Thread: Functions, pointers, structures - can't figure this out Newb

Not sure what is wrong here, I can't seem to compile. I am trying to pass the structure into the function to just print the list. I was asked to use "pass by value" instead of structure. Help is much appreciated, I am new to this and just read the chapter...

Also, is my structure initialization correct? Should I initialize it outside of the main function?

Code:

#include <stdio.h>
#include <string.h>

#define MAX 61
#define SIZE 5

struct name {
    char fname[MAX];
    char middle[MAX];
    char lname[MAX];
};

struct sin {
    int snumber;
    struct name info;
};

void printNames(struct sin * temp[]);

int main(void)
{
    int i = 0;
    struct sin * ptrData;

    struct sin data[SIZE] = {
        {302039823, {"Flossie", "Marie", "Dribble"}},
        {123456789, {"DeMar", "Test", "DeRozan"}},
        {987654321, {"John", "", "Doe"}},
        {736491092, {"Jane", "Cherry", "Dairy"}},
        {282398624, {"Andrea", "James", "Bargnani"}}
    };

    //for (i = 0; i < SIZE; i++)
       // ptrData[i] = &data[i];

    ptrData = &data;

    printNames(ptrData);


    return 0;
}

void printNames(struct sin * temp[])
{
    int i = 0;

    for(i = 0; i < SIZE; i++)
    {
        if(temp[i]->info.middle[0] != '\0')
        {
            printf("%s, %s %c. -- %d\n", temp[i]->info.lname, temp[i]->info.fname, temp[i]->info.middle[0], temp[i]->snumber);
        }
        else
        {
            printf("%s, %s %*-- %d\n", temp[i]->info.lname, temp[i]->info.fname, temp[i]->info.middle[0], temp[i]->snumber);
        }
    }
}

I wouldn't call you a newb per say, you've got the indentation/int main/return 0/pretty much everything else correct.

As for the actual passing by value, you want to declare your function so it can accept an array:

Code:

void printNames(struct sin temp[]);

Then, you can just pass "data" to the function without the pointer:

Code:

printNames(data);

----

By the way, your pointer initialization quite correct; because C treats pointers so similarly to arrays, there are two ways to do it:

Code:

ptrData = data; /* this is really all you need */
ptrData = &data[0]; /* it's pointing to the address of the first element, does the same thing */

Thanks for the reply memcpy.

Maybe I am not understanding my exercise properly.

Here is the question, I am trying to do part b. I did part a and declared my function to take a structure array but I don't/can't do part b. I thought I was suppose to do it using pointers

Write a program that creates a structure template with two members according to the following criteria:

a) The first member is a social security number. The second member is a structure with three members. Its first member contains a first name, its second member contains a middle name, and its final member contains a last name. Create and initialize an array of five such structures. Have the program print the data in this format:

Dribble, Flossie M. -- 302039823

Only the initial letter of the middle name is printed, and a period is added. Neither the initial (of course) nor the period should be printed if the middle name member is empty. Write a function to do the printing; pass the structure array to the function.

b) Modify part "a" by passing the structure value instead of the address.

Am I missing something, or is:

pass the structure array to the function

and:

assing the structure value instead of the address

the same thing?

---------------------

The closest thing to "passing by value" in regards to an array is this:

Code:

void printNames(struct sin temp[]);

struct sin temp[];
printNames(temp);

And passing by reference is this:

Code:

void printNames(struct sin *temp[]);

struct sin temp[];
printNames(&temp);

Ok so I am not that stupid then haha. I thought I didn't understand what the question was asking. It is doing something redundant but worded different.

Either case, I would still like to know why my code does not work why I try to pass by reference. It is very similar to what you mentioned but I get compile issues.

Code:

#include <stdio.h>
#include <string.h>

#define MAX 61
#define SIZE 5

struct name {
    char fname[MAX];
    char middle[MAX];
    char lname[MAX];
};

struct sin {
    int snumber;
    struct name info;
};

void printNames(struct sin * temp[]);

int main(void)
{
    int i = 0;
    struct sin * ptrData;

    struct sin data[SIZE] = {
        {302039823, {"Flossie", "Marie", "Dribble"}},
        {123456789, {"DeMar", "Test", "DeRozan"}},
        {987654321, {"John", "", "Doe"}},
        {736491092, {"Jane", "Cherry", "Dairy"}},
        {282398624, {"Andrea", "James", "Bargnani"}}
    };

    //for (i = 0; i < SIZE; i++)
       // ptrData[i] = &data[i];

    //ptrData = data;

    printNames(&data);  //changed to &data instead of assigning another pointer but I still can't get it to work


    return 0;
}

void printNames(struct sin * temp[])
{
    int i = 0;

    for(i = 0; i < SIZE; i++)
    {
        if(temp[i]->info.middle[0] != '\0')
        {
            printf("%s, %s %c. -- %d\n", temp[i]->info.lname, temp[i]->info.fname, temp[i]->info.middle[0], temp[i]->snumber);
        }
        else
        {
            printf("%s, %s %*-- %d\n", temp[i]->info.lname, temp[i]->info.fname, temp[i]->info.middle[0], temp[i]->snumber);
        }
    }
}

Your printf on line 56 is messed up, double check the format string.

And if your compiler is complaining about passing the struct, you can just typecast it:

Code:

printNames((struct sin **)&data);

In case you're wondering why I'm typecasting to a double pointer not a pointer to an array, it's because, technically, whenever you pass an array in C, you're only passing a pointer to it. Typecasting to a double pointer is perfectly fine in that respect.

Now my program crashes after the type cast

Regarding this part b:

b) Modify part "a" by passing the structure value instead of the address.

I have not seen that above. All are by address and can be incremented.

Thoughts:
Unlike arrays, a struct can be copied by assignment.
When you pass an object by value a copy is provided to the function.