Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

Chapter 5 ■ Programs with Repetition Logic
We can tell C we will be reading data from the file with this:
FILE * in = fopen("C:\\testdata\\input.txt", "r");
Recall that the escape sequence \\ is used to represent \ within a string. If the file is on a flash
drive with assigned letter E, we can use:
FILE * in = fopen("E:\\input.txt", "r");
5.8.1 fscanf
We use the statement (more precisely, the function) fscanf to read data from the file. It is used
in exactly the same way as scanf except that the first argument is the file pointer in. For example,
if num is int, the statement
fscanf(in, "%d", &num);
will read an integer from the file input.txt (the one associated with in) and store it in num. Note
that the first argument is the file pointer and not the name of the file.
When we have finished reading data from the file, we should close it. This is done with
fclose, as follows:
fclose(in);
There is one argument, the file pointer (not the name of the file). This statement breaks the
association of the file pointer in with the file input.txt. If we need to, we could now link the
identifier in with another file (paydata.txt, say) using:
in = fopen("paydata.txt", "r");
Note that we do not repeat the FILE * part of the declaration, since it has already been
declared as FILE *. Subsequent fscanf(in, ...) statements will read data from the file
paydata.txt.
5.8.2 Find Average of Numbers in a File
To illustrate the use of fscanf, let us rewrite Program P5.3 to read several numbers from a file and
find their average. Previously, we discussed how to find the average. We just need to make the
changes to read the numbers from a file. Suppose the file is called input.txt and contains several
positive integers with 0 indicating the end, for example,
24 13 55 32 19 0
Program P5.6 shows how to define the file as the place from which the data will be read and how
to find the average.
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