How to modify a value of a struct within a function? [C]

Question

Let's see this code as an example

#include <stdio.h>
typedef struct 
{
    int hp;
}Player;

void lessHp(Player a)
{
    printf("a.hp = %d\n", a.hp);
    (*Player) -> hp -= 1;
    printf("a.hp now = %d\n", a.hp);
}

int main()
{
    Player a;
    a.hp = 1;
    lessHp(a);
    printf("a.hp = %d\n", a.hp);
    return 0;
}

Now, what this program prints is:

a.hp = 1
a.hp now = 0
a.hp = 1

But how can I make it so that the lessHp function would actually be able to substract 1 from that value? When trying to do it by reference it tells me to use ("->"), but I really, really don't know what that is (I've only used simple pointers, the only thing that I've handled with pointers is dynamic memory allocation).

C passes arguments by value. When you give just a struct, it passes the value of the struct (a copy of it). Giving a pointer passes the location of your original struct, so you can write modifications at that location. — e0k
– e0k, Commented Apr 29, 2018 at 3:18
What is this (*Player) -> hp -= 1;? This does not compile, doesn't it? — alk
– alk, Commented Apr 29, 2018 at 10:08

Rivasa · Accepted Answer · 2018-04-29 03:16:05Z

2

You need to use a pointer instead of passing a copy. (That is you should be editing original.) You can fix like so:

#include <stdio.h>
typedef struct 
{
    int hp;
} Player;

void lessHp(Player* a)
{
    printf("a.hp = %d\n", a->hp);
    a->hp -= 1;
    printf("a.hp now = %d\n", a->hp);
}

int main()
{
    Player a;
    a.hp = 1;
    lessHp(&a);
    printf("a.hp = %d\n", a.hp);
    return 0;
}

With an output of:

a.hp = 1
a.hp now = 0
a.hp = 0

answered Apr 29, 2018 at 3:16

Rivasa

6,7803 gold badges40 silver badges67 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Yoakain Over a year ago

Thank you. You saved me a lot of hassle.

Jacek Cz Over a year ago

@Yoakain opening a book in ten first pages takes less time, than wainting help from web

Yoakain Over a year ago

I read a lot before asking for help, did not find my answer + I needed it quick. Your advice could've been helpful otherwise.

Collectives™ on Stack Overflow

How to modify a value of a struct within a function? [C]

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related