When writing code in a high level scripting language like Perl, Ruby or Python it is easy to take for granted what a nice feature it is to have automatic types like arrays or hashs. Variable that you can just dump data in to and pull it back out any way you like. These features were not always there, someone had to write the code to make it to this.
A linked list in C is a way to store values, as many as you like for as much memory as you have. Memory for a variable is created on the fly as needed and stored in a location that knows information about it's neighbour. A good linked list will have features to let you search, insert, delete from any point, move backwards and forwards in the list and most importantly a good list would protect your code from accidental memory bugs. My example today has non of those features.
This is the bare minimum of a simple linked list. It is a single linked list so you can only move forward. A double linked list lets you move backwards also.
#include<stdio.h>
#include<stdlib.h>
struct DataNode {
void *Load;
struct DataNode *Next;
};
struct DataNode *TheFirst = NULL;
struct DataNode *Current = NULL;
struct DataNode *Last = NULL;
void* NewList(void *Load) {
struct DataNode *Pointer = (struct DataNode*)malloc(sizeof(struct DataNode));
if(NULL == Pointer) {
return NULL;
}
Pointer->Load = Load;
Pointer->Next = NULL;
TheFirst = Last = Current = Pointer;
return(Pointer);
}
void* Add(void *Load) {
if(TheFirst == NULL) {
return (NewList(Load));
}
struct DataNode *Pointer = (struct DataNode*)malloc(sizeof(struct DataNode));
Pointer->Load = Load;
Pointer->Next = NULL;
Last->Next = Pointer;
Current = Last = Pointer;
return(Pointer);
}
void* First() {
Current = TheFirst;
if(Current == NULL) return(NULL);
return(Current->Load);
}
void* GetNext() {
struct DataNode *Pointer = Current;
// Advance to next
if(Pointer->Next) {
Current = Pointer->Next;
return(Current->Load);
}
return(NULL);
}
int DeleteFirst() {
int Return = 0;
if(TheFirst) {
if(TheFirst->Next) {
Current = TheFirst->Next;
Return = 1;
} else {
Current = NULL;
}
}
free(TheFirst);
TheFirst = Current;
return(Return);
}
int main(void) {
char* ToDoList;
Add("Pet the dog");
Add("Feed the fish");
Add("Let the cat out");
Add("Do the dishes");
Add("Gas up the car");
Add("Watch some TV");
ToDoList = First();
while(ToDoList != NULL) {
printf("%s\n", ToDoList);
ToDoList = GetNext();
}
while(DeleteFirst()) {}
return 0;
}
