Reading multiple text files into an array in C

Question

I am writing a program that opens numerous text files and reads from them parameters for planetary bodies. I have a problem when reading the text files.

sample text file

 2
 1, 1, 2
 3.5, 3, 4

The first number (2) refers to the number of bodies found in the file. The next 2 lines correspond to the planet's parameters (x and y coordinates and mass respectively). I have 4 text files containing different amounts of bodies and need to store all the data in a variable.

my code

struct body {

float x;
float y;
float mass;

};

int main()
{

struct body b[8];
FILE *fp;
int i, j, k;
int num, count = 0;
char fName[10];

for (i = 1; i < 5; i++)    
{ 
    sprintf(fName,"bodies%d.txt",i);
    fp = fopen(fName, "r");

    if (fp == NULL)
    {   
        printf("Can't open %s \n",fName);
        exit(-1);
    }

    fscanf(fp, "%d", &num);


    for (j = count; j < num; j++)
    {               
        fscanf(fp, "%f%*c %f%*c %f%*c", &b[j].x, &b[j].y, &b[j].mass);
        printf("%f %f %f\n", b[j].x, b[j].y, b[j].mass);

        count = j;
    }               


}

It is reading the numbers from the text files, but it is stopping after 6 readings and there are 8 in total.

What could be the problem?

sprintf(fName,"bodies%d.txt",i); <-- Buffer overflow. fName can hold a maximum of 9 chars +1 for \0 at the end. You copy 12 chars into it(\0 included). Why do you have count? — Spikatrix
– Spikatrix, Commented May 9, 2015 at 8:01
That's confusing and wrong. Think. When you do j = count, you check j < num . That's wrong. Use j=0 instead of j = count and it is better to use count++ instead of count=j. And in the loop, instead of b[j], use b[count]. And I don't like your way of hardcoding arrays(unless you know the number of bodies in each file beforehand). — Spikatrix
– Spikatrix, Commented May 9, 2015 at 8:15
suggest refactor functionality to a function eg. struct body *read_body(FILE *fp). and minimise your problem to one single call to this function — amdixon
– amdixon, Commented May 9, 2015 at 8:16
I understood the problem, very stupid mistake in fact. Corrected the loop, thanks! They are hardcoded because I already know the number of bodies in each file beforehand — Njal
– Njal, Commented May 9, 2015 at 8:20

Spikatrix · Accepted Answer · 2015-05-09 09:04:33Z

Your code has some problems:

fName is declared as
```
char fName[10];
```
and you use
```
sprintf(fName,"bodies%d.txt",i);
```
which writes 12 characters into fName(including the NUL-terminator) which can atmost hold 9 characters(+1 for the NUL-terminator).

The for loop:

for (j = count; j < num; j++)
{               
    fscanf(fp, "%f%*c %f%*c %f%*c", &b[j].x, &b[j].y, &b[j].mass);
    printf("%f %f %f\n", b[j].x, b[j].y, b[j].mass);

    count = j;
}

has many problems and is confusing too. When you do j = count, you check j < num. This makes no sense as count is not related to num.

Fixes:

For the first problem, allocate enough space for fName:
```
char fName[12];
```
instead of
```
char fName[10];
```

As for the second problem, use

for (j = 0; j < num; j++) //j should be initialized to 0
{               
    fscanf(fp, "%f%*c %f%*c %f%*c", &b[count].x, &b[count].y, &b[count].mass);
    printf("%f %f %f\n", b[count].x, b[count].y, b[count].mass); //Use b[count] instead of b[j]

    //count = j; Works, but the below is better
    count++;
}

Hoang Huynh · Accepted Answer · 2015-05-09 08:30:45Z

1

Try replacing j = count with j = 0 in the second for loop.

edited May 9, 2015 at 8:30

Hoang Huynh

1,4032 gold badges17 silver badges34 bronze badges

answered May 9, 2015 at 8:07

fassl

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