Myujth

As a novice programmer I started reading through The C Programming Language to learn more about pointers and structs.

I am currently learning about hashmaps in C. Following the example in the book, I created my own hashmap to hold key-value pairs:

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

#define MAX_STRING_SIZE 100
#define HASHSIZE 101

static struct map *hashtab[HASHSIZE]; /* pointer table */
unsigned hash(char *); /* hashing function: form hash value for string s. used by both lookup and install*/
char *strdup(char *);

struct map /* creating a map structure */
 struct map *next;
 char *KEY; /* KEY - pointer to a char - member of nlist*/
 char *object1; /* object - pointer to a char - member of nlist*/
;

/* lookup function takes a pointer to char - s as an argument and returns a pointer to map structure */
struct map *lookup(char *s) 
 struct map *np;

 for (np = hashtab[hash(s)]; np != NULL; np = np->next) 
 if (strcmp(s, np->KEY) == 0) 
 return np;
 
 
 return NULL;


/* install function takes a pointer to a char - KEY, object and returns a pointer to map structure */
struct map *install(char *KEY, char *object1) 
 /* install: put (name, defn) in */
 /* Install uses lookup to determine whether the KEY being installed
 is already present. Proceeds to create a new entry or update*/
 struct map *np;
 unsigned hashval;

 if ((np = lookup(KEY)) == NULL) 
 else 
 free((void *)np->object1);
 
 if ((np->object1 = strdup(object1)) == NULL) 
 return NULL;
 
 return np;

I then assign values to keys as follows:

int main(void) 

 struct map *table[4] = 
 (install("key1", "value1")),
 (install("key2", "value2")),
 (install("key3", "value3")),
 (install("key4", "value4"))
 ;

 int i;

 for (i = 0; i < 4; i++) 
 printf("%s->%sn", table[i]->KEY, table[i]->object);
 
 printf("n");
 return 0;

The code above works well, I am able to assign value1, ... , value4 to key1, ... ,key4 respectively. However, this does not allow me to assign multiple values to the same key.

Let's say I read the following from a text file:

key1 9000 600 Test1
key2 2000 600 Test2
key3 3000 120 Test3
key4 4000 120 Test4
.
.
key10 1000 560 Test10

I want to be able to store each key and assign multiple values to it. Since the number of columns is fixed, maybe I could have a structure that represents a row and put that into the map.

To do so, install needs to be modified to be able to add multiple values for the same key. Since the keys are unique, lookup and uninstall (function I created to remove keys) should remain unchanged.

The code above works perfectly, however I am looking for a general solution to add multiple values to the same keys.

How should I proceed to be able to call:

struct map *table[4] = 
 (install("key1", "9000" ,"600", "Test1")), //key, object1, object2, object3
 (install("key2", "2000" ,"600", "Test2")),
 (install("key3", "3000" ,"120", "Test3")),
 (install("key4", "4000" ,"120", "Test4"))
;

Another idea would be:

/* key points to this struct */
struct Value 
 int i;
 int k;
 char *c;
;

typedef struct map /* creating a map structure */
 struct map *next;
 char *KEY;
 struct Value value; /* place multiple values inside a struct*/
;

and here is where I am getting stuck:

struct map *insert(char *KEY, struct *Value) 
 struct map *np;
 unsigned hashval;

 if ((np = lookup(KEY)) == NULL) 
 else 
 free((struct Value*)np->value); //type cast cannot convert from 'Value' to 'Value*'

 
 if ((np->Value = strdup(Value)) == NULL) //map has no field value
 return NULL;
 

 return np;

I searched through the following questions, however was unable to extract the relevant information on how to implement this in C.

Hashmaps having multiple keys with multiple values

How can I assign multiple values to a hash key?

HashMap with multiple valued keys

How can I implement a hashmap which accepts multiple values assigned to the same key?

EDIT

As pointed out in the comments, the structure I am using may not be actuallly a hashmap but a linked list. However the book specifically says that page 144 is a hashmap.

This may not be the best possible solution, but thanks to the above comments I got something working. I am declaring multiple values as pointers to char. Please let me know if I can improve this.

I am yet to implement collision correction.

struct map /* creating a map structure */
 struct map *next;
 char *KEY;
 char *value1;
 char *value2;
 char *value3;
; 

I then pass these to the insert function.

struct map *insert(char *KEY, char *value1, char *value2, char *value3) 
 /* install: put (name, defn) in */
 /* Install uses lookup to determine whether the KEY being installed
 is already present. Proceeds to create a new entry or update*/
 struct map *np;
 unsigned hashval;

 if ((np = lookup(KEY)) == NULL) 
 else 
 free((void *)np->value1); //type cast cannot convert from 'Value' to 'Value*'
 free((void *)np->value2);
 free((void *)np->value3);
 
 if ((np->time = strdup(value1)) == NULL) //map has no field value
 return NULL;
 
 if ((np->description = strdup(value2)) == NULL) //map has no field value
 return NULL;
 
 if ((np->duration = strdup(value3)) == NULL) //map has no field value
 return NULL;
 

 return np;

Printing these will give the key pointing the the four values.

key1->value1 value2 value3