fgetc() can't read float - c

Question

I got a big problem using fgetc() and i can't figure it out... I try to parse a text file, everything compile but at the execution I got an infinite loop xor a segfault (Code::blocks), my text file is like that: {"USD_EUR": "0.8631364", "EUR_USD": "1.3964719"} with 16 rates change. I try to put all my float in rate[16]...
void read(float change[4][4], char* myFile)
{
FILE* file = NULL;
file = fopen(myFile, "r+");
int value,i;
float rate[16];
char* str = "";
if (file != NULL)
{
do
{
value = fgetc(file);
printf("%c \n",value);
while(value > 48 && value < 57)
{
value = fgetc(file);
strcat(str, value);
//printf("%s \n", str);
}
rate[i] = atof(str);
i++;
str = "";
}while(value != EOF);// 125 = }
change[0][1] = rate[5];
change[0][2] = rate[0];
change[0][3] = rate[15];
change[1][0] = rate[6];
change[1][1] = rate[14];
change[1][2] = rate[7];
change[1][3] = rate[10];
change[2][0] = rate[8];
change[2][1] = rate[2];
change[2][2] = rate[12];
change[2][3] = rate[4];
change[3][0] = rate[3];
change[3][1] = rate[13];
change[3][2] = rate[11];
change[3][3] = rate[9];
fclose(file);
}
else
{
printf("Unable to read the file!\n");
}
}
I also try with EOF but i only have the char before numbers then that goes out of the loop ex: {"USD_EUR": "

Answer 1

I suggest that you simply use fscanf.
E.g
FILE *file;
int i = 0, status;
float value;
float rate[16];
file = fopen(myFile, "r");
if(file == NULL){
printf("Unable to read the file!\n");
return ;
}
while((status=fscanf(file, "%f", &value))!=EOF){
if(status==1){
rate[i++] = value;
if(i==16)//full
break;
} else {
fgetc(file);//one character drop
}
}
fclose(file);

Answer 2

Problem 1:
char* str = "";
Declares str as a pointer to a static string. This creates a literal "" in memory and points str to it, which isn't anything you can safely change. You want something like
char str[30] = "";
Problems 2 and 3:
strcat(str, value);
Attempts to to append to str, which isn't safe or right. Also, as guest notes, you are trying to strcat(char *, int), which isn't the correct usage. strcat(char *, char *) is correct. Note - this doesn't mean that you should strcat(str, (char *) &value); - you need to understand how strings are implemented as char arrays in C, particularly with regard to zero termination.
Problem 4:
str = "";
See user3629249's comment above. Given a proper declaration,
str[0] = '\0';
Would be correct.
Problem 5:
Again, with credit to user3629249,
in 'change', the position change[0][0] is being skipped.

Answer 3

In addition to the solutions provided in the other answers, when faced with a messy line of input to read, it may be easier to use the line-oriented input functions provided by libc (e.g. fgets or getline). Reading the data one line at a time into a buffer, often (not always) allows greater flexibility in parsing the data with the other tools provided by libc (e.g. strtok, strsep, etc..)
With other data, character-oriented input is a better choice. In your case, the lines were interlaced with numerous '"', ':', ' ' and ',''s. This made it difficult to construct a fscanf format string to read both exchange rates in a single call or use any of the string parsing tools like strtok. So this was truly a tough call. I agree, BluePixyes' solution for parsing a single float in a fscanf call is a good solution. The line-oriented alternative is to read a line at a time, and then using strtof to convert the float values found in the line. The only advantage that strtof provides is error checking on the conversion that allows you to verify a good float conversion. This is one approach for a line-oriented solution:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <limits.h>
int main () {
FILE* file = NULL; /* aways initialize variables */
float rate[16] = {0.0}; /* market rates 1st & 2nd */
char myFile[50] = {0}; /* input filename */
char line[128] = {0}; /* input line buffer */
char *p = NULL; /* pointer to parse line */
char *ep = NULL; /* pointer to parse line */
size_t idx = 0; /* index for rate array values */
size_t it = 0; /* general index iterator */
/* prompt for filename */
printf ("\n Please enter filename to read rates from: ");
scanf ("%[^\n]%*c", myFile);
/* open & validate file */
file = fopen (myFile, "r");
if (!file) {
fprintf(stderr, "error: Unable to read the file!\n");
return 1;
}
/* using line-oriented input to read line, then parse */
while (fgets (line, 127, file) != NULL)
{
if (idx == 16) {
fprintf (stderr, "warning: array full.\n");
break;
}
p = line; /* parse line for floats */
while (*p) { /* find first digit or end */
while (*p && (*p < 48 || *p > 57) ) p++;
if (!*p) break; /* validate not null */
rate[idx++] = strtof (p, &ep); /* get float, set end-ptr */
if (errno != 0 || p == ep) /* validate conversion */
fprintf (stderr, "discarding: rate[%zd] invalid read\n", --idx);
p = ep; /* set ptr to end-ptr */
}
}
fclose (file);
printf ("\n The exchange rates read from file:\n\n");
for (it = 0; it < idx; it++)
printf (" rate[%2zd] = %9.7f\n", it, rate[it]);
printf ("\n");
return 0;
}
sample input:
$ cat dat/rates.txt
"USD_EUR": "0.8631364", "EUR_USD": "1.3964719"
"USD_AGT": "0.9175622", "EUR_USD": "1.0975372"
"USD_BRZ": "0.8318743", "EUR_USD": "1.1713074"
"USD_COL": "0.9573478", "EUR_USD": "1.0537964"
"USD_GIA": "0.7904234", "EUR_USD": "1.5393454"
output:
$ ./bin/read_xchgrates
Please enter filename to read rates from: dat/rates.txt
The exchange rates read from file:
rate[ 0] = 0.8631364
rate[ 1] = 1.3964719
rate[ 2] = 0.9175622
rate[ 3] = 1.0975372
rate[ 4] = 0.8318743
rate[ 5] = 1.1713074
rate[ 6] = 0.9573478
rate[ 7] = 1.0537964
rate[ 8] = 0.7904234
rate[ 9] = 1.5393454
Note: check your strtof man page for any additional #define's your compiler may require.

Answer 4

the code has the following sequence:
fgetc results in 'U',
that is not a value inside the range 0...9 exclusive,
so drops through to try and convert str to rate[i]
(where 'i' has not been initialized to a known value)
Since no digits have been saved where str points,
some unknown offset from rate[] gets set to 0
(this is undefined behaviour)
then the unknown value 'i' gets incremented
and the following line: str = "" is executed
which has no effect on string
(unless each literal is at a different location in the .const section)
and the outer loop is repeated.
Eventually, a char in the range 1...8 is input
Then, in the inner loop, that first digit is SKIPPED
and another char is read.
from your example that next char is a '.'
Which could cause the inner loop to be exited
However,
the line: strcat(str, value);
should cause a seg fault event
due to trying to write to the .const section of the executable