CODEWITHSUNDEEP
carraysstruct
lolly22
lolly22

Reputation: 21

Writing an array structure to file

I am trying to set up an array of structs for a few sample parts which then need to output to a file. It seems to work ok but will only print the first 3 parts and then crashes.

    #include "stdio.h"

main()
{
struct stock{
    char name[20];
    int partNum;
    int quantity;
};

struct stock someStock[3];

strcpy(someStock[0].name, "License plate");
someStock[0].partNum = 1234;
someStock[0].quantity= 4;

strcpy(someStock[1].name, "Head lamp");
someStock[1].partNum = 1111;
someStock[1].quantity= 2;

strcpy(someStock[2].name, "Rear wiper");
someStock[2].partNum = 2222;
someStock[2].quantity= 6;

strcpy(someStock[3].name, "Tyres");
someStock[3].partNum = 3333;
someStock[3].quantity= 10;

struct stock *ptr = &someStock[0];
int i;
FILE *file_ptr;
file_ptr = fopen("stock.dat", "w");

for(i=0;i<4;i++)
{
    fprintf(file_ptr, "%s %d %d\n",(*ptr).name,(*ptr).partNum, (*ptr).quantity);
    ptr++;
}

fclose(file_ptr);
return 0;

}

Upvotes: 1

Views: 169

Answers (3)

weston
weston

Reputation: 54781

As others said:

struct stock someStock[4];

Some other tips for you:

struct stock *ptr = &someStock[0];

Is equivalent to:

struct stock *ptr = someStock;

As Daniel pointed out this is since the array name evaluates to a pointer to its first element in that context.

And:

fprintf(file_ptr, "%s %d %d\n",(*ptr).name,(*ptr).partNum, (*ptr).quantity);

Is equivalent to:

fprintf(file_ptr, "%s %d %d\n", ptr->name, ptr->partNum, ptr->quantity);

-> is known as the structure pointer operator, it dereferences a pointer and accesses a member all in one, as oppose to the . which is the structure member operator and so requires the indirection operator * as well when you have a pointer.

Upvotes: 1

Muhammad Usama
Muhammad Usama

Reputation: 3107

struct stock someStock[3]; means you have three elements in array so the array index can be from 0 to 2 so strcpy(someStock[3].name, "Tyres"); is crashing your program as 3 is out of range array index

Upvotes: 0

Daniel Fischer
Daniel Fischer

Reputation: 183858

struct stock someStock[3];

declares an array of 3 struct stock, but you're using four. The number in brackets is the number of elements, not the highest valid index. Declare

struct stock someStock[4];

Upvotes: 4

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