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POHow do I allocate memory and determine array sizes dynamically in C?
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2010-07-14T08:47:36.120
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I am trying to teach myself C from a python background. My current mini-problem is trying to do less hard-coding of things like array lengths and allocate memory dynamically based on input. I've written the following program. I was hoping for suggestions from the community for modifying it in the following ways: 1.) Make <code>first</code> and <code>last</code> <code>Name</code> elements variable length. Currently their length is hardcoded as <code>MAX_NAME_LENGTH</code>. This will involve both change <code>Name</code>s <code>struct</code>declaration as well as the way I'm assigning values to its elements. 2.) Bonus: Figure out some way to progressively add new elements to the <code>name_list</code> array without having to determine its length beforehand. Basically make it an expandable list. <pre><code>/* namelist.c Loads up a list of names from a file to then do something with them. */ #include <stdlib.h> #include <stdio.h> #include <memory.h> #define DATAFILE "name_list.txt" #define DATAFILE_FORMAT "%[^,]%*c%[^\n]%*c" #define MAX_NAME_LENGTH 100 typedef struct { char first[MAX_NAME_LENGTH]; char last[MAX_NAME_LENGTH]; } Name; int main() { FILE *fp = fopen(DATAFILE, "r"); // Get the number of names in DATAFILE at runtime. Name aName; int lc = 0; while ((fscanf(fp, DATAFILE_FORMAT, aName.last, aName.first))!=EOF) lc++; Name *name_list[lc]; // Now actually pull the data out of the file rewind(fp); int n = 0; while ((fscanf(fp, DATAFILE_FORMAT, aName.last, aName.first))!=EOF) { Name *newName = malloc(sizeof(Name)); if (newName == NULL) { puts("Warning: Was not able to allocate memory for ``Name`` ``newName``on the heap."); } memcpy(newName, &aName, sizeof(Name)); name_list[n] = newName; n++; } int i = 1; for (--n; n >= 0; n--, i++) { printf("%d: %s %s\n", i, name_list[n]->first, name_list[n]->last); free(name_list[n]); name_list[n] = NULL; } fclose(fp); return 0; } </code></pre> Sample contents of <code>name_list.txt</code>: <pre><code>Washington,George Adams,John Jefferson,Thomas Madison,James </code></pre> Update 1: I've implemented a linked list and some helper functions as @Williham suggested, results are below. <pre><code>#include <stdlib.h> #include <stdio.h> #include <string.h> #define DATAFILE "name_list.txt" #define MAX_NAME_LENGTH 30 #define DATAFILE_FORMAT "%29[^,]%*c%29[^\n]%*c" static const int INPUT_BUFFER_SIZE_DEFAULT = sizeof(char) * MAX_NAME_LENGTH; typedef struct _Name Name; struct _Name { char *first; char *last; Name *next; }; int get_charcount(char *str); Name * create_name_list(char *filename); void print_name_list(Name *name); void free_name_list (Name *name); int main() { // Read a list of names into memory and // return the head of the linked list. Name *head = create_name_list(DATAFILE); // Now do something with all this data. print_name_list(head); // If you love something, let it go. free_name_list(head); head = NULL; return 0; } int get_charcount (char *str) { int input_length = 1; while (str[input_length] != '\0') { input_length++; } return input_length; } Name * create_name_list(char *filename) { FILE *fp = fopen(DATAFILE, "r"); char *first_input_buffer = malloc(INPUT_BUFFER_SIZE_DEFAULT); char *last_input_buffer = malloc(INPUT_BUFFER_SIZE_DEFAULT); Name *firstNamePtr; Name *previousNamePtr; while ((fscanf(fp, DATAFILE_FORMAT, last_input_buffer, first_input_buffer))!=EOF) { Name *newNamePtr = malloc(sizeof(Name)); if (previousNamePtr) { previousNamePtr->next = newNamePtr; previousNamePtr = newNamePtr; } else { firstNamePtr = previousNamePtr = newNamePtr; } char *temp_buffer = malloc(get_charcount(first_input_buffer)); strcpy(temp_buffer, first_input_buffer); newNamePtr->first = malloc(get_charcount(first_input_buffer)); strcpy(newNamePtr->first, temp_buffer); realloc(temp_buffer, get_charcount(last_input_buffer)); strcpy(temp_buffer, last_input_buffer); newNamePtr->last = malloc(get_charcount(last_input_buffer)); strcpy(newNamePtr->last, temp_buffer); free(temp_buffer); temp_buffer = NULL; } previousNamePtr->next = NULL; previousNamePtr = NULL; free(first_input_buffer); free(last_input_buffer); first_input_buffer = NULL; last_input_buffer = NULL; fclose(fp); return firstNamePtr; } void print_name_list (Name *name) { static int first_iteration = 1; if (first_iteration) { printf("\nList of Names\n"); printf("=============\n"); first_iteration--; } printf("%s %s\n",name->first, name->last); if (name->next) print_name_list(name->next); else printf("\n"); } void free_name_list (Name *name) { if (name->next) free_name_list(name->next); free(name->first); free(name->last); name->first = NULL; name->last = NULL; name->next = NULL; free(name); name = NULL; } </code></pre>
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<c><dynamic><arrays><struct><malloc>
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How do I allocate memory and determine array sizes dynamically in C?
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