How do I read and edit a .txt file in C? - c

Question

I'm writing a program for an ATM. My .txt file is an account balance amount (in this case, 1500.00). How do I read in the .txt file, edit the account balance, and then save it to the file?
For example, if I were to ask a user to enter a deposit an amount of 300.00, I want to be able to add that 300.00 to the existing 1500.00 in the file, and then overwrite the 1500.00 with the total amount of 1800.00.
This is what I have so far.
float deposit;
float var;
printf("Current account balance:");
if ( (file_account = fopen ("account.txt", "r")) == NULL)
{
printf ("Error finding account balance.\n");
return;
}
while ( (fscanf (file_account, "%c", &var)) != EOF)
{
printf ("%c", var);
}
printf ("\n");
fclose (file_account);
for (deposit=0; deposit>0; deposit++)
{
if (deposit > 0)
{
printf ("Enter amount to deposit:");
scanf ("%f", &deposit);
//file_account + deposit;
fprintf (file_account, "Your new account balance is: %f", deposit);
}
else
{
printf ("Amount must be 0 or more.");
}
fclose (file_account);
}

Answer 1

You should use a File pointer to open & read the file, modify the contents according to your will and write back to file.
For example:
File *fp; // creates a pointer to a file
fp = fopen(filename,"r+"); //opens file with read-write permissions
if(fp!=NULL) {
fscanf(fp,"%d",&var); //read contents of file
}
fprintf(fp,"%d",var); //write contents to file
fclose(fp); //close the file after you are done

Answer 2

You need a few steps here:
int triedCreating = 0;
OPEN:
FILE *filePtr = fopen("test.txt", "r+");
if (!filePtr)
{
// try to create the file
if (!triedCreating)
{
triedCreating = 1;
fclose(fopen("test.txt", "w"));
goto OPEN;
}
fprintf(stderr, "Error opening file %i. Message: %s", errno, strerror(errno));
exit(EXIT_FAILURE);
}
// scan for the float
float value = 0.0f;
fscanf(filePtr, "%f", &value);
printf("current value: %f\nvalue to add: ", value);
// add the new value
float add = 0.0f;
scanf("%f", &add);
value += add;
// write the new value
fseek(filePtr, 0, SEEK_SET);
fprintf(filePtr, "%f", value);
fclose(filePtr);
You may wish to have different formatting for your printf()'s so that it looks nicer when read into a normal text editor.

Answer 3

you can read and edit a file with the help of basic FILE I/O(Input/ output) function in C i an simple manner like for example,writing to a file if it exist and if doesn't create a file with a name and then write to it.
A basic tutorial can be found at
http://www.tutorialspoint.com/cprogramming/c_file_io.htm
Now if you are talking about the complex .txt file having a lot of content in it and then you need to find a particular word and change it, i find it a bit difficult rather possible in Linux where you can call a script to read a file , edit it using SED,( stream editor for filtering and transforming text).You can find tutorials for it at below links
http://www.panix.com/~elflord/unix/sed.html