalisation for a string which is unknown at compile time would require you to replace the above with a loop:

// look for t at the start of s
for (int i=0; i<s.length(); i++)
{
  if (s[i]!=t[i])
    return false;
}

Answer 2

Look to the Boost's String Algo library, that has a number of useful functions, such as starts_with, istart_with (case insensitive), etc. If you want to use only part of boost libraries in your project, then you can use bcp utility to copy only needed files

Answer 3

I am really fed up when people demands : " I want to fly up to London but I don't want to use an airplane, neither a rocket and I even don't want to be transformed into a bird."

When I ask them "Any particular reason?", they say: "Airplane will burn a lot of fuel and cause pollution, the rocket is too fast for me and I don't want to loose my human-ship". These are the symptoms of pre-mature optimization and as Donald Knuth beautifully puts up

"We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil."

So, in a nutshell, please tell us "What you want to achieve" and not "How you want to achieve".

There is nothing wrong in using substr and string comparisons, so why not use it.

Answer 4

I would use compare method:

std::string s("xyzblahblah");
std::string t("xyz")

if (s.compare(0, t.length(), t) == 0)
{
// ok
}

Answer 5

if (mystring.find("xyz", 0) == 0)
{
   // you found it
}
else
{
   // you didn't find it, or it doesn't start with xyz
}