Array of file names (such that you call fopen with array[i])

Hello,

I am trying to create an array of file names such that when I want to open one of the files, who's name is given in the array, I can call that element of the array.

My code for making the array is the following,

Code:

char file_name[40];
char *file_locations[N];         // array of N names (*) 


for(ii=0; ii<=N-1; ii++){        // printing the names of all the files P1_8mag_1.txt - P1_8mag_N.txt 
    sprintf(file_name, "P1_8mag_%i.txt", ii+1);
    file_locations[ii] = file_name;
}

then to open a given file I run

Code:

FILE*fp1=fopen(file_locations[i], "r");     // open the file

The problem with this is that in the first step: all elements of the array "file_locations" are the name of the last file in the loop (in this case "P1_8mag_N.txt")

What is wrong with this?

Ps thanks for all of you comments, I am new to C and programming.

BN

You have created a 1D array of char pointers and then proceeded to point all of them to file_name, which of course is the last string you created with sprintf.

You can either create a 2D array of N x 40 or you can malloc() the memory for each string created and point each element to their respective malloc'd string.

If you do the former, you'll need to use the function strcpy() and not the assignment operator =.

Thank you Tclausex,

Is there any benefit to using one or the other? Or is it simply preference?

Code:

char file_name[40];
char file_locations[N][40];         // array of 16 pointers (*) 


for(ii=0; ii<=N-1; ii++){        // printing the names of all the files P1_8mag_1.txt - P1_8mag_16.txt 
    sprintf(file_name, "P1_8mag_%i.txt", ii+1);
    strcpy(file_locations[ii], file_name);
}

gives error : warning: incompatible implicit declaration of built-in function ‘strcpy’
BN

malloc would only allocate enough memory needed for each string, but your strings seem to be within a few characters of the same length, so not much benefit really. Unless your N is very large and you might not use all N rows.

Sorry there is no error, I forgot to inlude

Code:

#include<string.h>

Thanks again for you help

Originally Posted by bnickerson

Code:

char file_locations[N][40];         // array of 16 pointers (*)

That comment is completely wrong. file_locations is an array of arrays, not an array of pointers.

Pointers and arrays are very very different things. Potential confusion comes about because, in some circumstances, they can be used interchangeably. But that does not make them the same thing.

If your teacher is telling you pointers and arrays are the same thing, get a new teacher. If your textbook is telling you they are the same thing, burn it.

If you are jumping to the conclusion that "can be used interchangeably in some circumstances" means "a pointer is an array", rethink.

Originally Posted by grumpy

That comment is completely wrong. file_locations is an array of arrays, not an array of pointers.

Pointers and arrays are very very different things. Potential confusion comes about because, in some circumstances, they can be used interchangeably. But that does not make them the same thing.

If your teacher is telling you pointers and arrays are the same thing, get a new teacher. If your textbook is telling you they are the same thing, burn it.

If you are jumping to the conclusion that "can be used interchangeably in some circumstances" means "a pointer is an array", rethink.

I understand that they are not the same. However, is there any error in the code I have posted? It gives me what I want namely an array "file_locations" consisting of elements

Code:

P1_8mag_1.txt
P1_8mag_2.txt
P1_8mag_3.txt
P1_8mag_4.txt
.
.
.
P1_8mag_N.txt

which can then be used to open each of the locations

Code:

FILE*fp=fopen(file_locations[i], "r");     // open the file
.
.
.
.
fclose(fp);

Originally Posted by bnickerson

I understand that they are not the same. However, is there any error in the code I have posted? It gives me what I want namely an array "file_locations" consisting of elements

Code:

P1_8mag_1.txt
P1_8mag_2.txt
P1_8mag_3.txt
P1_8mag_4.txt
.
.
.
P1_8mag_N.txt

which can then be used to open each of the locations

Code:

FILE*fp=fopen(file_locations[i], "r");     // open the file
.
.
.
.
fclose(fp);

Yup, file_locations is an array of arrays NOT and array of pointer to character. If you understood that, remove the comment from your code. It'll help with future misunderstandings.

Before you rush off and create an array of 1000's of filenames, you should be aware that most operating systems restrict the number of open files per process.
Eg.
c++ - Is there a limit on number of open files in Windows - Stack Overflow

Originally Posted by Salem

Before you rush off and create an array of 1000's of filenames, you should be aware that most operating systems restrict the number of open files per process.
Eg.
c++ - Is there a limit on number of open files in Windows - Stack Overflow

I read a bit about this on another post and that is why I wanted to save all of the file names to an array. In doing so I can then easily open and close each file in a loop while editing them.

Thank you everyone for your comments!

P.s. How do I edit my post? Indeed grumpy and hex_dump pointed out that I do not fully understand the relation between pointers and arrays and I would like to remove the related comment. However, the edit button is no longer showing.

Originally Posted by bnickerson

Hello,

I am trying to create an array of file names such that when I want to open one of the files, who's name is given in the array, I can call that element of the array.

Code:

char file_name[40];
char *file_locations[N];         // array of N names (*) 

for(ii=0; ii<=N-1; ii++){        // printing the names of all the files P1_8mag_1.txt - P1_8mag_N.txt 
    sprintf(file_name, "P1_8mag_%i.txt", ii+1);
    file_locations[ii] = file_name;
}

The problem with this is that in the first step: all elements of the array "file_locations" are the name of the last file in the loop (in this case "P1_8mag_N.txt")

What is wrong with this?

The problem here is that every time through the loop, you are assigning the same address (file_name) to the array. A simple way to solve this is strdup.

Code:

for(int ii=0; ii<=N-1; ii++){        // printing the names of all the files P1_8mag_1.txt - P1_8mag_N.txt 
	sprintf(file_name, "P1_8mag_%i.txt", ii+1);
	if ((file_locations[ii] = strdup(file_name)) == NULL) {
		printf("not enough memory!\n");
		return 1;
	}
}

// ...

// all done: clean up each i in file_locations[i]
for (int i=0; i < N; i++)
	free(file_locations[i]);

strdup means will duplicate the string data in your temporary buffer file_name into a new location before assigning it into your file_locations[i] array member. If there is not enough memory, NULL is returned and the program aborts with an error message (unlikely but necessary to check for it)

Originally Posted by bnickerson

P.s. How do I edit my post? Indeed grumpy and hex_dump pointed out that I do not fully understand the relation between pointers and arrays and I would like to remove the related comment. However, the edit button is no longer showing.

After some time (AFAIK 1 hour) you can't edit you post anymore.

Bye, Andreas

There's no need to edit your post. Just take it out of your code and be done with it. You can post your new code when you're ready.