Structure demo

Need a little help understanding why the addDecade function doesn't work.

It is supposed to add 10 years to the birthdate, but it doesn't do that.

Code:

---

//structuredemo2.c//demonstrating Structures and functions
#include<stdio.h>
#include<stdlib.h>


struct Date
{
        int Month; //members
        int Day;
        int Year;
};//end structure definition


void AddDecade(struct Date);


int main(int argc, char *argv[])
{
        struct Date BDay;


        char buffer[50];


        printf("What month were you born?  ");
        BDay.Month = atoi(fgets(buffer, 50, stdin));


        printf("What day were you born?  ");
        BDay.Day = atoi(fgets(buffer, 50, stdin));


        printf("What year were you born?  ");
        BDay.Year = atoi(fgets(buffer, 50, stdin));


        printf("You were born on %d, %d, %d.\n", BDay.Month, BDay.Day, BDay.Year);


        AddDecade(BDay);


        printf("You will be 10 years older on %d, %d, %d\n", BDay.Month, BDay.Day, BDay.Year);


}//end main


void AddDecade(struct Date Target)
{
        Target.Year += 10;
}//end AdDecade

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Look up the return value of fgets.

Read from here the swap that uses an extra variable. You will find one that does not use one in the link, but that's not the point. Then try to explain why your code behaves like that.

Quote:

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Originally Posted by Tclausex

Look up the return value of fgets.

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This is not the problem of course. On success, the function returns the buffer. Check it on the manual too!
If you want to do some error handling or you were reading a file, then you would have to change the code at this part.....

You need to pass a pointer to your structure to the function, to modify it.

The way it stands, you're passing the structure itself, which creates a local copy within that function. Any changes you make only occur to that local copy (not the structure in "main()"), then the local copy is destroyed when the function ends.

Quote:

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Originally Posted by std10093

This is not the problem of course. On success, the function returns the buffer. Check it on the manual too!
If you want to do some error handling or you were reading a file, then you would have to change the code at this part.....

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LOL Derp!

Quote:

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Originally Posted by Matticus

You need to pass a pointer to your structure to the function, to modify it.

The way it stands, you're passing the structure itself, which creates a local copy within that function. Any changes you make only occur to that local copy (not the structure in "main()"), then the local copy is destroyed when the function ends.

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Oh it would be better for him to find out himself after checking the swap example I provided.. For sure he has been taught that and would remind him of what is happening (I hope)

Quote:

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Originally Posted by std10093

Oh it would be better for him to find out himself after checking the swap example I provided.. For sure he has been taught that and would remind him of what is happening (I hope)

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Perhaps. Another possibility might be that the OP would gloss over the link and not notice such a subtle clue, since on the surface, a swap example doesn't appear to answer their question.

Code:

---

//structuredemo2.c
//demonstrating functions and functions
#include<stdio.h>
#include<stdlib.h>


struct Date
{
        int Month; //members
        int Day;
        int Year;
};//end structure definition


struct Date AddDecade(struct Date);//can return a date


int main(int argc, char *argv[])
{
        struct Date BDay;//using the datatype we just created


        char buffer[50];//char array of fifty charaters


        printf("What month were you born?  ");//enter what month you were born in
        BDay.Month = atoi(fgets(buffer, 50, stdin));//ascii string to integer takes in input


        printf("What day were you born?  ");
        BDay.Day = atoi(fgets(buffer, 50, stdin));


        printf("What year were you born?  ");
        BDay.Year = atoi(fgets(buffer, 50, stdin));


        printf("You were born on %d, %d, %d.\n", BDay.Month, BDay.Day, BDay.Year);//prints your DOB


        BDay = AddDecade(BDay);//calls function to add ten years to DOB


        printf("You will be 10 years older on %d, %d, %d\n", BDay.Month, BDay.Day, BDay.Year);


}//end main


struct Date AddDecade(struct Date Target)
{
        Target.Year += 10;//add ten years to DOB
        return Target;
}//end AdDecade

---

it works now

Yes it works, but I have to say that you have to think that when you return a struct, you return a copy of it. In cases where the struct is big (like in real programs), you might not want to pay this cose, because coping is linear to amount of information you copy :). On the other hand, passing a pointer is cheap.

But of course, what you have done is correct in terms will it work or not :)

Hey, man. If you know a way to make this work faster and more efficiently, preach on.
THanks

Matticus was right. You didn't check on the link.
Pass a reference like this

Code:

---

#include<stdio.h>
#include<stdlib.h>
 
 
struct Date
{
    int Month; //members
    int Day;
    int Year;
};//end structure definition
 
 
void AddDecade(struct Date*);
 
 
int main(int argc, char *argv[])
{
    struct Date BDay;
 
 
    char buffer[50];
 
 
    printf("What month were you born?  ");
    BDay.Month = atoi(fgets(buffer, 50, stdin));
 
 
    printf("What day were you born?  ");
        BDay.Day = atoi(fgets(buffer, 50, stdin));
 
 
    printf("What year were you born?  ");
        BDay.Year = atoi(fgets(buffer, 50, stdin));
 
 
    printf("You were born on %d, %d, %d.\n", BDay.Month, BDay.Day, BDay.Year);
 
 
    AddDecade(&BDay);
 
 
    printf("You will be 10 years older on %d, %d, %d\n", BDay.Month, BDay.Day, BDay.Year);
 
    return 0;
}//end main
 
 
void AddDecade(struct Date* Target)
{
    Target->Year += 10;
}//end AdDecade

---

The reason is in #10 post.

Actually, I did look at your link. I look at a lot of your code. You are a very skilled programmer and I'm afraid I didn't understand the code.
What is the "->" operator?

That is referred to as the "arrow operator." It's a short cut for referencing a structure member via a pointer to the structure. Consider the following three examples.

Code:

---

// Example 1
// Referencing a structure member locally in "main()" with the "dot operator"

#include <stdio.h>

struct Test
{
    int x;
};

int main(void)
{
    struct Test sTest;

    sTest.x = 2;

    printf("x = %d\n",sTest.x);

    return 0;
}

---

Code:

---

// Example 2
// Referencing a structure member in a function, using the "dot operator"
//    and standard pointer notation

#include <stdio.h>

struct Test
{
    int x;
};

void updateStruct(struct Test *y);

int main(void)
{
    struct Test sTest;

    sTest.x = 2;

    printf("x = %d\n",sTest.x);

    updateStruct(&sTest);

    printf("x = %d\n",sTest.x);

    return 0;
}

void updateStruct(struct Test *y)
{
    (*y).x = 3;  // line of interest
}

---

Code:

---

// Example 3
// Referencing a structure member in a function, using the "arrow operator"

#include <stdio.h>

struct Test
{
    int x;
};

void updateStruct(struct Test *y);

int main(void)
{
    struct Test sTest;

    sTest.x = 2;

    printf("x = %d\n",sTest.x);

    updateStruct(&sTest);

    printf("x = %d\n",sTest.x);

    return 0;
}

void updateStruct(struct Test *y)
{
    y->x = 3;  // line of interest
}

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Compare the "line of interest" from examples 2 and 3.

I'm beginning to understand how this works. I'm going to go over it a little more, but you guys have been a great help. Thanks a lot.