Reading a text file into a character array in c - c

Question

I'm writing a program that uses the command-line arguments to receive the name of a text file from the user. The text file is a very simple CSV file such as:
Bob's experiment,12,14,15,16
Mary's experiment,16,15,18
I just want it to print the experiment name then the average of all the numerical values. I'm attempting to do this by putting all the numbers and commas into a char array and I don't know where I've gone wrong.
This is what I have:
int main(int argc, char *argv[])
{
if(argc == 2) {
FILE *txt_file;
txt_file=fopen(argv[1], "rt");
char str[4096];
if(!txt_file) {
printf("File does not exist.\n");
return 1;
}
while(!feof(txt_file)){
char s;
s = fgetc(txt_file);
//prints experiment name
if(s != ',' && (!isdigit(s))) {
printf("%c", s);
}
if(isdigit(s) || s == ',') {
fgets(str, 4096, txt_file);
}
}
fclose(txt_file);
return 0;
}

Answer 1

There are a number of ways to do this, but you should tailor your input routine to the type of data you are reading from your file. Here you are reading lines of data, so you should focus on line-oriented input routines (fgets, getline, or a shoehorned scanf). The basic approach is to read a line of input from your file into a buffer and then parse the line as needed. You can do this dynamically allocating all storage needed, or you can define a maximum value that should be large enough to handle your data.
Next you will need to parse the buffer read from the file to get the experiment name and each of the values associated so that an average can be calculated. Again, there are many ways to do this, but strtol is tailor made for this purpose. It takes a pointer to the string to convert and returns an endptr to the next character that is not a number. This allows you to read a values and set pointer = endptr+1 which sets you up to read your next number.
I have put these pieces together in the example below. It is commented to help you follow along. Drop a comment if you have any additional questions:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXEXPS 256
int main (int argc, char* argv[])
{
if (argc < 2) {
fprintf (stderr, "error: insufficient input. Usage %s <filename>\n", argv[0]);
return 1;
}
char *line = NULL; /* line read from file (getline allocates if NULL) */
size_t n = 0; /* number of characters to read (0 - no limit) */
ssize_t nchr = 0; /* number of characters actually read by getline */
char *p = NULL; /* pointer to use parsing values from line */
char *lnp = NULL; /* second pointer to use parsing values from line */
char *expname[MAXEXPS] = {0}; /* array of MAXEXPS pointers for experiment names */
int expavg[MAXEXPS] = {0}; /* array of MAXEXPS ints to hold averages */
int val = 0; /* val returned by each call to strtol */
int eidx = 0; /* experiment index */
int idx = 0; /* value index */
FILE *txt_file = fopen(argv[1], "r");
if (!txt_file) {
fprintf (stderr, "error: unable to open file '%s'\n", argv[1]);
return 1;
}
while ((nchr = getline (&line, &n, txt_file)) != -1) /* read each line in file */
{
p = strchr (line, ','); /* find first ',' */
*p = 0; /* set it to null (zero) */
expname[eidx] = strdup (line); /* copy exp name to array (strdup allocates) */
lnp = ++p; /* set lnp to next char */
int sum = 0; /* reset sum to 0 */
idx = 0; /* reset idx to 0 */
while ((val = (int)strtol (lnp, &p, 10)) != 0 && lnp != p) /* read next number */
{
sum += val; /* add val to sum */
lnp = ++p; /* set lnp to next char */
idx++; /* inc idx */
}
expavg[eidx++] = (idx > 0) ? sum / idx : 0; /* calc avg for experiment */
}
fclose (txt_file);
/* print the averages of experiments */
n = 0;
printf ("\n Experiment Avg\n");
printf (" -----------------------\n");
while (expname[n])
{
printf (" %-18s %d\n", expname[n], expavg[n]);
n++;
}
printf ("\n");
/* free all allocated memory */
n = 0;
if (line)
free (line);
while (expname[n])
free (expname[n++]);
return 0;
}
output:
$ ./bin/csvavgfixed dat/csvavg.dat
Experiment Avg
-----------------------
Bob's experiment 14
Mary's experiment 16
memory allocation/free summary:
==22148== HEAP SUMMARY:
==22148== in use at exit: 0 bytes in 0 blocks
==22148== total heap usage: 4 allocs, 4 frees, 723 bytes allocated
==22148==
==22148== All heap blocks were freed -- no leaks are possible
==22148==
==22148== For counts of detected and suppressed errors, rerun with: -v
==22148== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

Answer 2

I think this would do what you want.
#include <stdio.h>
int main(int argc, char* argv[])
{
int n = 0, count = 0, t = 0;
if(argc == 2) {
FILE *txt_file;
txt_file=fopen(argv[1], "rt");
char str[4096];
if(!txt_file) {
printf("File does not exist.\n");
return 1;
}
while(!feof(txt_file)){
char s;
s = fgetc(txt_file);
//prints experiment name
if(s != ',' && (!isdigit(s))) {
if(n!=0) {
printf("%d\n", n / count);
n = 0;
count = 0;
}
printf("%c", s);
}
if(s == ',') {
fscanf(txt_file, "%d", &t);
n+=t;
count++;
}
}
printf("%d\n", n / count);
fclose(txt_file);
return 0;
}
}