Files in C, cannot copy the elements from a file to another - c

Question

I'm trying to copy the elements from a file to another if the line is an odd number;
for example I have in first file:
6
5 6 8
3
6 9 32
And the output should be
5 6 8
6 9 32
but I get nothing.
Here is my code:
int main() {
FILE *data_input = fopen("data.in", "r");
FILE *data_out = fopen("data.out", "w");
char str;
int nr = 0;
if (data_input == NULL || data_out == NULL)
printf("error");
while ((str = fgetc(data_input)) != EOF) {
str = fgetc(data_input);
if (str == "\n")
nr++;
if (nr % 2 != 0)
fputc(str, data_out);
}
fclose(data_input);
fclose(data_out);
return 0;
}

Answer 1

You don't need to read another time after the while. By doing that you are discarding half the characters (You read 2 but only write one).
You compare with with '\n' (with single quotes, not double).
You should initialize nr with one, since the first row probably isn't the count as the number 0.
int main() {
FILE *data_input = fopen("data.in", "r");
FILE *data_out = fopen("data.out", "w");
char str;
int nr = 1;
if (data_input == NULL || data_out == NULL)
printf("error");
while ((str = fgetc(data_input)) != EOF) {
if (str == '\n')
nr++;
if (nr % 2 != 0)
fputc(str, data_out);
}
fclose(data_input);
fclose(data_out);
return 0;
}

Answer 2

First, what do you start counting from? 0 or 1? The following code starts from zero, so the first line is even.
In addition to the other issues, you emit your newline with the wrong line, which could make the last line of the file incorrectly empty because you emitted the newline for the last even line if it's empty:
int main() {
FILE *data_input = fopen("data.in", "r");
FILE *data_out = fopen("data.out", "w");
char str;
int nr = 0;
if (data_input == NULL || data_out == NULL)
printf("error");
while ((str = fgetc(data_input)) != EOF) {
str = fgetc(data_input);
if (str == "\n")
nr++; <--- this should be *AFTER* emitting the output
if (nr % 2 != 0)
fputc(str, data_out);
}
fclose(data_input);
fclose(data_out);
return 0;
}
would be better as
int main() {
FILE *data_input = fopen("data.in", "r");
FILE *data_out = fopen("data.out", "w");
int nr = 0;
if (data_input == NULL || data_out == NULL)
printf("error");
// note that this is now easier to read and -
// importantly - to *debug* along with being
// **MUCH** less bug-prone
for (;;) {
int str = fgetc(data_input);
if (str == EOF)
break;
if (nr % 2 != 0)
fputc(str, data_out);
if (str == '\n')
nr++;
}
fclose(data_input);
fclose(data_out);
return 0;
}

Answer 3

There are multiple problems:
you do not exit the program if the files cannot be opened
you have a redundant fgets() inside the loop
fgetc() returns an int, do not use a char variable to store it and test for EOF
naming a byte str is very confusing
you should test and output the byte before testing for the end of line: the trailing newline is part of the line.
line numbers are usually 1 based. The first line has number 1 which is odd. From your expected output, you want the lines with an even line number.
Here is a modified version:
#include <stdio.h>
int main() {
FILE *data_input = fopen("data.in", "r");
FILE *data_out = fopen("data.out", "w");
int c;
int lineno = 1; // the first line is line 1
if (data_input == NULL || data_out == NULL) {
printf("error opening files\n");
return 1;
}
while ((c = fgetc(data_input)) != EOF) {
if (lineno % 2 == 0) // output lines with an even line number
fputc(c, data_out);
if (c == "\n") // update the line number after the newline
lineno++;
}
fclose(data_input);
fclose(data_out);
return 0;
}