Convert Numbers to Words (Dev C++ Code)

/*-----------------------------------------------------------
 * Description: Program that converts numbers to words.
 *-----------------------------------------------------------*/

#include <stdio.h> 
#include <stdlib.h> // for atoi() and calloc() 
#include <string.h> // for strlen()

char *tens[]={"ten", "eleven", "twelve", "thirteen", "fourteen","fifteen", "sixteen", "seventeen", "eighteen", "nineteen"}; 
char *ones[]={"zero","one","two","three","four","five","six","seven","eight","nine"}; 	// multi D Array [][] is the same as *[] 
char *teen[]={"","","twenty", "thirty", "forty", "fifty","sixty", "seventy", "eighty", "ninety"};

void ten(int);
void tee(int);	//	Functions Prototype 
void hun(int);				
void read(FILE*,int *,int *); // function to read from the file
void write_display(FILE*);  // function to write in the file and display to stdout (standard output)

static FILE *OUT;   	// declare a file pointer and make it accessible to every function so we can write flexibly 

int main(void)
{
	char b[10];// 9 digits and 1 for the '\0'
	int size,i,*s,n=0;	// size for file size // i counter // *s points to the first element of the array // n number of elements read from the file
	FILE *IN=fopen("input.txt","r"); // declare a file pointer and open it in the read mode 
	OUT=fopen("output.txt","w");	// open the output file in the write mode 
	fseek(IN, 0, SEEK_END); 		 // seek to the end of the file 
	size=ftell(IN);       			// Get the exact size of the file 
	fseek(IN,0,SEEK_SET); 			// seek back to the beginning to make the reading process possible 
	s=(int*)calloc(size,sizeof(int));  // Dynamic Memory Allocation
	read(IN,&n,s);						 // read from the file "input.txt"
	puts("Welcome to the number to words converter:\nOpening files...\n");
	for (i=0;s[i]!=-1;i++)
	{
		fprintf(OUT,"Number %d: %d\n",i+1,s[i]);
		itoa(s[i],b,10);   		// To convert integer to string "10" means decimal so we can calculate the lenght using strlen() 
		switch (strlen(b))
		{
			case 1:
				fprintf(OUT,"%s", ones[s[i]]);
				break;
			case 2:
				((s[i]/10)%10==1) ? ten(s[i]) : tee(s[i]);	
				break;
			case 3:
				hun(s[i]);
				break;
			case 4:	
				fprintf(OUT,"%s thousand ",ones[(s[i]/1000)]);
				((s[i]%1000)<100) ? ((s[i]%1000)/10) ? ten((s[i]%1000)) : tee((s[i]%1000)) : hun((s[i]%1000));	// double selection 
				break;
			case 5:
				((s[i]/10000)==1) ? ten((s[i]/1000)) : tee((s[i]/1000));
				fprintf(OUT,"thousand ");
				hun(s[i]%1000);	
				break;
			case 6:
				hun(s[i]/1000);
				fprintf(OUT,"thousand ");
				(s[i]%1000!=0) ? hun(s[i]%1000) : printf("");
				break;
			case 7:
				fprintf(OUT,"%s million",ones[s[i]/1000000]);
				hun((s[i]/1000)%1000);
				(((s[i]/1000)%1000)!=0) ? fprintf(OUT,"thousand ") : fprintf(OUT,"");
				(s[i]%1000!=0) ? hun(s[i]%1000) : printf("") ;	
				break;
			case 8:
				(s[i]/10000000==1) ? ten(s[i]/1000000) : tee(s[i]/1000000);
				fprintf(OUT,"million ");
				hun((s[i]/1000)%1000);
				(((s[i]/1000)%1000)!=0) ? fprintf(OUT,"thousand ") : fprintf(OUT,"");
				hun(s[i]%1000);
				break;
			case 9:
				hun(s[i]/1000000);	
			    fprintf(OUT,"million ");
			    hun((s[i]/1000)%1000);
			    (((s[i]/1000)%1000)!=0) ? fprintf(OUT,"thousand\n") : fprintf(OUT,""); 
			    hun(s[i]%1000);
			    break; 
			default :	
				puts("The number is more than 9 digits conversion is impossible.");
		}
		fprintf(OUT,"\n");
	}
	fclose(OUT); // close the output file after writing all the numbers
	write_display(OUT);  // display what was writeen in the "output.txt" file to stdout the screen stream
	puts("-1 is encountered\nAll numbers in words have been stored in the output file\nClosing files...");
	return 0;
}
void ten(int s)
{
	fprintf(OUT,"%s ",tens[s%10]);
}
void tee(int s)
{
	((s%10)==0) ? fprintf(OUT,"%s ",teen[(s/10)%10]): fprintf(OUT,"%s %s ",teen[(s/10)%10],ones[s%10]);
}
void hun(int s)
{
	(s/100 !=0) ? fprintf(OUT,"%s hunderd ",ones[(s/100)]) : fprintf(OUT,"");
	(((s%100)/10)==1) ? ten(s%100) : tee(s%100);	
}
void read(FILE *IN,int *n,int *s)
{
	int j; // counter
 	for (j=0;!feof(IN);j++)  // loop until the end of file (eof)
	{
		fscanf(IN,"%d",&s[j]);
		(*n)++;     // get the number of elements stored in the file 
	}
	fclose(IN); // close the input file
}	
void write_display(FILE *OUT)
{
	int j; // counter
	char r[100][100];
	if ((OUT=fopen("output.txt","r"))==NULL)    // check if null pointer 
		puts("Couldn't open the file");
	else 
	{ 
	for (j=0;!feof(OUT);j++)   // loop until the end of file "EOF"
	{
			fgets(r[j],100,OUT);
			printf("%s ",r[j]);
			puts(" ");   // newline 
	}
		fclose(OUT);
	}
}