comparing string elements

[MC felon]

what's the best way to compare two string elements?

std::string cstr;
if ( cstr.at(3) == cstr.at(2)) //do something
or
std::string cstr;
if ( cstr[3] == cstr[2]) //do something

?

[Gianni Mariani]

Re: comparing string elements

MC felon wrote:

what's the best way to compare two string elements?
>
std::string cstr;
if ( cstr.at(3) == cstr.at(2)) //do something
or
std::string cstr;
if ( cstr[3] == cstr[2]) //do something

That depends.

Do you know that cstr has at least 4 chars ? Do you want it to throw an
exception if it does not ?

operator[] will not check for out of bounds access while at should.

I think for readability, operator[] is probably easier but it really
makes very little practical difference.

[Jim Langston]

Re: comparing string elements

MC felon wrote:

what's the best way to compare two string elements?
>
std::string cstr;
if ( cstr.at(3) == cstr.at(2)) //do something
or
std::string cstr;
if ( cstr[3] == cstr[2]) //do something
>
?

[] is a *little* faster, but should only be used when it is known that the
length of the string is greater than the index. If it is not sure, .at()
should be used.

I.E.

if ( cstr.size() < index )
{
if ( cstr[index] == cstr[index] // do something
}

Of course, that check for size() brings it's own overhead, so in that case
just use .at(). However, at lot of times at the beginning of a function I
will validate sizes and throw an error or return with an empty string. So
in my base I know the size indexes will fit, so then I will just use []

Realistically, I never use .at() because I always check the length of the
strings before I do operations on them. I would rather check on error
conditions in code rather than have to depend on throws.

--
Jim Langston
tazmaster@rocke tmail.com

[MC felon]

Re: comparing string elements

Thanks. The problem is i'm programming a Tic Tac Toe game and the AI
just isn't working! I think there's something wrong with the way i'm
comparing the strings.

[James Kanze]

Re: comparing string elements

On Feb 20, 9:55 am, Gianni Mariani <gi4nos...@mari ani.wswrote:

operator[] will not check for out of bounds access while at should.

That's a quality of implementation issue. In the libraries I use
most often, operator[] does bounds checks, aborting the program
if the index is out of bounds. At() guarantees an exception,
which is rarely what you want.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
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