strcmp

[michy]

hi there,
a newbie is back to ask some newbie question again =)..

i have a line of code:

user *temp, *temp2;
if (strcmp(temp->username[0], temp2->username[0]) != 0)

where temp and temp2 is struct.

struct user{
	char username[1];
	user *next;
};

and i get error for it.
cannot convert parameter 1 from 'char' to 'const char *'

I dun really understand what is about "const" i did my research but still dun understand pls explain.

Thanks for reading...

[kempofighter]

strcmp is for comparing strings but you are passing a single character by value to a function that expects a pointer.

Your username is one character in length which seems strange. You can compare characters directly.
if(temp->username[0] == temp2->username[0])

If they were strings you could do this. But they aren't. With only 1 character there is no room for at least one character plus the null. Did you mean for the array to be of a bigger size?

if (strcmp(temp->username, temp2->username) != 0)
{
  // not equal
}
else
{
 // equal
}

[jsmith]

temp->username[0] has type "char", right?

strcmp() wants as its two parameters C strings which are of type const char* (or just char* for
the sake of a newbie).

[michy]

yea i got it
if (strcmp(temp->username, temp2->username) != 0)
This is what i want, but can explain whats the different between the code and code below
if (strcmp(temp->username[0], temp2->username[0]) != 0)

I tot I has declay "username" as array rite?
so by comparing the 1st value isn't it a correct way?

[Zhuge]

When you add the [0], you are no longer getting an array, rather just a single character out of it.

[Disch]

I'm surprised nobody mentioned this yet.

struct user{
	char username[1];  // only 1 character in size!
	user *next;
};

This means your buffer is only large enough to hold a single character. Note that C style strings need to be null terminated to mark the end of the string, which means that 'username' only has enough room for ZERO characters. IE: it can't hold anything and is pretty much completely worthless.

If you try to put a larger string (ie: more than zero characters) in it, you'll have a buffer overflow, memory corruption, and all sorts of other terribly nasty problems.

The easy solution here is to use std::string to represent strings, rather than char arrays:

struct user{
	string username;  // use std::string instead
	user *next;
};

// now you don't need strcmp, either, you can compare with ==
if(temp->username == temp2->username)
{
  // equal
}

if you must use a char array, you'll need to make it bigger than [1]:

char username[100];  // can now have a name up to 99 characters long, but no longer 

[michy]

oh i see, it is good to use string then char.
when i use char it's so complicated to convert here and there = =
and i used string, everything gets easy.
but now i a question.
when do we use char? and when do we use string?
or just simply depend on programmer?

[Zhuge]

You can pretty much prefer string everywhere. Unless you are in C, which doesn't have them.