pointer and struct issue

Question

function find_young() have to receive pointer p as an actual parameter. And p have to point a struct s which has youngest person. There is no error message but the program isn't work. Please give me some advice.

typedef struct
{
    char *name;
    int age;
} PERSON;

void find_young(PERSON **ip)
{
    PERSON s[3] = {{"John", 25}, {"Brandon", 28}, {"Alex", 30}};
    int i;
    int min = s[0].age; 
    for(i = 0; i < 3; i++)
    {
        if(min > s[i].age)
            **ip = s[i];
    }
}

int main()
{
    PERSON *p;
    find_young(&p);
    printf("The youngest person is %s.\n", p->name);
}

a advice PERSON *p; --> PERSON p; also s[0] is not set when the minimum. — BLUEPIXY
– BLUEPIXY, Commented Nov 30, 2015 at 7:52
Your pointer has not been set should pointer point to the Object(PERSON Object). E.g PERSON aMan; PERSON *p = &aMan; — BLUEPIXY
– BLUEPIXY, Commented Nov 30, 2015 at 8:11

Haris · Accepted Answer · 2015-11-30 08:20:12Z

2

You need to make a number of changes in your code to make it work

1) You need to allocate memory to the struct, and for the char* name in the struct

int main()
{
    PERSON *p = malloc(sizeof(PERSON));
    p->name = malloc(100);
    find_young(&p);
    printf("The youngest person is %s with age %d.\n", p->name, p->age);

    free(p->name);
    free(p);
}

2) Then you need to copy both the name and age values to the passed struct properly

void find_young(PERSON **ip)
{
    PERSON s[3] = {{"John", 25}, {"Brandon", 28}, {"Alex", 30}};
    int i;

    int min = s[0].age;             //copying the first value
    (**ip).age = s[0].age;
    strcpy((**ip).name, s[0].name);

    for(i = 1; i < 3; i++)          //starting from the next values to compare
    {
        if(min > s[i].age)
        {
            (**ip).age = s[i].age;
            strcpy((**ip).name, s[i].name);
        }
    }
}

edited Nov 30, 2015 at 8:20

answered Nov 30, 2015 at 7:52

Haris

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2 Comments

Magisch Over a year ago

Mind you its best practice to include free() statements after the printf in the main() function. Otherwise if the program doesn't terminate after that you have a memory leak.

Haris Over a year ago

@Magisch, I always miss that. :)

Sourav Ghosh · Accepted Answer · 2015-11-30 07:52:14Z

0

In your code,

either, for p, before passing &p to the function
or, for *ip, inside the function

you need to allocate memory. Other wise, you'll end up dereferencing invalid pointer, which invokes undefined behavior.

answered Nov 30, 2015 at 7:52

Sourav Ghosh

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