Thread: Problem using scanf with array

Hello everyone!
I'm a beginner and I was facing a problem here.

I am trying to use scanf to input filenames as strings to different indexes in the array char **filenames.

For example:

filenames[0] = "lol1.txt";
filenames[1] = "lol2.txt";
filenames[2] = "lol3.txt";
filenames[3] = "lol4.txt";
filenames[4] = "lol5.txt";

But my code doesn't seem to work:

Code:

	int totalfiles = 0;

	printf ("Enter the total number of files to be merged:");
	scanf ("%d", &totalfiles);

	char **filenames;
	filenames = (char**)calloc(sizeof(char**), totalfiles);


	// I tried using the code below but it messes up the program

	int n = 0;

	while ( n < totalfiles )
	{
		printf ("File number %d:", n+1);
		scanf ("%s", filenames[n]); // I'm assuming the problem is in this line
		n++;
	}

Any ideas on what I'm doing wrong here?

Thanks!

How about malloc()'ing storage for the arrays that each element of filenames[] points to.
Note that the arguments to calloc() are switched around, so better fix 'em.

Oh..., you are doing all sorts or errors. First of all, you need to learn the difference between arrays and pointers, second you need to understand the difference between multidimmensional arrays and pointer to pointer, these are not equal, AND have totaly different purposes.

Give it an other try, repost if you can't get it right, and I'll give some additional leeds.

itCbitC: I does not matter if changes the paramters, the calloc statement is still wrong. The sizeof(char**) will yield 4, which is the size of the pointer and not the sizeof an element in the multi dimmensional array. If he was supposed to something close to correct, it would be something like (not compiled and tested):

Lets say that max file name is 10 chars.

Code:

char **fileNames = NULL;

fileNames = (char**)calloc(totalfiles, sizeof(char) * 10);

But still there is a lot of errors in the code.

Originally Posted by Elysia

This is wrong. You are allocating 10 chars, but store it in a pointer to pointer.

No, its not, you are allocating totalFiles, which is the input, times 10 which is the size of one element the array times the size of the element type.

However, you are right about the pointer to pointer.

Woah...thanks for the overwhelming response everyone!

I'll take everyone's post into consideration, read the links, do some more studying myself and then get back here!

I am actually trying to make a merge and split program in C as part of my university's course project. This is my first semester and we just started learning C.

Thanks again!

I think I solved the problem:

Code:

void main ()
{

	clrscr();
	int totalfiles = 0;

	printf ("Enter the total number of files to be merged:");
	scanf ("%d", &totalfiles);

	char filenames[10][10];

	int i;

	for (i=0; i < totalfiles; i++)
	{
	printf ("Enter file number %d:",i+1);
	scanf ("%s", filenames[i]);
	}

	printf ("\n\n");

	int p = 0;

	printf ("Analysing the following %d files\n", totalfiles);
	while ( p < totalfiles )
	{
		printf ("%s\n", filenames[p]);
		p++;
	}

As you can see, I used a simple two dimensional string array instead:
char filenames[10][10];


Everything seems to work now.

Originally Posted by BEN10

But I'm curious to know how we can do it using pointer to pointer to char. I tried to do that way but always resulted in segfault.

Starting out malloc() storage for the char** variable.
This creates an array of char* pointers with the char** pointing at the base.
Finally, malloc() storage for an array of char - one per char* pointer.