How to compare command line params with strings?

[Rox]

I am wondering how I can compare the command line arguments with predefined strings/char arrays.

int main(int argc, char *argv[])
{

   if(argv[1] == 'hello') // error since 'hello' is a char array and argv[1] is a pointer
      cout << "hello" << endl;

   char param = argv[1]; 
   if(param == 'hello') // error since 'hello' is a char array and param is not. 
      cout << "hello" << endl;
   return 0;
}

How can I make the check?

[hamsterman]

'hello' is not a char array. "hello" is. There is a difference.
Well, actually, "hello" is a const char*.

One way (using C standard library)
if( strcmp( argv[1], "hello" ) == 0 ) ...
You need to include <cstring>

Another (using C++ standard library)
if( std::string(argv[1]) == "hello" ) ...
You need to include <string>

[lnk2019]

You code doesn't work because you are using single quotes around your strings. These are usually used to enclose single characters (as far as I know).

You want to use double quotes ( "Your String" ). Also I'm pretty sure that direct comparison between character strings ( char* == char* ) is not possible. I found this explanation on the stackoverflow forums:

stackoverflow wrote:

Explanation: in C, a string is a pointer to a memory location which contains bytes. Comparing a char* to a char* using the equality operator won't work as expected, because you are comparing the memory locations of the strings rather than their byte contents. A function such as strcmp() will iterate through both strings, checking their bytes to see if they are equal. strcmp() will return 0 if they are equal, and a non-zero value if they differ.

Try strcmp(...) function.

More information on it can be found here: http://www.cplusplus.com/reference/clibrary/cstring/strcmp/

For example:

bool fIsEqual = strcmp(argv[1], "hello") == 0;

I hope this helps!

EDIT: Too late :(

[andywestken]

In case someone else reads this late.

This code doesn't work...

if(argv[1] == 'hello')
{
    ....

as it is testing the address of the argument string (as in, pointer to a null terminated string) against the address of a constant string. Even if the value of argv[1] is "hello", the condition will fail as the two "hello"s are stored at different memory addresses, and the pointer values therefore differ.

[hamsterman]

@andywestken, no, 'hello' is not a string. It's a character. I don't think it should compile though.. It doesn't of VC++.

[andywestken]

Thanks hamsterman.

And sorry! I did meant to type

if(argv[1] == "hello")
{
    ....

(in a belated - failed? - attempt to re-clarify an earlier comment.)

Andy

P.S. I have just today bumped into some open source making exactly this mistake! As I habitually crank the warning up level to max, the bad code was immediately flagged. But it seems to be a pretty common mistake to make!

[moorecm]

Once again, to clearify further, 'hello' is invalid syntax and should never compile according to the standard. Character literals are exactly one character--never more and never less. ('\n' is consider a single character.)