Converting hex string to unsigned int issue C++ [closed]

Question

I am currently designing a code and one of the instructions given is to get the first two digits of a hex string and do a desired operation. Everything that is less than A is taken into account. Anything greater than that is not, it just takes the next number. Which messes up the code.

Here is the line which converts it:

int x = atoi(hex.c_str);

What am I doing wrong?

Answer 1

I'm assuming this is what you meant - you'd like to convert only the first two hexadecimal digits of a string to an unsigned integer. I'm also assuming that the string contains only valid hexadecimal digits. If you want to validate the string, you need to write some extra code for that.

Use strtoul to convert a hex string to unsigned integer instead. Also use substr() method of the string class to extract only the initial 2 digits of the hexadecimal string.

#include <string>
#include <cstdio>
#include <cstdlib>

int main()
{
    std::string myhex = "abcdef";
    unsigned int x = strtoul(myhex.substr(0, 2).c_str(), NULL, 16);
    printf("x = %d\n", x);
}

This would be your output (i.e. 0xab = 171):

x = 171

Answer 2

the atoi function requires base 10

instead use e.g. strtol

it is generally a good idea to read the documentation. just write the function's name into AltaVista (or, for that matter, google). and it will find some documentation for you

Answer 3

int x = strtol(hex.c_str(), NULL, 16);

You can also use sscanf if you want to read a partial string. You mentioned you might want to just take the first two characters:

int x;
if( 1 == sscanf(hex.c_str(), "%2x", &x) ) {
    printf( "Value is: %d\n", x );
} else {
    printf( "Conversion failed\n" );
}

Note the above isn't really C++. You can use the std::hex stream modifier for that (note that I called your string mystr this time to avoid confusion (particularly if you've imported the namespace std):

int x;
std::istringstream iss(mystr);
bool ok = (iss >> std::hex >> x);

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[edit] I notice in your comment you ask about converting to an unsigned integer. In that case, why are you not explicitly declaring your integer as unsigned int?

For unsigned, you should use strtoul.