Overwrting a record

[mossy]

Hey,

I'm wondering if anyone can help with this problem. I'm trying to overwrite a record on a customer.dat file. the code is as follows

int updatebal(float bal){
struct Customer_rec // Set up the structure
{
  long id;
  char name[25];
  float balance;
  }struct Customer_rec Customer;

        FILE *ifp;// file pointers

	if ((ifp=fopen("C:\\CUSTOMER.DAT","r+")) == NULL){
	      printf("Can't open file.");
	      fclose(ifp);
	      return(-1);
	}
//Trying to find the correct start pt.
	fseek(ifp,sizeof(struct Customer_rec)*3,SEEK_SET);
	fread(&Customer,sizeof(struct Customer_rec),1,ifp);
     //Change value for the balance
	Customer.balance = bal;

                 fseek(ifp,sizeof(struct Customer_rec)*3,SEEK_SET);
//Write it back to the file
                 fwrite(&Customer,sizeof(struct Customer_rec),1,ifp);
//Print the updated balance
                 if((fread(&Customer,sizeof(struct  Customer_rec),1,ifp)) != 1)
	printf("\n Error in reading file");
	else
                printf("\n%ld %s %7.2f\n",Customer.id,Customer.name,Customer.balance);
	fclose(ifp);			
}

You'll notice that i'm bringing in a float which i want to use to replace the Customer.balance value. What i'm getting is garbage been written over in the wrong place.
Any ideas

thanks a million

[Salem]

> //Print the updated balance
Two things you need to do
1. fflush(ifp);
This ensures that the file is up to date - especially important if you're mixing reads and writes

2. fseek(ifp,sizeof(struct Customer_rec)*3,SEEK_SET);
Reposition the pointer to the start of the record you just wrote.

Checking the status results of the functions would be a good idea.

[mossy]

thanks for getting back Salem, i've tried your suggestions and i adapted the code

 
int updatebal(float bal){

struct Customer_rec // Set up the structure
{
  long id;
  char name[25];
  float balance;
  };
struct Customer_rec Customer;

	FILE *ifp;// file pointers

			if ((ifp=fopen("CUSTOMER.DAT","r+")) == NULL){
				printf("Can't open file.");
				fclose(ifp);
				return(-1);
			}
			//Trying to find the correct start pt.
			fseek(ifp,sizeof(struct Customer_rec)*3,SEEK_SET);
			fread(&Customer,sizeof(struct Customer_rec),1,ifp);
			//Change value for the balance
			Customer.balance = bal;

			fseek(ifp,sizeof(struct Customer_rec)*3,SEEK_SET);
			//Write it back to the file
			fwrite(&Customer,sizeof(struct Customer_rec),1,ifp);
			//Print the updated balance
 			//fflush(ifp);
			fseek(ifp,sizeof(struct Customer_rec)*3,SEEK_SET);
			if((fread(&Customer,sizeof(struct Customer_rec),1,ifp)) != 1)
				printf("\n Error in reading file");
			else
			 printf("\n%ld %s %7.2f\n",Customer.id,Customer.name,Customer.balance);
			fclose(ifp);			
}

I inputted 15 for the id and 15 for the new balance.
I commented out the flush and i got this as the output written to Customer.dat file:
ID NAME BALANCE
12343 F.Jones 0.00
1432 P.Kelly 150.50
15011 L.Byrne @0.00
15 P.Curran 850.00
1234 O.Lynch 1092.00

the orignal value for L.Byrne's balance was 1000.00


I have to be honest i'm not really that clued in to fread / fwrite and fseek so the '*3' i put in fseek is because the amount of members in my structure is that correct? I'm thinking now it might have something to do with the result i got. Plus it seems that it won't recognize the value?

[Salem]

The 3 is the record number, starting at 0

Imagine the file as
struct Customer_rec Customers[10];

With a real array, you just do things like
Customers[3].balance = 1234.5;

With a file, you use fseek to locate the record you want
fseek(ifp,sizeof(struct Customer_rec)*3,SEEK_SET);
then you read/modify/write the record you want.

Put the fflush() back in and try again. You're going to see odd things if you don't do that between fwrite() and fread()

[mossy]

I put the flush back in and i'm getting the same results. Its putting garbage in, in the wrong place on file.

ie The orginal was '1000.00'

after i insert '45' it becomes '[email protected]' on file and '45.00' on screen. So it writes it correctly to screen but not to file?