Copying file and write the info of file backwards - c

Question

Need create a copy of the file that was previously created with fifteen random numbers inside, and in this copy write those numbers backwards.
I tried to read the file with for() and write to the array, which will also fill in the newly created file with for(), but the IDE stops the script as soon as it goes to for().
#include <stdio.h>
#include <stdlib.h>
int main(){
char a[20], sf[20], tf[20];
FILE *source, *target;
int i;
printf("Enter name of file to copy\n");
gets(sf);
source = fopen(sf, "r");
if (source == NULL){
printf("Press any key to exit...\n");
exit(1);
}
printf("Enter name of target file\n");
gets(tf);
target = fopen(tf, "w");
if (target == NULL){
fclose(source);
printf("Press any key to exit...\n");
exit(1);
}
for(i=0;i<15;i++){
fscanf(source, "%d", a[i]);
//printf("%d\n", a[i]);
}
for(i=15;i>0;i--){
fprintf(target, "%d\n", a[i]);
}
//printf("File copied successfully.\n");
return 0;
}

Answer 1

Some remarks :
as it is said in remark you read int so you need an array of int, not an array of char to save them, currently you will write out of a with an undefined behavior
if you need to save 15 values an array for 15 is enough, 20 (supposing int) is useless
never use gets, it is deprecated (since years) because dangerous, use fgets to not take the risk to write out of the receiving string, and do not forget to remove the probable newline
may be the input and output files will be the same, so read first before to open output file and write its contents
fscanf(source, "%d", a[i]); is invalid and must be fscanf(source, "%d", &a[i]);
check the result of fscanf to manage error cases in the input file
fprintf(target, "%d\n", a[i]); must be fprintf(target, "%d\n", a[i-1]);
to explicitly fclose the output file is better in case you later transform your program to something more complex
A proposal :
#include <stdio.h>
#include <string.h>
#define N 15
FILE * openIt(const char * msg, const char * dir)
{
char fn[256];
printf(msg);
if (fgets(fn, sizeof(fn), stdin) == NULL)
/* EOF */
return NULL;
char * p = strchr(fn, '\n');
if (p != NULL)
*p = 0;
FILE * fp = fopen(fn, dir);
if (fp == NULL)
fprintf(stderr, "cannot open '%s'\n", fn);
return fp;
}
int main()
{
FILE * fp;
if ((fp = openIt("Enter name of file to copy\n", "r")) == NULL)
return -1;
int a[N], i;
for (i = 0; i != N; ++i) {
if (fscanf(fp, "%d", &a[i]) != 1) {
fprintf(stderr, "invalid integer rank %d\n", i);
fclose(fp);
return -1;
}
}
fclose(fp);
if ((fp = openIt("Enter name of target file\n", "w")) == NULL)
return -1;
while (i--) {
fprintf(fp, "%d\n", a[i]);
}
fclose(fp);
return 0;
}
Compilation and execution :
pi#raspberrypi:/tmp $ gcc -pedantic -Wall -Wextra r.c
pi#raspberrypi:/tmp $ cat in
1 2 3 4 5
6 7 8 9 10
11 12
13
14
15
not read
pi#raspberrypi:/tmp $ ./a.out
Enter name of file to copy
in
Enter name of target file
out
pi#raspberrypi:/tmp $ cat out
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
pi#raspberrypi:/tmp $ ./a.out
Enter name of file to copy
out
Enter name of target file
out
pi#raspberrypi:/tmp $ cat out
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
pi#raspberrypi:/tmp $