This is the post I’m keeping on the down-low since my site is 99% about Python.  Along the way, in order to better understand the underpinnings of Python (which was written in C as were other languages), I decided I need to learn enough C to be able to function the in the language.  I’m taking a course from Udemy but the instruction leaves a lot to be desired and for every hour spent listening to lectures I’m spending about 4 or 5 reading stuff on the internet pertaining to the current subject of study. One big hurdle was trying to integrate my understanding of how pointers, structures and functions work together.  I could never find a single, unified example.  So here I present one so I will know at least one coherent example exists on the internet.

/* USING POINTERS WITH STRUCTURES AND FUNCTIONS */
#include <stdio.h>
#include <stdlib.h>

/* Declare struct structure first */
struct item {
char name[30]; /* Could create a pointer here with: char *itemName; */
               /* but if we do it that way we will need malloc to allocate later */
               /* the advantage to creating just a pointer is we do not use up memory here */
int quantity;
float price;
float amount;
};

/* ...then declare function - struct must already exist */
void readItem(struct item *); /* * says pass a pointer - could add tag name, ex: *pMyPtr
                                 could make it a constant with (const struct item *pMyPtr) */

int main()
{
struct item itemx, *itemPtr;    /* Create struct instance and pointer */
itemPtr = &itemx;               /* initialize pointer to struct INSTANCE */
                           /* if only created pointer above, we would use malloc here with: */
                           /* itemPtr->name = (char *) malloc(30 * sizeof(char));
                           /* then check for null with:
                                  if(itemPtr == NULL)
                                       exit(-1);
                           */
readItem(itemPtr);     /* call function - then print proof of performance */
printf("\nReturned from function\nData inserted by pointer: \n");
printf("Name: %s\n", itemx.name);
printf("Price: %.2f\n", itemx.price);
printf("Quantity: %d\n", itemx.quantity);
printf("Amount (from pointer): %.2f\n", itemx.amount);
printf("Compare to Amount calclated: %.2f\n", itemx.price * itemx.quantity);

free(itemPtr->name);     /* need to free memory if we allocate it */

return 0;
}

void readItem(struct item *itemPtr)
{
printf("As prompted, please enter name, price, and quantity.\n");
printf("Item name: ");    /* note no '\n needed in inputs */
gets(itemPtr->name);    /* gets captures strings with spaces
                           scanf would be: scanf("%s", itemPtr->name); */
printf("Enter price: ");
scanf("%f", (&itemPtr->price));
printf("Enter quantity: ");
scanf("%d", (&itemPtr->quantity));

/* Demo pointer efficacy */
printf("\nName: %s\n", itemPtr->name);         /* the -> operator selects element and deref's pointer */
printf("Price: %.2f\n", (*itemPtr).price);     /* alternative construction not using the -> operator */
printf("Quantity: %d\n", itemPtr->quantity);
printf("\nTesting __________\n");
printf("Pointer arithmetic: \n");
float sum = (itemPtr->price)*(itemPtr->quantity);
printf("sum is: %.2f\n", sum);      /* could assign amount directly with */
itemPtr->amount = sum;              /* itemPtr->amount = (float)itemPtr->quantity * itemPtr->price; */
return;
}