How to check whether a given integer already exists in a file - c

Question

How do I write a C program to enter a number from the keyboard and store it in a text file called number.dat. If the number exists in the file, display an error message. Program should allow to input numbers until the user inputs -99.
I tried this question, but I could not find how to check whether the integer already exists in the file.
This is my C code answer:
#include <stdio.h>
int main(void) {
int num;
FILE *xPtr;
xPtr = fopen("number.dat", "a");
while (num = -99) {
printf("Enter a number : ");
scanf("%d", &num);
if (num == -99)
break;
fprintf(xPtr, "%d\n", num);
}
return 0;
}
I could not find how to check whether an integer already exists in the file.
Can you please give me a solution for this matter?

Answer 1

There are a few things that could be improved. First of all, your while loop is using an assignment operator for the conditional expression:
while(num = -99)
Instead of assigning it to 99 at the beginning of the loop each time, you could do this:
while(num != -99)
Since you want the block inside of the loop to execute when num is not equal to -99. I would also convert it to a do-while loop, since you are not assigning num before you enter the loop the first time. When you are in the loop, you should be opening an closing the file so it can keep track of any numbers you add. In order to check your file you will need to read AND append, it is now currently set to only append. To read and append you will do the following:
xPtr = fopen("number.dat", "a+");
It might be helpful to add a function called has_num or whatever you choose, which takes the FILE* and checks for an int returning 1 if found and 0 if not. So you will modify your if statement to be
if (num != -99 && !has_num(xPtr,num))
{
fprintf(xPtr, "%d\n", num);
}
So with all of those changes, your code will become
#include <stdio.h>
int has_num(FILE* file, int num)
{
int curr;
int fnd = 0;
while (!fnd && (fscanf(file, "%d\n", &curr) != EOF))
{
fnd = (curr == num);
}
return fnd;
}
int main(void)
{
int num;
FILE *xPtr ;
do
{
xPtr = fopen("number.dat", "a+");
printf("Enter a number : ");
scanf("%d", &num);
if (num != -99 && !has_num(xPtr,num))
{
fprintf(xPtr, "%d\n", num);
}
fclose(xPtr);
} while (num != -99);
return 0;
}

Answer 2

#include <stdio.h>
#include <stdlib.h>
int main(){
FILE *fp; //create a pointer to FILE
char c,n,num;
printf("Enter a Number : ");
scanf("%c",&n);
if(n== -99){
// printf("Enter number greater than -99");
exit(0);
}
else{
fp = fopen("numbers.dat", "r+"); //read file
while ((c = getc(fp)) != EOF) {
num = c;
if(num==n){
printf("Included Number");
exit(0);
}
}
fp = fopen("numbers.dat","w+"); //write new number to file
putc(n, fp);
}
}

Answer 3

You program should successfully append numbers typed by the user to the file, but it has a few problems:
you do not check if fopen() successfully opened the file, you have undefined behavior if fopen() returns NULL.
you do not check if scanf() encountered an input failure. Entering a character that os not a number will cause undefined behavior, most likely an infinite loop appending the previously entered number if any.
you do not close the file. This will not have any adverse effect because all files are automatically closed upon program exit, but it is good style to close them explicitly.
of course you do not check if the number already exists in the file.
For the last problem, you should fopen() the file for both read and write, rewind the file and read all lines, checking if the number is present.
Here is a modified version:
#include <stdio.h>
int main(void) {
int c, num, num1, found;
FILE *xPtr = fopen("number.dat", "w+");
if (xPtr == NULL) {
printf("cannot open file number.dat\n");
return 1;
}
for (;;) {
printf("Enter a number: ");
if (scanf("%d", &num) != 1) {
if (feof(stdin))
break;
printf("invalid input\n");
while ((c = getchar()) != EOF && c != '\n')
continue;
if (c == EOF)
break;
} else {
if (num == -99)
break;
rewind(xPtr);
found = 0;
for (;;) {
/* skip all characters that cannot start a number */
if (fscanf(xPtr, "%*[^-+0-9]") == EOF)
break;
if (fscanf(xPtr, "%d", &num2) == 1) {
if (num == num2) {
found = 1;
break;
}
} else {
getc(xPtr); // consume a byte
}
}
if (found) {
printf("number is already in the file\n");
} else {
/* append the number */
fseek(xPtr, 0L, SEEK_END);
fprintf(xPtr, "%d\n", num);
}
}
}
fclose(xPtr);
return 0;
}