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Project Notes

Implementing a Dictionary

An example of using struct-based C to implement a somewhat object-oriented dictionary.


This is based on an example from 21st Century C Chapter 11: Object-Oriented Programming in C, Implementing a Dictionary, p210.

It is not intended to be ready for prime-time; this problem has been solved before and there a re reliable implementations available such as GHashTable.

A dictionary (aka hash table) is an easy structure to generate, given what we have in struct-based C. It represents associations between keys and values so that given a key the value can be found quickly.


keyval.{h,c} implements the core key-value data structure and some supporting manipulation methods. It just stores the name and a pointer to the value:

typedef struct keyval {
   char *key;
   void *value;
} keyval;

dict.{h,c} is the dictionary implementation. It holds the list of keyval and keeps track of the list length:

typedef struct dictionary {
   keyval **pairs;
   int length;
} dictionary;

dict_use.c demonstrates the use. The basics:

int zero = 0;
dictionary *d = dictionary_new();
dictionary_add(d, "an int", &zero);
printf("The integer I recorded was: %i\n", *(int*)dictionary_find(d, "an int"));

Building with Make

I’ve added a Makefile that builds the project with minimal fuss. It produces two binaries: dict_test and dict_use.

$ make all
$ ./dict_test
/set1/new test: OK
/set1/copy test: OK
/set2/fail test: OK
$ ./dict_use
Finding: 'key1' (int)    .. found: 1
Finding: 'key2' (float)  .. found: 2.000000
Finding: 'key3' (string) .. found: three
Finding: 'bogative'      .. not found

Building with Automake

The script wraps up the commands to build with Automake. This will copy and re-generate the project in a sub-folder called hash.

$ ./
ALL DONE! Trying it out...
The integer I recorded was: 0
The string was: two

Credits and References

About LCK#92 C
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This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

LittleCodingKata is my collection of programming exercises, research and code toys broadly spanning things that relate to programming and software development (languages, frameworks and tools).

These range from the trivial to the complex and serious. Many are inspired by existing work and I'll note credits and references where applicable. The focus is quite scattered, as I variously work on things new and important in the moment, or go back to revisit things from the past.

This is primarily a personal collection for my own edification and learning, but anyone who stumbles by is welcome to borrow, steal or reference the work here. And if you spot errors or issues I'd really appreciate some feedback - create an issue, send me an email or even send a pull-request.