std::vector performance

[Steve]

Hi,

I'm using a std::vector to store a list of user defined objects. The vector
may have well over 1000 elements, and I'm suffering a performance hit. If I
use push_back I get a much worse perfomance than if I first define the
vector of a given size, then write to the elements with myvec[i] =

However, I'm currently thinking that it isn't feasible to obtain the vector
size, so really need to resize the vector dynamically as I go. Is push_back
the fastest function to add elements to the end of a vector, or is there a
faster way (a different container, maybe)?

I've also seen this with the std::string object. I have a function called
GetFloat, which extracts a space delimited float from a text string:

float GetFloat(const string& Str, const int& Spaces, bool& Valid)
{
int ColPosition = GetNewPosition( Str, Spaces); // convert spaces to
chars

if(ColPosition == -1) {
Valid = false;
return 0.0;
}

std::string s;
std::string::co nst_iterator p = Str.begin() + ColPosition;

while(*p != ' ' && p != Str.end()) {
if(*p == 'I') { Valid = false; return 0.0; }
if(*p != ' ') s.push_back(*p) ; <============
alternatives below
++p;
}
Valid = true;
return atof(s.c_str()) ;
}

for s, if I make it a fixed length and use s[i] = ... it runs rather faster
than the above. Also s+=(*p) runs slightly faster than above, but not as
fast as s[i] = ...

This function may be called thousands of times during my program (loading a
data file) and perhaps this is why the difference is significant. Is this
what I should expect to see, I thought they should be similar?
Thanks for your time,
Steve

[John Harrison]

>

Hi John,

Thanks again. I hope I'm not being a no-brainer, but I can't find strtof.
I'm using Borland C++Builder5 Compiler, and I've searched the include
folders for files containing strtof, the only hit being the VCL's version of StringToFloat. (you're going to tell me to use a 'real' compiler now, aren't you <g>)

strtof is in C99 standard, which I guess means many compilers won't support
it yet.

Use strtod and a cast from double to float instead (or just switch to double
throughout, normally double is to be preferred over float).

If you don't have strtod then I really would suggest getting a real
compiler.

john

[Steve]

Thanks John, found it!

Steve