Structure Arranging in C - c

Question

I am trying to make a program that will arrange my structures by people ids.
I used long double because IDs are 20 digits long.
For example if I introduce 3 persons:
1.Alex Alex / id = 219(...)
2.John John / id = 200(...)
3.Robert Robert / id = 199(...)
I want my program to rearrange so Robert comes first, John second and Alex third.
I am having a problem on "for" structure - I don't exactly know how to swap two structures since I have chars and ints combined.
typedef struct
{
char name;
char prename;
long double id;
int j;
} PERSON;
int main()
{
int n,i,j;
printf ("How many people = ");
scanf("%d", &n);
PERSON v[n];
for(i=1;i<=n;i++)
{
printf("For person number nr. %d\n", i);
printf("name = ");
scanf("%s", &v[i].name);
printf("Prename = ");
scanf("%s", &v[i].prename);
printf("id = ");
scanf("%d", &v[i].id);
}
for(int i=0; i<n; i++)
{
for(int j=0; j<n-1; j++)
{
if( v[i].id > v[j+1].id )
{
int temp = v[j].id;
char temp2[100];
char temp3[100];
strcpy(v[j].prename,temp3);
strcpy(v[j].name,temp2);
v[j].id = v[j+1].id;
v[j+1].id = temp;
}
}
}
return;
}

Answer 1

Ok, because there is so many things you've done here that are not obvious to a new C developer, I want to point them out to help you learn:
typedef struct
{
// These need to be arrays if they are to be strings
// Preferably using a constant for size or dynamic
// if you want them to be variable sized to match input.
char name;
char prename;
// Do not use floating point numbers to represent integer values.
// IRL, you'd use a library, but here, you may want to use an array of
// some sort of integer type instead.
long double id;
// This is a really poor name for a struct variable and probably shouldn't be here.
int j;
} PERSON;
int main()
{
int n,i,j;
printf ("How many people = ");
// Dropping raw output from scanf into a memory allocation is crazy dangerous.
// At least error check the results to be sure it is meaningful.
scanf("%d", &n);
// This is a variable length array and often is not supported directly.
// You probably want to use a malloc()/free() pair to handle this.
// (Update: VLA is now part of newer standards, but are not safe because they cannot fail gracefully on out of memory.)
PERSON v[n];
// Anytime in C I see an array start at 1 and use <= for condition, I get very nervous because
// it tends to lead to indexing errors.
for(i=1;i<=n;i++)
{
printf("For person number nr. %d\n", i);
printf("name = ");
// Oops - and this is why. You just skipped the first entry at 0
// and will overwrite memory on the last loop.
// Also, scanf into a string without a length can blow up memory...
scanf("%s", &v[i].name);
printf("Prename = ");
// Ditto
scanf("%s", &v[i].prename);
printf("id = ");
// Ditto - worse because you've crossed your types - %d doesn't go into a long double.
scanf("%d", &v[i].id);
}
// Should be its own function to make it easier to swap later to a better sort.
for(int i=0; i<n; i++)
{
// Bubble sort usually wants j=i here.
for(int j=0; j<n-1; j++)
{
if( v[i].id > v[j+1].id )
{
// Make a swap function here. Makes it clearer what you want to do.
int temp = v[j].id;
// What is 100? How do you know that is enough?
// These are called magic numbers and lead to code death.
char temp2[100];
char temp3[100];
// Ah, strcpy - 3 things wrong here.
// 1 - You have the parameters backwards - you are copying temp3 to your struct.
// 2 - You have no way to know if the destination will fit the source because it copies until it finds a '\0' - very dangerous.
// 3 - Because your parameters are backwards and temp123 is not initialized, this very well could copy forever.
// strncpy (also crazy dangerous) at the least should be used and consider using better means like strlcpy() and such.
strcpy(v[j].prename,temp3);
strcpy(v[j].name,temp2);
v[j].id = v[j+1].id;
v[j+1].id = temp;
// You kinda forgot to swap the strings, but the program is already dead so no worries.
}
}
}
// Please enable compiler warnings - you must return a value here.
return;
}
In seriousness, I'm sure I missed a few other things, but this is good enough for a free internet code review and learning session. :)
Info on strcpy() and strncpy()
Why is strncpy insecure?
Info on scanf() safety
How to prevent scanf causing a buffer overflow in C?
Info on Variable Length Array safety:
Is it safe to use variable-length arrays?

Answer 2

The main problem with your code is here:
typedef struct
char name; <--- just a char! no room for a string
char prename; <--- just a char! no room for a string
long double id;
int j;
} PERSON;
You need to make these arrays so that they can hold the names.
Like:
#define MAX_NAME_LEN 100
typedef struct {
char name[MAX_NAME_LEN];
char prename[MAX_NAME_LEN];
long double id;
int j;
} PERSON;
Besides that, you should always check the value returned by scanf and never do scanf("%s", &v[i].prename); as it may lead to buffer overflow. Instead do scanf("%99s", v[i].prename); but even better use fgets
And for sorting... just use qsort - see http://man7.org/linux/man-pages/man3/qsort.3.html
BTW: A long double can't be scan'ed using scanf("%d", &v[i].id); - %d is for integers. That said, I doubt you want to use long double. You probably want some interger type - maybe long long unsigned or uint64_t