Problem reading in string in C for dynamic number inputs - c

Question

I'm a Noob, struggling with C. I'm having a hard time wrapping my head around pointers, arrow -> and dot . notation. I am working on a very simple program that has 1 struct, but I am unsure of how to get users to input strings without using the <cs50.h> header (which I need to understand for the harder problem I am also trying to wrap my head around).
//#include <cs50.h>
#include <stdio.h>
#include <string.h>
// create a struct, call it pupils
typedef struct
{
char *name;
char *dorm;
}
pupils;
int main(void)
{
// Allocate space for students - generally dynamic, here static
int enrollment = 2;
// create variable 'idinfo', which contains (enrollment) number of structs of type pupils
pupils idinfo[enrollment];
// Prompt for students' names and dorms
for (int i = 0; i < enrollment; i++)
{
// Not the way to do this given various error codes.....
char idinfo[i].Name[20];
char Dorm[20];
// gets idinfo[i].Name
scanf("%s", idinfo[i].Name);
//idinfo[i].name = printf("%s", Name);
// Below syntax works fine when <string.h> header included
idinfo[i].dorm = get_string("Dorm: ");
}
// Print students' names and dorms
for (int i = 0; i < enrollment; i++)
{
printf("%s is in %s.\n", idinfo[i].name, idinfo[i].dorm);
}
}

Answer 1

Assuming you do not want to use the cs50 magic, you will have to allocate and deallocate strings. From your code I will also assume that the input strings are supposed to be shorter than 20 characters. The following code does not even ensure that it is true but will not crash either but will simply give unexpected results.
Code could be:
int main(void)
{
// Allocate space for students - generally dynamic, here static
int enrollment = 2;
// create variable 'idinfo', which contains (enrollment) number of structs of type pupils
pupils idinfo[enrollment];
// Prompt for students' names and dorms
for (int i = 0; i < enrollment; i++)
{
// allocate memory for idinfo[i] pointers:
idinfo[i].name = malloc(20);
scanf("%19s", idinfo[i].name);
idinfo[i].dorm = malloc(20);
scanf("%19s", idinfo[i].dorm);
}
// Print students' names and dorms
for (int i = 0; i < enrollment; i++)
{
printf("%s is in %s.\n", idinfo[i].name, idinfo[i].dorm);
}
// free allocated memory
for (int i = 0; i < enrollment; i++)
{
free(idinfo[i].name);
free(idinfo[i].dorm);
}
}

Answer 2

You probably want something like this:
#include <cs50.h> // needed for get_xxx functions and other cs50 stuff
...
// Prompt for students' names and dorms
for (int i = 0; i < enrollment; i++)
{
idinfo[i].name = get_string("Name: ");
idinfo[i].dorm = get_string("Dorm: ");
}
But this should be covered in the documentation you were given.
My advice is not using scanf at all but using only the cs50 get_xxx input methods.
And beware: the cs50 string is not a real "string" type but merely the same thing as char *.

Answer 3

Thanks to Serge, I was able to get my code compiling, which is below.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
// create a struct, call it pupils
typedef struct
{
char *name;
char *dorm;
}
pupils;
int main(void)
{
// Allocate space for students - generally dynamic, here static
int enrollment = 2;
// create variable 'idinfo', which contains (enrollment) number of structs of type pupils
pupils idinfo[enrollment];
// Prompt for students' names and dorms
for (int i = 0; i < enrollment; i++)
{
// allocate memory for idinfo[i] pointers:
idinfo[i].name = malloc(20);
idinfo[i].dorm = malloc(20);
printf("Enter student %d's Name ", i+1);
scanf("%19s", idinfo[i].name);
printf("Enter student %d's dorm ", i+1);
scanf("%19s", idinfo[i].dorm);
}
// Print students' names and dorms
for (int i = 0; i < enrollment; i++)
{
printf("%s is in %s.\n", idinfo[i].name, idinfo[i].dorm);
}
// free allocated memory
for (int i = 0; i < enrollment; i++)
{
free(idinfo[i].name);
free(idinfo[i].dorm);
}
}