The difference between fgets() and scanf() - c

Question

This is my code. I didnt want to use scanf() to read the name and I tried using fgets() but after I put the first name and age, the second and third time my program runs the for loop it doesnt take age .
#include <stdio.h>
#include <string.h>
struct student
{
char name[15];
int age;
};
int main()
{
int i;
struct student s[A];
for (i = 0 ; i < A ; i++)
{
printf("enter the names and age\n");
fgets(s[i].name, 10, stdin);
scanf("%d", &s[i].age);
}
printf("\n\n");
for (i = 0 ; i < A ; i++)
{
printf("%s\t%d\n", s[i].name, s[i].age);
}
return 0;
}
It doesnt work, why?
But when I replace fgets with
scanf("%s%d",s[i].name,&s[i].age);
It works fine

Answer 1

The difference between fgets() and scanf()
fgets(...) typically reads until receiving a '\n'
scanf("%d", ...) typically:
1. Reads and discards leading white-space.
2. Reads numeric input (sign,digits) until scanning a non-digit.
3. Non-digit is put back into stdin for the next input function.
Example:
JohnEnter
"John\n" is read by fgets() into s[0].name.
21Enter
21 is read by scanf("%d",...) into s[0].age. '\n' put back into stdin
"\n" is read by fgets() into s[1].name.
M
"M" is read by scanf("%d",...), nothing is put in s[1].age. 'M' put back into stdin.
aryEnter
"Mary\n" is read by fgets() into s[2].name.
19Enter
19 is read by scanf("%d",...) into s[2].age. '\n' put back into stdin
"\n" is read by fgets() into s[3].name.
Alternative: To read 2 lines, call fgets() twice, then parse:
int Scan_student(struct student *dest) {
char buffer[2][80];
dest->name[0] = '\0';
dest->age[0] = -1;
printf("enter the names and age\n");
for (int i=0; i<2; i++) {
if (fgets(buffer[i], sizeof buffer[i], stdin) == NULL) {
return EOF; // stdin was closed, no more input (or input error)
}
buffer[i][strcspn(buffer[i], "\r\n")] = '\0'; // lop off potential trailing \n
}
// parse the 2 buffers: MANY options here - something simple for now.
if (sscanf(buffer[0], " %14[-'A-Za-z ]", dest->name) != 1) {
return 0;
}
if (sscanf(buffer[1], "%d", &dest->age) != 1) {
return 0;
}
return 1;
}
int i;
struct student st[3];
for (i = 0 ; i < sizeof(st) / sizeof(st[0]) ; i++) {
if (Scan_student(&st[i]) != 1) break;
}

Answer 2

If you input both the name and the age in a single line, then that's the normal behavior because fgets() will read the whole line until 9 bytes (in your case) are read or a '\n' is found.
You need one of the two, for instance you could only use fgets() like this
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct student
{
char name[100];
int age;
};
int main()
{
int i;
struct student s[1];
for (i = 0 ; i < sizeof(s) / sizeof(*s) ; i++)
{
char number[100];
char *unconverted;
printf("Name > ");
fgets(s[i].name, sizeof(s[i].name), stdin);
if ((unconverted = strchr(s[i].name, '\n')) != NULL)
*unconverted = '\0'; // Remove the trailing '\n'
printf("Age > ");
fgets(number, sizeof(number), stdin);
s[i].age = strtol(number, &unconverted, 10);
if ((*unconverted != '\0') && (*unconverted != '\n'))
s[i].age = -1; // Invalid value indicating input error
}
for (i = 0 ; i < sizeof(s) / sizeof(*s) ; i++)
printf("Name: %s\nAge : %d\n", s[i].name, s[i].age);
return 0;
}
Or scanf() only
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct student
{
char name[100];
int age;
};
int main()
{
int i;
struct student s[1];
for (i = 0 ; i < sizeof(s) / sizeof(*s) ; i++)
{
printf("Name Age > ");
if (scanf("%99s%d", s[i].name, &s[i].age) != 2)
{
fprintf(stderr, "input error\n");
s[i].name[0] = '\0';
s[i].age = -1;
}
}
for (i = 0 ; i < sizeof(s) / sizeof(*s) ; i++)
printf("Name: %s\nAge : %d\n", s[i].name, s[i].age);
return 0;
}
and you can then input both name and age in a single line.
The fgets() method is better because you don't need to deal with the '\n' that scanf() doesn't pick up from stdin. A combination would work if you are careful to force the user to input the values in separate lines.

Answer 3

To be honest fgets() isn't required here as you've specified to read from stdin you should simply use gets() this reads from stdin by default. If your moving between a scanf() statement and a gets() statement you should use fflush(stdin) to clear out the input stream, example below:
scanf("%99s", s[i].name);
fflush(stdin);
gets(s[i].age);
In general your better sticking with either scanf() or gets() and not combining them.