How to move file pointer back by one integer?

Question

Say I have a file containing integers in the form

1 57 97 100 27 86 ...

Say that I have a input file stream fin and I try to read the integers from the file.

ifstream fin("test.txt");
int val;
fin>>val;

Now I am doing this action in a while loop where at one period of time, I want to move my file pointer exactly one integer back. That is if my file pointer is about to read the integer 27 when I do fin>>val, I want to move the file pointer such that it can read the integer 100 when I do fin>>val. I know we can use fin.seekg() but I have used it only to move the file pointers by characters, not by integers.

Probably this is a naive question. But can someone please help me out?

Answer 1

You can use tellg after each read to save the pointer to be used later on with a seekg.

You could also take the implementation of << and modify it with a function that also returns the number of characters you have advanced each time. Where to find the source code of operator<< is not something where I could easily help you with.

Answer 2

In your case it is not an integer, but a text representing a number. Because of this you will have to move backward character by character until you find a non-digit one (!isdigit(c)).

As one of the commenters below pointed out, you may also pay attention to a the 'minus' sign in case your numbers can be negative.