How to save a modified array to a file - c

Question

So I created an array which is saved in a file. I have the number 4 printed out 100 times in the file. Now everytime the an element in the array is accessed it the value will decrease by 1. So if A[1] = 4 is accessed then it will become A[1] = 3 and be saved on the file. The thing is I can't save the modified array to the file.
I already tried moving the placement of FILE pointer.
void buildingB4()
{
system("CLS");
FILE *input, *output;
int i, B4[100], room;
input = fopen("B4.txt", "r");
if (input == NULL)
{
output = fopen("B4.txt", "w");
for (i = 1; i <= 100; i++)
{
B4[i] = 4;
fprintf(output, "%d\n", B4[i]);
}
fclose(output);
for (i = 1; i <= 100; i++)
{
if (i % 10 == 0)
{
printf("\n\n");
}
printf("B-4-%d(%d)\t", i, B4[i]);
}
}
else
{
for (i = 1; i <= 100; i++)
{
fscanf(input, "%d\n", &B4[i]);
if (i % 10 == 0)
{
printf("\n\n");
}
printf("B-4-%d(%d)\t", i, B4[i]);
}
fclose(input);
printf("\nPlease choose a room:B-4-");
scanf("%d", &room);
B4[room] = B4[room] - 1;
output = fopen("B4.txt", "a");
fprintf(output, "%d\n", B4[i]);
studentDetails();
}
}
say if A[1] = 4
and when user input is 1, 1 is saved in a variable called room.
so A[room] = A[room] -1
so the result would be A[1] = 3 and it modify the A[1] saved in the file.

Answer 1

Use fclose after the line
fprintf(output, "%d\n", B4[i]);

Answer 2

I've found little mistakes in your code and here is your fixed code:
#define HOME_SIZE 100
void show_rooms(int B[]){
for (int i = 0; i < HOME_SIZE; i++){
if (i % 10 == 0){
printf("\n\n");
}
printf("B-4-%3d(%d) ", i + 1, B[i]);
}
}
void buildingB4()
{
FILE *input, *output;
input = fopen("B4.txt", "r");
unsigned int B[HOME_SIZE], room;
if (input == NULL){
fclose(input);
// Setting all homes to 4.
for (int i = 0; i < HOME_SIZE; ++i){
B[i] = 4;
}
output = fopen("B4.txt", "w");
for(int i = 0; i < HOME_SIZE; ++i)
fprintf(output, "%d\n", B[i]);
fclose(output);
show_rooms(B);
}
else{
for (int i = 0; i < HOME_SIZE; ++i){
fscanf(input, "%d", &B[i]);
}
fclose(input);
show_rooms(B);
printf("\nPlease choose a room:B-4-");
scanf("%d", &room);
if (room > 0 && room <= HOME_SIZE)
B[room - 1] -= 1;
output = fopen("B4.txt", "w");
for(int i = 0; i < HOME_SIZE; ++i)
fprintf(output, "%d\n", B[i]);
}
}
Notice that :
In C index starts from 0 not 1.
close using file after your work in order to save it properly.
Don't use '%d\n' format for scanf, it will ignore ' ' and '\n' automatically.
Tips for future development :
Try using feof function to know if your file is ended or not instead of using constant size for your inputs.