Printing out values in an array not working as expected - c

Question

I was tasked with inputting student information based on a given struct, where each field of information is to be typed in one line, separated by a space, then the student id is sorted incrementally, and then print out the information, each student on a new line. The problem is while I thought my code was good, the print part keeps giving fractured results and overall just not printing out the correct values. Where should I fix this?
Here's my code:
#include <stdio.h>
typedef struct
{
char id[8];
int year;
}student;
int main() {
student std[100];
int i, j, num, tmp;
printf("So sinh vien:\n");
scanf("%d", &num);
printf("Nhap thong tin sinh vien:\n");
for(i=0; i <= num; i++)
{
scanf("%c %d\n", &std[i].id, &std[i].year);
}
for(i=0; i < num; i++)
{
for (j=1; j< num; j++)
{
if (std[i].id > std[j].id)
{
tmp = *std[i].id
*std[i].id = *std[j].id;
*std[j].id = tmp;
}
}
}
for(i=0; i < num; i++)
{
printf("%c ", std[i].id);
printf("%d\n", std[i].year);
}
return 0;
}
My output is
So sinh vien:
3
Nhap thong tin sinh vien:
12324521 2003
12341552 2002
12357263 2001
Σ 12324521
≡ 3
ⁿ 2341552

Answer 1

Check the return value of scanf() otherwise you may be operating on uninitialized variables.
Check that num less than the 100 records you allocated for student, or even better use a vla along with a check to avoid large input from smashing the stack.
You input num then read num+1 records but later you only print num records.
As you read a character with "%c" the first input with be the \n from the previous scanf().
The struct contains a char id[8] but you only read a single character into it. Read a string instead.
In sort you use > to compare the first letter of id. You probably want to use strcmp() to compare strings.
In sort section you use a int tmp for storing a character of id (which is ok) but then you write an int which is no good.
In sort you only swap the ids. You probably want to swap the entire record not just the ids.
It seems to be an exchange sort. Use a function, and also at least for me the algorithm didn't work as the inner loop variable should start at j=i+1 not 1.
In your print char id[8] as a single char instead of string.
Moved print functionality to a function. This allows you, for instance, to print the students before and after the sort() during debugging.
Minimizing scope of variables (i and j are now loop local, tmp is only used in the swap() function). This makes code easier to reason about.
#include <stdio.h>
#include <string.h>
#define ID_LEN 7
#define str(s) str2(s)
#define str2(s) #s
typedef struct {
char id[ID_LEN+1];
int year;
} student;
void swap(student *a, student *b) {
student tmp = *a;
*a = *b;
*b = tmp;
}
void print(size_t num, student std[num]) {
for(size_t i=0; i < num; i++)
printf("%s %d\n", std[i].id, std[i].year);
}
// exchange sort
void sort(size_t num, student std[num]) {
for(size_t i=0; i < num - 1; i++)
for (size_t j=i+1; j < num ; j++)
if(strcmp(std[i].id, std[j].id) > 0)
swap(&std[i], &std[j]);
}
int main() {
printf("So sinh vien:\n");
size_t num;
if(scanf("%zu", &num) != 1) {
printf("scanf() failed\n)");
return 1;
}
if(num > NUM_MAX) {
printf("Too many students\n");
return 1;
}
student std[num];
printf("Nhap thong tin sinh vien:\n");
for(size_t i=0; i < num; i++)
if(scanf("%" str(ID_LEN) "s %d", std[i].id, &std[i].year) != 2) {
printf("scanf() failed\n");
return 1;
}
sort(num, std);
print(num, std);
}
and here is an example run:
So sinh vien:
3
Nhap thong tin sinh vien:
aaa 1
zzz 2
bbb 3
aaa 1
bbb 3
zzz 2

Answer 2

printf("%c ", std[i].id);
should be
printf("%s ", std[i].id);
%c means a single char.