Linked lists and arrays - c

Question

I have to make linked lists that contain all the non zero values from an array and print it out. However, my code is printing out the same value for how many non zero elements there are in the array.
The creation of the linked list and printing of the linked list are two separate functions.
The addLink function creates the linked list
void addlink(DataPtr *start, int element, double value) {
Data last = *start;
Data newPtr;
newPtr = malloc(sizeof(Data));
newPtr->element = element;
newPtr->usage = value;
newPtr->next = NULL;
if(*start == NULL) {
*start = newPtr;
return;
newPtr->element = element;
newPtr->usage = value;
newPtr->next = NULL;
}
while(last->next != NULL) {
last = last->nextPtr;
last->next = newPtr;
return;
}
}
The print function prints the linked list
void print(Data *start) {
Data current = *start;
while(current != NULL) {
printf("%d ", current->element);
printf("%.3lf", current->value);
current = current->next;
}
printf("\n");
}
This is how i call it in my main
for(k = 0; k < 50; k++) {
if(values[k] != 0) {
value = values[k];
addlink(&start,k,value);
print(&start);
}
}
struct data{
int element;
double value;
struct data *next;
};
typedef struct data Data;
typedef Data *DataPtr;
DataPtr start = NULL;

Answer 1

Here is correct implementation of your list:
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
typedef struct data {
int element;
double value;
struct data *next;
} data_t;
void print(data_t *start);
void addlink(data_t **start, int element, double value);
int main()
{
int values[50] = { 0 };
int value = 0;
int k;
data_t *start = NULL;
//Give values some value
values[0] = 100;
values[1] = 250;
for(k = 0; k < 50; k++)
{
if(values[k] != 0)
{
value = values[k];
addlink(&start,k,value);
print(start);
}
}
}
void addlink(data_t **start, int element, double value)
{
data_t *newPtr;
newPtr = malloc(sizeof(data_t));
newPtr->element = element;
newPtr->value = value;
*start = newPtr;
}
void print(data_t *start)
{
data_t *current = start;
while(current != NULL)
{
printf("%d ", current->element);
printf("%f", current->value);
current = current->next;
}
printf("\n");
}
Your DataPtr idea, is no good. The "data_t" implementation is generally what you will see when seeing linked lists. It is easiest implemented this way.
If you are trying to implement a list that adds the newest data_t to the end of the list...
Check out http://www.learn-c.org/en/Linked_lists for further explanation.

Answer 2

A correct function implementation can look the following way
int addlink( DataPtr *start, int element, double value )
{
DataPtr newPtr = malloc(sizeof(Data));
int success = newPtr != NULL;
if ( success )
{
newPtr->element = element;
newPtr->value = value;
newPtr->next = NULL;
while ( *start != NULL ) start = &( *start )->next;
*start = newPtr;
}
return success;
}
If you want to add new elements at the beginning of the list then the function can look like
int addlink( DataPtr *start, int element, double value )
{
DataPtr newPtr = malloc(sizeof(Data));
int success = newPtr != NULL;
if ( success )
{
newPtr->element = element;
newPtr->value = value;
newPtr->next = *start;
*start = newPtr;
}
return success;
}
As for showed by you the function implementation then it shall not even compile. For example in this code snippet
Data last = *start;
Data newPtr;
newPtr = malloc(sizeof(Data));
the expression *start has the type DataPtr while the initialized variable last has the type Data. The same problem exists for the variable newPtr.
The function print can look like
void print( DataPtr *start )
{
for ( DataPtr current = *start; current != NULL; current = current->next )
{
printf("%d ", current->element);
printf("%.3lf", current->value);
}
printf("\n");
}
And it is called like
print( &start );