Extracting string from character array

Question

I've tried memcpy / strncpy / std::copy but neither of them seem to work resulting the program to crash.

Here's what i need to do:

I'm trying to parse arguements from user input for instance. "message -add 0xAE"

i need to fetch the 0xAE part into an integer, heres some pseudo-code _input will hold the full string "message -add 0xAE"

if(strstr(_input,"message -add 0x")){
    char* _temp;
    std::copy(strlen("message -add 0x"),strlen("message -add 0x")+(strlen(_input)-strlen("message -add  0x")),_temp);
    /* or */
    memcpy(_temp,_input+strlen("message -add 0x"),strlen(_input)-strlen("message -add 0x"));
    int _value = (int)_temp;
    CLog->out("\n\n %d",_value);
}

Edit: Thanks Alan!

if(strstr(_input,"message -add 0x")){
            char* _temp = new char[strlen(_input)-strlen("message -add 0x")];
            memcpy(_temp,_input+strlen("message -add 0x"),strlen(_input)-strlen("message -add 0x"));
            int _value = atoi(_temp);
            CLog->out("\n\n %d",_value);
}

I can't see the part of code where you attempt to convert a string to an integer — David Heffernan
– David Heffernan, Commented Oct 8, 2011 at 19:05
Whilst you don't want to use any 3rd party code and therefore are ruling out all good options parsers, your attempts to do this yourself are liable to end with many many defects. Why would you wish to reinvent this particular wheel? Parsing options is surprisingly hard. Unless you are a highly skilled developer you should be happy to use 3rd party code that works. — David Heffernan
– David Heffernan, Commented Oct 8, 2011 at 19:16
@David Hefferman Sorry if I sounded a bit offensive, i didn't mean any offense. I'd like to thank you for your time. Others might say you're wasting it, but passing knowledge is far better than mindlessly watching football all night.At least in my honest opinion.You have answered all of my questions so far so I'd like you to know how much I appreciate it! Keep up the good work! — NT_SYSTEM
– NT_SYSTEM, Commented Oct 8, 2011 at 19:30
No offense taken at all. I'm just not sure what level you are at and what your goals are. Keep it up yourself!! — David Heffernan
– David Heffernan, Commented Oct 8, 2011 at 19:34

Ry- · Accepted Answer · 2011-10-08 19:20:25Z

1

Are you looking for:

int value = 0;
char c;
for(int i = strlen("message -add 0x"); c = *(_input + i); i++) {
    value <<= 4;
    if(c > '0' && c <= '9') {
        // A digit
        value += c - '0';
    } else if(c >= 'A' && c < 'G') {
        // Hexadecimal
        value += 10 + c - 'A';
    }
}

?

edited Oct 8, 2011 at 19:20

answered Oct 8, 2011 at 19:05

Ry-♦

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8 Comments

NT_SYSTEM Over a year ago

It doesn't work properly. If i enter message -add 0x1 .. it returns 203 or something.

David Heffernan Over a year ago

@minitech looks like you forgot to initialise value to 0

Carey Gregory Over a year ago

Won't work; value isn't initialized. You're also treating the input as decimal when it's hex.

Ry- Over a year ago

@CareyGregory: I'm too used to VB.NET right now, sorry. And I was thinking the alphabet came right after the numbers. I do read questions properly, even if I don't think straight all the time :)

Carey Gregory Over a year ago

@minitech: My condolences on the vb.net thing ;-)

|

D.Shawley · Accepted Answer · 2011-10-08 19:57:08Z

If you want to use C++, then steer clear of the various C functions and all of that nasty memory management. I would recommend reading Accelerated C++. It really is a top-notch book for learning C++ and actually using C++. Here's another solution to your problem without the C string parsing routines:

#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>


int
main()
{
    std::string const TARGET("message -add 0x");
    char const _input[] = "message -add 0xAE23";

    std::string input(_input);
    std::string::size_type offset = input.find(TARGET);
    if (offset != std::string::npos) {
        std::istringstream iss(input);
        iss.seekg(offset + TARGET.length());
        unsigned long value;
        iss >> std::hex >> value;
        std::cout << "VALUE<" << value << ">" << std::endl;
    }

    return 0;
}

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