Command Line Argument Comparison

Hmm, I'm having some trouble with this.

I want to check if the second argument (argv[1]) is equal to "-debug".

so what I've done is...well, see below. None of the if statements register as true.

Code:

---

char *debugArg = argv[1];

if (debugArg == "-debug")
if (debugArg == "-debug\0")
if (debugArg == "-debug\n")

---

None of them work! The middle one with the \0 should...I tested it manually to make sure that letter was a \0, and it is. What's wrong with this?

The only solution I have right now is, unfortunately,:

Code:

---

if (debugArg[0] == '-' &&
  debugArg[1] == 'd' &&
  debugArg[2] == 'e' &&
  debugArg[3] == 'b' &&
  debugArg[4] == 'u' &&
  debugArg[5] == 'g')

---

...but that kinf of sucks. What's the solution to this? Maybe I could use strings somehow...

--Ashiq

Quote:

---

char *debugArg = argv[1];

if (debugArg == "-debug")
if (debugArg == "-debug\0")
if (debugArg == "-debug\n")

---

The problem here is that you're comparing POINTERS, not strings. if (debugArg == "-debug") is testing if the memory location where the first string resides is the same as the memory location in which a string literal resides, and it can never happen.

2 options:

1) The C-way - use strcmp().
2) The C++ way - do the following:

#include <string>

std::string debugArg = argv[1];

if (debugArg == "-debug"){
/*...*/
}

http://faq.cprogramming.com/cgi-bin/...&id=1043284392
and
http://faq.cprogramming.com/cgi-bin/...&id=1043284385

if(!strcmp(debugArg, "-debug")) ... or equivalently ...
if(strcmp(debugArg, "-debug") == 0)
is the C way of doing it.

You also might want strcpy() instead of assignment (debugArg = argv[1]) for the same reason Cat mentioned. It will work that way (as they point to the same place), but when not intentional, its good not to have two values pointing at the same place (in the event that one gets modified, the other would also be modified as it points to the same place).

>>in the event that one gets modified
Normally, you wouldn't want to actually change the command line args, mostly you just read them. There's no need to make a copy if you're only reading them... (personal preference of course ;) )

>> Normally, you wouldn't want to actually change the command line args, mostly you just read them.

Sure there is! In case you want to change the args to what the user "meant" to put in. ;)

>>Sure there is! In case you want to change the args to what the user "meant" to put in. ;)
In which case you just ignore them and go off doing whatever you want, just like any other Windows program :D

Ack...sorry. Hammer: the FAQ is for character array manipulation...which is cool...but strings would've been better :(

Anways...I realized the solution, and yes, I remember that it's pointers I'm comparing...damn stupid thing to do...but here's what I did anyway.

Code:

---

char *debugArg = argv[1];
string debugStr = debugArg;

if (debugStr.compare("-debug\0") == 0)
{ ... }

---

...which worked. Thanks for all the help!

--Ashiq

You don't need the explicit \0 in there... "-debug" will work just fine. Also, since you are using string, operator== will work just fine (since you are no longer comparing pointers).

>> In which case you just ignore them and go off doing whatever you want, just like any other Windows program :D

Alright, just need to change the parameters explicitly for Linux programs then. :D