You are given a data file EMPLOYEE.DAT with the following record structure:

struct employee {
 int empno ;
 char name[30] ;
 int basic, grade ;
 } ;

Every employee has a unique empno and there are supposed to be no gaps between employee numbers. Records are entered into the data file in ascending order of employee number, empno. It is intended to check whether there are missing employee numbers. Write a program segment to read the data file records sequentially and display the list of missing employee numbers. 

Ans:

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<string.h>
#include<Windows.h>

struct employee
{
	int empno;
	char name[30];
	int basic, grade;
};

void addemp(int, char*, int, int, char*);
void display(FILE*);
void miss(FILE*);
int main()
{
	FILE *fp;
	addemp(1,"Kishor Dass", 100,  1, "File (o).txt");
 	addemp(2,"Amit Kumar Mandal", 101,2, "File (o).txt");
 	addemp(3,"Prakash babu Sharma", 100, 3, "File (o).txt");
 	addemp(4, "Mohd Seraj Alam", 123, 1, "File (o).txt");
 	addemp(5, "Priya Saxena", 100, 2, "File (o).txt");
 	addemp(9, "Varun Taneja", 120, 3, "File (o).txt");
	addemp(13, "Shahrukh", 123, 3, "File (o).txt");
	addemp(20, "Paras Chugh", 452, 1, "File (o).txt");
	puts("\nList of the employees.\n\n");
	fp = fopen("File (o).txt", "rb+");
	display(fp);
	_getch();
	system("cls");
	miss(fp);
	_getch();
	fclose(fp);
	return 0;
}

void addemp(int eid, char *name, int bas, int grad, char *file)
{
	FILE *fp;
	struct employee dat;
	fp = fopen(file, "rb+");
	if (fp == NULL)
	{
		fp = fopen(file, "wb");
		if(fp == NULL)
		{
			printf("\nCannot open the file.\n");
			exit(1);	
		}
	}
	dat.empno = eid;
	strcpy(dat.name, name);
	dat.basic = bas;
	dat.grade = grad;
	fseek(fp, 0, SEEK_END);
	fwrite(&dat, sizeof(dat), 1, fp);
	fclose(fp);
}

void display(FILE *fp)
{
	struct employee dat;
	while (fread(&dat, sizeof(dat), 1, fp) == 1)
	{
		printf("%2d", dat.empno);
		printf("\t%-20s", dat.name);
		printf("\t % 4d", dat.basic);
		printf("\t % 3d\n", dat.grade);
	}
}

void miss(FILE *fp)
{
	int i = 1;
	struct employee dat;
	rewind(fp);
	puts("\nEmployees with following IDs are missing.");
	while (fread(&dat, sizeof(dat), 1, fp) == 1)
	{
		while(dat.empno != i)
		{
			printf("%d, ", i);
			i++;
		}
		i++;
	}
}