array of strings

**jordanguyoflove** · 11-19-2008

Hi,

In java you can create an array of strings.
For ex: string[10], which can store 10 array of characters.
how do you do this in C?

is it char str[x][y];

and then how do you assign a string to the array?
when i write this: str[0][0]= "some string";
str[1][0] = "another string";
i get something bizzare.

Tanks you.

**hauzer** · 11-19-2008

You use strcpy().

Code:

char str[10][100]; //A string array that can hold 10 strings of 100 characters

int main(void)
{
    strcpy(str[0],"Hello ");
    strcpy(str[0],"World!");

    return 0;
}

**C_ntua** · 11-19-2008

In C you would have to have a maximum size for a string. So for 10 stings indeed you would do char str[10][MAXSIZE]
You use functions (strncpy, strcpy) and NEVER the = operator. So you would do strcpy(str[2], "Hey") for the Java str[2] = "Hey"
In C++ there is a string type by the way. There are other ways around if you don't want a maximum size. That is using strlen() function and malloc() to allocate space for a char* str[10]

**MK27** · 11-19-2008

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
	char string[]="hello world";	// a single string
	char *list[3];			// an array of three strings
	list[0]=malloc(12);
	strcpy(list[0],"hello world");
	list[1]=malloc(6);
	strcpy(list[1],"world");
	list[2]=malloc(6);
	strcpy(list[2],"hello");
	printf("%s\n%s %s\n%s\n",list[0],list[2],list[1],string);
}

**laserlight** · 11-19-2008

Tsk, MK27, you forgot to free() what you malloc()ed.

**bbebfe** · 11-19-2008

Unlike Java automatic garbage collection mechanism, you need to free the memory allocated by malloc manually in C.

char *p = (char*)malloc(100);
strcpy(p, "hello");
free(p); //free the memory

**MK27** · 11-19-2008

Originally Posted by laserlight

Tsk, MK27, you forgot to free() what you malloc()ed.

Yes, and I didn't error check my malloc calls either!

Code:

if ((list[0]=malloc(12)) == NULL) perror("for list[0]: ");           // for example
[...]
free(list[0]);
free(list[1]);
free(list[2]);
return 0;             // musn't forget!
}      // end of main()

**jordanguyoflove** · 11-19-2008

ok thanks.

now i have to use the string in execvp.

Code:

char str[10][100]; //A string array that can hold 10 strings of 100 characters

    strcpy(str[0],"ls");
    strcpy(str[1],"dir");
fork()...
...

execvp(str[0], str)

but it's not working.

any idea why?

Thanks.

**matsp** · 11-19-2008

Describe "not working".

--
Mats

**MK27** · 11-19-2008

The trick with execvp is twofold:

1st) the first argument is a string which is really a path to the program. Since execvp checks the PATH env. variable, you can just use the command name ("ls"). The
2nd argument is a list, but the first element of the list must be the file/command name (without a path), so in this case it will be the same thing ("ls"). Using execlp, which works the same way but with a series of strings, you would have execlp("ls","ls",NULL);

2nd) In either case, the last element must be a NULL pointer. With execvp, that means the final element of your list.

Code:

#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
	char *args[3];
	args[0]=malloc(3);
	strcpy(args[0],"ls");
	args[1]=malloc(2);
	strcpy(args[1],"/");
	args[2]=NULL;
	if (execvp("ls",args)==-1) perror("execvp");
	return 0;
}

Notice that "ls" again appears twice in this.

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