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copied!<p>This is the relevant part of the code currently on display:</p> <pre><code> cfPtr = fopen("credit.dat", "rb+"); fread(&client, sizeof(struct clientData), 1, cfPtr); client.balance -= serviceCharge; fseek(cfPtr,(client.acctNum - 1) * sizeof(struct clientData), SEEK_CUR); fwrite(&client, sizeof(struct clientData), 1, cfPtr); </code></pre> <p>I will assume that error checking on the function return values is 'not needed'; it should be there, but it will complicate things slightly. However, I wouldn't trust myself to write a program without the error checking - too many things can go wrong for my tastes (and bitter experience).</p> <ul> <li>You read a record at offset 0 into the file. You then update the balance, seek to a position based on the client account number relative to the current position, and then write the data back.</li> </ul> <p>At another time, we can discuss the wisdom of the binary data format you are using; it is sufficient for the time being.</p> <p>Here's code that 'works':</p> <pre><code>#include <stdio.h> struct clientData { int acctNum; char lastName[15]; char firstName[10]; double balance; }; static void print_client(struct clientData *data) { printf("%04d %-15s %-15s %6.2f\n", data->acctNum, data->lastName, data->firstName, data->balance); } int main(void) { FILE *cfPtr; struct clientData client = {0, "Me", "Mine", 50.0}; double serviceCharge = 5.0; /* Initialize */ cfPtr = fopen("credit.dat", "wb"); fwrite(&client, sizeof(struct clientData), 1, cfPtr); fclose(cfPtr); /* Update */ cfPtr = fopen("credit.dat", "rb+"); fread(&client, sizeof(struct clientData), 1, cfPtr); print_client(&client); client.balance -= serviceCharge; fseek(cfPtr, 0, SEEK_SET); fwrite(&client, sizeof(struct clientData), 1, cfPtr); fclose(cfPtr); /* Validate */ cfPtr = fopen("credit.dat", "rb"); fread(&client, sizeof(struct clientData), 1, cfPtr); fclose(cfPtr); print_client(&client); return 0; } </code></pre> <hr> <p>This works too...it doesn't skip 0 client IDs because it doesn't generate any. But you can deal with that.</p> <pre><code>#include <stdio.h> struct clientData { int acctNum; char lastName[15]; char firstName[10]; double balance; }; static void print_client(const struct clientData *data) { printf("%04d %-15s %-15s %6.2f\n", data->acctNum, data->lastName, data->firstName, data->balance); } /* Initialize file */ static void init_file(const char *filename) { FILE *cfPtr = fopen(filename, "wb"); int i; for (i = 1; i <= 30; i++) { struct clientData client = { i, "", "", i * 50.0}; sprintf(client.lastName, "Me (%d)", i); sprintf(client.firstName, "Mine (%d)", i); fwrite(&client, sizeof(struct clientData), 1, cfPtr); } fclose(cfPtr); } static void print_file(const char *filename) { struct clientData client; FILE *cfPtr = fopen(filename, "rb"); while (fread(&client, sizeof(struct clientData), 1, cfPtr) == 1) print_client(&client); fclose(cfPtr); } int main(void) { FILE *cfPtr; double serviceCharge = 5.0; const char *filename = "credit.dat"; struct clientData client; init_file(filename); printf("Post-initialization: %s\n", filename); print_file(filename); /* Update */ cfPtr = fopen(filename, "rb+"); while (fread(&client, sizeof(struct clientData), 1, cfPtr) == 1) { client.balance -= serviceCharge; fseek(cfPtr, -1 * (long)sizeof(struct clientData), SEEK_CUR); fwrite(&client, sizeof(struct clientData), 1, cfPtr); fseek(cfPtr, 0, SEEK_CUR); } fclose(cfPtr); printf("Post-update: %s\n", filename); print_file(filename); return 0; } </code></pre> <p>See also the comments...</p>

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