Thread: Passing Array of Strings to a function ??

Kindly indent your code more consistently and post the exact error message.

For whatever reason (and someone please correct me if I'm wrong!), in C you can only allow pass and modify a char list (2D array) if it is malloced onto the heap like this:

Code:

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

int testfunc (char **list) {
	strcpy(list[1],"!!");
	return 0;
}

int main () {
	int i;
	char *list[3];
	for (i=0;i<3;i++) {
		list[i]=malloc(3);
		sprintf(list[i],"#&#37;d",i+1);
	}
	testfunc(list);
	for (i=0;i<3;i++) {
		printf("%s\n",list[i]);
		free(list[i]);
	}
	return 0;
}

Output:

Code:

#1
!!
#3

In other words you cannot pass char list[40][40] and expect your function to change "list" in main(). You cannot even use a pointer to list to do this. "list" must be heap allocated. This is the only way (unless someone has a better one).

I've heard this isn't an issue in C++, and I'm certain the phenomenon is very, very poorly explained in virtually all of the C documentation I've seen, and discussions of it usually seem to get bogged down in what "pass by reference" means.

A more normative method is to return a pointer, and if you need an int to pass that by ref:

Code:

char **testfunc (char list[3][3], int *retv)

I just noticed you call "convert" in main's for loop, and indeed the function itself isn't processing an array of strings, it's a single string! So rags_to_riches has a more appropriate solution. You do not really need to "pass an array of strings".

Still, if anyone can confirm or deny about what I wrote re: heap allocating arrays of strings for functions, it would be much appreciated

> Still, if anyone can confirm or deny about what I wrote re: heap allocating arrays of strings for functions, it would be much appreciated
You're just wrong, that's all.
Arrays, no matter how many dimensions, and no matter how you dress up the declaration, are always passed by reference.


Aside from the long-winded code, the OP only needed
array[i]=convert(name);

Originally Posted by Salem

You're just wrong, that's all.

How? In the sense that there is another method of passing and modifying an array without using the return value? Show me.

Arrays, no matter how many dimensions, and no matter how you dress up the declaration, are always passed by reference.

I'm not denying that. It seems to me the issue is where the reference is to.

Originally Posted by Adak

There is no difference between changing a malloc'd array, and changing an array on the stack. You didn't get the array to change because you copied the whole array in that function.

I'm not going to give you code to confirm this, because I don't believe in it - I believe you learn best by discovering it yourself - with guidance.

You are not going to give me code that won't work, is what you mean. You cannot pass a (2D) stack char array to a function, modify it, and have the changes take place back in main, unless you return a new pointer.

ps. If by "you", you mean me, I did change the array and you should read more carefully....so there

Originally Posted by MK27

You are not going to give me code that won't work, is what you mean. ]You cannot pass a stack char array to a function, modify it, and have the changes take place back in main, unless you return a new pointer.

ps. If by "you", you mean me, I did change the array and you should read more carefully....so there

Good - get angry! I guess it helps you in the discovery process.

A char array is the same as an integer array! In one case the words are 8 bits, and in the other case, the words are 16 or 32 bits. They're both just strings of binary words to the computer.

I pass stack char array's all the time. Fact is, I don't malloc all that much. I'm not sure I know what you mean by "and have the changes take place back in main".

The changes take place in RAM, or some other physical device. Main has nothing to do with that.

Consider

Code:

void foo ( char a[10][20] ) {
    strcpy( a[0], "hey, it works" );
    a[1][0] = '!';
}
void bar ( char a[][20] ) {
    strcpy( a[0], "hey, it works" );
    a[1][0] = '!';
}
void baz ( char (*a)[20] ) {
    strcpy( a[0], "hey, it works" );
    a[1][0] = '!';
}

int main ( ) {
    char arr[10][20];
    foo( arr );
    bar( arr );
    baz( arr );
    return 0;
}

Anything you can do in main() with the array, you can also do in the function as well. So if the array is modifyable in main (not declared const), then you can modify it in the function as well.

Note that foo(), bar() and baz() are functionally identical, despite the different declarations of the parameter. They all mean the same thing to the compiler. The lack of any "*" in the first two does NOT mean you're getting a copy of the array all of a sudden.

The only things which would be different would be:
- you can INITIALISE the array in main, but you can't reinitialise the array inside the function.
- sizeof(arr) in main gives a different answer to sizeof(a) in the function.



