Advanced C - Extra Credit - Array of Strings and Files

README.md

Array of Strings and Files

Two Solutions to Problem 3, page 270 of Perry, John. "Advanced C Programming by Example" Belmont, CA: PWS Publishing, 1998.

The first copies the strings.

The second reuses the pointers in the array passes to it.

by-reference.c

//
// Created by Noah Cardoza on 2019-03-21.
//

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

/**
 * verifies that an array of files exists
 * @param filenames a double array of
 * pointers to c-strings
 * @return a subset of filenames only
 * including files that exist
 */

char **checkDelete(char **filenames) {
     // a pointer used to traverse filenames
     char    *filename = NULL;
     // contains all the files that exist
     char   **existing_files = NULL;
     // the number of files found
     int      found = 0;

     // in the case where there are no valid
     // filenames we need to still return a
     // NULL terminated char* array
     existing_files = (char**)malloc(sizeof(char*));
     assert(existing_files); // assure the malloc was successful
     *existing_files = NULL;

     while ((filename = *filenames)){
         // returns -1 if the filename is not "ok" aka not readable/writable/non-existent
         if(access(filename, F_OK) != -1) {
             // add 2 because we need a null terminator
             existing_files = (char**)realloc(existing_files, (found + 2) * sizeof(char*));
             assert(existing_files); // assure the realloc was successful
             // reuses the pointers passes to it
             existing_files[found - 1] = filename;
             existing_files[found + 1] = NULL;
             found += 1;
         }
         filenames++;
     }
    return existing_files;
}

int main(void){
    char **ptr, **head;
    char *files[80] = {
            "main.c",
            "test_file/that/does/not.exist",
            "ec03",
            "ec03/EC_Tables.c",
            "hw04",
            "non.existent",
            NULL
    };

    head = ptr = checkDelete(files);

    while (*ptr) {
        printf("%s\n", *ptr);
        ptr++;
    }

    free(head);
    return 0;
}

/* Output:
 * main.c
 * ec03
 * ec03/EC_Tables.c
 * hw04
 */

file-system.md

/
  main.c
  ec03/
    EC_Tables.c
    INPUT.TXT
  hw04/

strcpy.c

//
// Created by Noah Cardoza on 2019-03-21.
// St
//

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

/**
 * verifies that an array of files exists
 * @param filenames a double array of
 * pointers to c-strings
 * @return a subset of filenames only
 * including files that exist
 */

char **checkDelete(char **filenames) {
     // a pointer used to traverse filenames
     char    *filename = NULL;
     // contains all the files that exist
     char   **existing_files = NULL;
     // the number of files found
     int      found = 0;

     // in the case where there are no valid
     // filenames we need to still return a
     // NULL terminated char* array
     existing_files = (char**)malloc(sizeof(char*));
     assert(existing_files); // assure the malloc was successful
     *existing_files = NULL;

     while ((filename = *filenames)){
         // returns -1 if the filename is not "ok" aka not readable/writable/non-existent
         if(access(filename, F_OK) != -1) {
             // add 2 because we need a null terminator
             existing_files = (char**)realloc(existing_files, (found + 2) * sizeof(char*));
             assert(existing_files); // assure the realloc was successful
             existing_files[found] = (char*)malloc((strlen(filename) + 1) * sizeof(char));
             assert(existing_files[found]); // assure the malloc was successful
             // copy the string so it can be safely mutated later
             strcpy(existing_files[found], filename);
             existing_files[found + 1] = NULL;
             found += 1;
         }
         filenames++;
     }
    return existing_files;
}

int main(void){
    char **ptr, **head;
    char *files[80] = {
            "main.c",
            "test_file/that/does/not.exist",
            "ec03",
            "ec03/EC_Tables.c",
            "hw04",
            "non.existent",
            NULL
    };

    head = ptr = checkDelete(files);

    while (*ptr) {
        printf("%s\n", *ptr);
        free(*ptr);
        ptr++;
    }

    free(head);
    return 0;
}

/* Output:
 * main.c
 * ec03
 * ec03/EC_Tables.c
 * hw04
 */