C program to put the output into a new file - c

Question

So i have een solving a problem where i have to find the prime numbers from the input file and save those prime numbers in an output file called output.txt.But If there are no prime numbers in the input file, i'll have to write “No prime numbers found” in the output file.So when i completed the code when there is no prime number it shows No prime numbers found 5-6 times and i only want it to appear 1 time.What is my mistake here?I am totally a noob here
#include <stdio.h>
#include <stdlib.h>
int Primecheck(const int number);
int main()
{
FILE* Number,
* Prime_N;
int num;
char sentence[50] = "No prime numbers found";
int length = strlen(sentence);
int i;
Number = fopen("input.txt", "r");
Prime_N = fopen("output.txt", "w");
if (Number == NULL || Prime_N == NULL)
{
printf("Unable to open file.\n");
exit(EXIT_FAILURE);
}
printf("File opened and Reading Done \n\n");
while (fscanf(Number, "%d", &num) != -1)
{
if (Primecheck(num) == 1)
fprintf(Prime_N, "%d\n", num);
else
for (i = 0; i < length; i++)
{
fputc(sentence[i], Prime_N);
}
}
fclose(Number);
fclose(Prime_N);
printf("Overwrite Success.");
return 0;
}
int Primecheck(const int number)
{
int i;
if (number < 0)
return 0;
for (i = 2; i <= number / 2; i++)
{
if (number % i == 0)
{
return 0;
}
}
return 1;
}

Answer 1

instead of
else
for(i=0;i<length;i++)
{
fputc(sentence[i] ,Prime_N);
}
you can just write
else
printf("No prime numbers found");

Answer 2

Well , as I see from your code , each time you read a number from the file you check if prime write it or write sentence . so of course you get multiple output ,
you should write the number to char* then check if changed after reading from Number and checking if prime write the number in the char* after the loop just check if the length of the char* that you stored the primes in it if changed write it else write your sentence ... Done
something like this
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <math.h>
typedef struct {
char* data;
size_t size;
size_t capacity;
} string;
void init_string(string* s, size_t size){
s->data = (char*)malloc(sizeof(char) * size);
s->capacity = size;
s->size = 0;
}
void append(string* s,const char* str){
if(s->capacity - s->size < strlen(str)){
s->data = (char*)realloc(s->data, sizeof(char)* 4 * strlen(str));
}strcat(s->data, str);
}
void free_string(string* s){
free(s->data);
}
int is_odd(int n){
return n%2;
}
int main(){
int num[] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19};
string str;
init_string(&str,1);
char* snum;
for(int i=0; i<18; i++){
if(!is_odd(i)){
sprintf(snum,"%d",num[i]);
append(&str, snum);
append(&str, "\n");
}
//printf(snum);
//strcat(str, snum);
}
printf("%s",str.data);
free_string(&str);
//printf(str);
return 0;
}