Read a text file into a struct array with functions - c

Question

so I have this file:
Jane
18
5.3
John
23
5.8
and I need to create a program to store this two persons details on a array of structs.
I have done this:
#include <stdio.h>
typedef struct
{
char name[100];
int age;
float height;
} PERSON;
main()
{
PERSON *X = NULL;
FILE *f;
char ch;
int lines = 1;
f = fopen("filename.txt", "r");
while ((ch = fgetc(f)) !=EOF)
{
if (ch == '\n')
lines++;
}
rewind(f);
X = (PERSON*) malloc ((lines/3) * sizeof(PERSON));
StoreInArray(X, f, lines);
}
StoreInArray(PERSON *X, FILE *f, int lines)
{
int i = 0;
for (i=0; i < lines/3; i++)
{
fscanf(f, "%s%d%f", (*(X+i)).name[100], &(*(X+i)).age, &(*(X+i)).height);
}
//for testing//
for (i=0; i < lines/3; i++)
printf("%s\n%d \n%f\n",X[i].name[100], X[i].age, X[i].height);
}
But all it prints is:
(null)
0
0.000000
If you could help me figure out what is wrong I'd be very appreciated!
Thank you in advance.

Answer 1

I'd compile with -Wall -O2 to catch warnings, which would have pointed out an error.
In your fscanf and printf, you don't want name[100] but name. You want to point to the name array, but name[100] is a single char [fetching past the end of the array (i.e.) undefined behavior], and would be flagged by the compiler.
Here's a version of your code with the bugs annotated/corrected:
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char name[100];
int age;
float height;
} PERSON;
void StoreInArray(PERSON * X, FILE * f, int lines);
int
main()
{
PERSON *X = NULL;
FILE *f;
char ch;
int lines = 1;
f = fopen("filename.txt", "r");
while ((ch = fgetc(f)) != EOF) {
if (ch == '\n')
lines++;
}
rewind(f);
X = (PERSON *) malloc((lines / 3) * sizeof(PERSON));
StoreInArray(X, f, lines);
return 0;
}
void
StoreInArray(PERSON * X, FILE * f, int lines)
{
int i = 0;
for (i = 0; i < lines / 3; i++) {
#if 0
fscanf(f, "%s%d%f", (*(X + i)).name[100], &(*(X + i)).age, &(*(X + i)).height);
#else
fscanf(f, "%s%d%f", (*(X + i)).name, &(*(X + i)).age, &(*(X + i)).height);
#endif
}
//for testing//
for (i = 0; i < lines / 3; i++) {
#if 0
printf("%s\n%d \n%f\n", X[i].name[100], X[i].age, X[i].height);
#else
printf("%s\n%d \n%f\n", X[i].name, X[i].age, X[i].height);
#endif
}
}
Note that using *(X + i)).name is cumbersome, so here's a simplified and more readable version that uses an extra pointer variable:
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char name[100];
int age;
float height;
} PERSON;
void StoreInArray(PERSON * X, FILE * f, int lines);
int
main()
{
PERSON *X = NULL;
FILE *f;
char ch;
int lines = 1;
f = fopen("filename.txt", "r");
while ((ch = fgetc(f)) != EOF) {
if (ch == '\n')
lines++;
}
rewind(f);
X = (PERSON *) malloc((lines / 3) * sizeof(PERSON));
StoreInArray(X, f, lines);
return 0;
}
void
StoreInArray(PERSON * X, FILE * f, int lines)
{
int i = 0;
PERSON *p;
p = X;
for (i = 0; i < lines / 3; i++, p++)
fscanf(f, "%s%d%f", p->name, &p->age, &p->height);
//for testing//
p = X;
for (i = 0; i < lines / 3; i++, p++)
printf("%s\n%d \n%f\n", p->name, p->age, p->height);
}
It isn't really necessary to preread the file to get a line count. It is possible to grow your array dynamically, as lines are read in, using realloc:
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char name[100];
int age;
float height;
} PERSON;
int
main()
{
PERSON *X = NULL;
FILE *f;
int i;
PERSON *p;
int count = 0;
f = fopen("filename.txt", "r");
while (1) {
X = realloc(X,sizeof(PERSON) * (count + 1));
p = &X[count];
if (fscanf(f, "%s%d%f", p->name, &p->age, &p->height) == EOF)
break;
++count;
}
fclose(f);
// trim the array
X = realloc(X,sizeof(PERSON) * count);
p = X;
for (i = 0; i < count; i++, p++)
printf("%s\n%d \n%f\n", p->name, p->age, p->height);
return 0;
}
Here's a further refinement that reduces the number of realloc calls needed:
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char name[100];
int age;
float height;
} PERSON;
int
main()
{
PERSON *X = NULL;
FILE *f;
int i;
PERSON *p;
size_t count = 0;
size_t alloc = 0;
f = fopen("filename.txt", "r");
while (1) {
if (count >= alloc) {
alloc += 100;
X = realloc(X,sizeof(PERSON) * alloc);
}
p = &X[count];
if (fscanf(f, "%s%d%f", p->name, &p->age, &p->height) == EOF)
break;
++count;
}
fclose(f);
// trim the array to what was actually used
X = realloc(X,sizeof(PERSON) * count);
p = X;
for (i = 0; i < count; i++, p++)
printf("%s\n%d \n%f\n", p->name, p->age, p->height);
return 0;
}