How to read comma-separated csv file with `sscanf()` - c

Question

I'm attempting to print an array of structures read from a CSV file in Excel. However, only the students' IDs are printed; other information was also printed but some confusing rare characters. Can you please tell me what could be wrong with this code?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct student {
char ID[8];
char name[32];
int score;
} student;
int main(int argc, char *argv[]) {
student student_list[100];
FILE *source = fopen("students.csv", "r");
if (source == NULL) {
perror("Unable to open the source file.");
exit(1);
}
char buffer[1024];
fgets(buffer, sizeof(buffer), source);
int num_student = 0;
while (!feof(source)) {
student *one_student = student_list + num_student;
sscanf(buffer, "%8[^,] %32[^,] %3[^,]",
&one_student->ID, &one_student->name, &one_student->score);
fgets(buffer, sizeof(buffer), source);
num_student++;
}
for (int i = 0; i < num_student; i++) {
printf("ID: %s name: %-9s score: %-3d\n",
student_list[i].ID, student_list[i].name, student_list[i].score);
}
fclose(source);
return 0;
}
This is a sample input file students.csv:
B213350,John Adam Smith,80
B191835,Mary Elizabeth Smith,71
B201304,Yamazaki Fuyumi,95
B201832,Liam,57
B201834,Alfonso Hernández,65

Answer 1

There are multiple problems:
you should not use feof(). Read Why is “while ( !feof (file) )” always wrong?
Use this loop instead:
while (fgets(buffer, sizeof buffer, source)) {
// handle the line
}
the sscanf() format string is incorrect: the character counts are too large and the , are missing. It should be " %7[^,\n], %31[^,\n], %d" and you should check that the return value is 3, the number of successful conversions expected.
you should stop when the student array is full.
Here is a modified version:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct student {
char ID[8];
char name[32];
int score;
} student;
int main(int argc, char *argv[]) {
student student_list[100];
FILE *source = fopen("students.csv", "r");
if (source == NULL) {
fprintf(stderr, "Cannot open file students.csv: %s\n", strerror(errno));
return 1;
}
char buffer[1024];
int num_student = 0;
while (num_student < 100 && fgets(buffer, sizeof(buffer), source)) {
student *one_student = &student_list[num_student];
if (sscanf(buffer, " %7[^,\n], %31[^,\n], %d",
one_student->ID, one_student->name,
&one_student->score) == 3) {
num_student++;
} else {
printf("invalid CSV line: %s", buffer);
}
}
for (int i = 0; i < num_student; i++) {
printf("ID: %-9s name: %-32s score: %-3d\n",
student_list[i].ID, student_list[i].name,
student_list[i].score);
}
fclose(source);
return 0;
}
Note that this approach to parsing CSV files cannot handle empty fields. Parsing the line with strtok() would not work either because consecutive commas would be handled as a single separator. You need a different approach using strcspn() or strchr().