variadic function- reading files (c) - c

Question

i need to create a variadic function (stdarg library) which will loop through all the files i pass it and will count words similiar to a word i pass as a parameter,
#include <stdio.h>
#include <stdarg.h>
void countWords(char* name, FILE* file, ...){
va_list params;
FILE* currentFile;
FILE* f;
int words = 0;
va_start(params, file);
currentFile = file;
while (currentFile != NULL)
{
f = fopen(currentFile, "r+"); //which file should i open every time? this doesnt compile
// comparing words in each file code
currentFile = va_arg(params, FILE*);
}
va_end(params);
}
i cant read the file (no metter what i try it doesnt compile),
and how can i loop through each file comparing my word? i would really appreciate guidance
thank you!

Answer 1

If you are passing the file name or more precisely the path to the file, then this
FILE *currentFile;
currentFile = va_arg(params, FILE *);
should be
char *currentFile;
currentFile = va_arg(params, char *);
If you pass FILE pointers, you should not open them because if the rest of the program is correct then they shall be already opened inside the funcion, otherwise it doesn't make any sense to pass FILE *'s.
So the function should probably be
#include <stdio.h>
#include <stdarg.h>
void countWords(char *word, char *filename, ...)
{
va_list params;
FILE *file;
int words;
words = 0;
va_start(params, file);
while (filename != NULL)
{
file = fopen(filename, "r+");
// comparing words in each file code
filename = va_arg(params, char *);
}
va_end(params);
}
You would call it like this
countWords("example", "/path/to/file/1", ..., "/path/to/file/n", NULL);
and you should be careful with string literals probably use the const qualifier in this situation, because even if the parameters are not string literals it wouldn't make sense to modify them inside countWords() so to prevent accidentally modifying them const could help, although you can always modify them anyway. Even if modifying a string literal invokes undefined behavior you cannot completely forbid your program from doing so.

Answer 2

i need to create a variadic function
Unless this is a homework assignment about variadic functions, you don't need a variadic function at all. Variadic functions are not type safe and type safety is a desirable property.
Others have already pointed this out: You need a function that takes a filename and counts the occurrences of a word in a single file
size_t occurrences(const char *word, const char *filename) { ... }
You can then easily loop over an array of files, e.g.:
size_t count = 0;
for (int i = 1, i < argc; i++) {
size += occurrence("pink", argv[i]);
}
If you like, you can wrap this in a separate function which takes an array of file names with a file count. This is easily done, because whether you take your files from a command line or from somewhere else, you already have them stored in some kind of array.
Now consider the variadic variant, which makes sense only if you know the files you want to process at compile time, e.g.:
size_t n = var_occurrences("banana",
"alpha.txt", "beta.txt", "gamma.txt", NULL);
The same can be achieved with the non-variadic approach:
const char *w = "banana";
size_t n = occurrences(w, "alpha.txt")
+ occurrences(w, "beta.txt")
+ occurrences(w, "gamma.txt");
This is a bit more typing, but everything else is much more straightforward. If you must use variadic functions, go ahead and look at iharob's answer. But variadic functions are not a good solution to your task.