My guess is you tried this once with a
char *msg = "hello";
kinda thing once, and then tried to modify your string literal in a function, and got a segfault for your trouble.

Only you didn't understand the real reason for the problem, and so decided it was a problem with the parameter passing mechanism instead.

Originally Posted by Adak

I pass stack char array's all the time. Fact is, I don't malloc all that much. I'm not sure I know what you mean by "and have the changes take place back in main".

I pass stack char arrays all the time. My point is there is some significant syntax difference, in C, between a char array and a char 2D array. If you don't know what I meant by "having the changes take place in main", then obviously you don't use the arrays in this manner altho you may pass them all the time. This is the difference between:

Code:

void func (char **string);
//to call:
char *string;
func(&string);

and

Code:

void func (char *string);
//to call:
void func(string);

Do you understand what I mean by "having the changes take place in main" (or wherever func is called from) now? In the first case, whatever happens to *string in func will apply back in main() after func is called. In the second case, whatever happened to string in func WILL NOT apply back in main() after func is called.

Now, try and apply this logic to a 2D "list" (char list[40][40]) and you will find out what I mean by syntax differences, because the corallary to the first case is what was in my initial post.

The changes take place in RAM, or some other physical device. Main has nothing to do with that.

You're a kook. So about the heap...

Okay I'm wrong.

And since none has been kind enough to post some indented code, I'll show you how it's done.

Code:

#include <stdio.h>

int convert(char name[40][40]);

int main()
{
	char name[40][40];
	static int array[11];	
	int i;
	for (i = 0; i <= 11; i++)
	{
		printf("Please enter note &#37;d in the series:",i+1);
		//scanf("%s",name[i]);
		array[i] = convert(name[i]);
	}
	printf("\n\t");
	for (i = 0; i<= 11; i++)
	{
		printf("%d\n\t", array[i]);
	}
}

int convert(char name[40][40])
{
	int x;
	if ( strcmp(name, "C") == 0 || strcmp(name, "c") == 0 || strcmp(name, "B#") == 0 || strcmp(name, "b#" ) == 0)
		x = 0;
	else if( strcmp( name, "C#" ) == 0 || strcmp( name, "c#")==0||strcmp( name, "Db" ) == 0 || strcmp( name, "db")==0 )
		x=1;
	else if( strcmp( name, "D" ) == 0 || strcmp( name, "d")==0)
		x=2;
	else if( strcmp( name, "D#" ) == 0 || strcmp( name, "d#")==0||strcmp( name, "Eb" ) == 0 || strcmp( name, "eb")==0 )
		x=3;
	else if( strcmp( name, "E" ) == 0 || strcmp( name, "e")==0||strcmp( name, "Fb" ) == 0 || strcmp( name, "fb")==0 )
		x=4;
	else if( strcmp( name, "F" ) == 0 || strcmp( name, "f")==0||strcmp( name, "E#" ) == 0 || strcmp( name, "e#")==0 )
		x=5;
	else if( strcmp( name, "F#" ) == 0 || strcmp( name, "f#")==0||strcmp( name, "Gb" ) == 0 || strcmp( name, "gb")==0 )
		x=6;
	else if( strcmp( name, "G" ) == 0 || strcmp( name, "g")==0 )
		x=7;
	else if( strcmp( name, "G#" ) == 0 || strcmp( name, "g#")==0||strcmp( name, "Ab" ) == 0 || strcmp( name, "ab")==0 )
		x=8;
	else if( strcmp( name, "A" ) == 0 || strcmp( name, "a")==0 )
		x=9;
	else if( strcmp( name, "A#" ) == 0 || strcmp( name, "a#")==0||strcmp( name, "Bb" ) == 0 || strcmp( name, "bb")==0 )
		x=10;
	else if( strcmp( name, "B" ) == 0 || strcmp( name, "b")==0||strcmp( name, "Cb" ) == 0 || strcmp( name, "cb")==0 )
		x=11;
	else 
	{
		printf("Not a Vaild Entry!!!\n");
		printf("Please enter note in the series AGAIN:");
		//scanf("%s",name);
		convert(name);		
	}
	return x;
}

Also took the liberty to remove the lot of unnecessary lines.
And then I'll hand out the usual helpful links.
Do not use scanf to read strings, or use it properly: https://apps.sourceforge.net/mediawi...tle=Scanf_woes
Do not use implicit main: http://cpwiki.sourceforge.net/Implicit_main