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

Chapter 5 ■ Programs with Repetition Logic
The process is similar to reading from a file. We must declare a “file pointer” (we use out) and
associate it with the actual file (output.txt, say) using fopen. This can be done with
FILE * out = fopen("output.txt", "w");
This tells C to “open the file output.txt for writing”; "w" indicates writing. When this
statement is executed, the file output.txt is created if it does not already exist. If it exists, its
contents are destroyed. In other words, whatever you write to the file will replace its original
contents. Be careful that you do not open for writing a file whose contents you wish to keep.
5.9.1 fprintf
We use the statement (more precisely, the function) fprintf to send output to the file. It is
used in exactly the same way as printf except that the first argument is the file pointer out. For
example, if sum is int with value 143, the statement
fprintf(out, "The sum is %d\n", sum);
will write
The sum is 143
to the file output.txt.
Note that the first argument is the file pointer and not the name of the file.
When we have finished writing output to the file, we must close it. This is especially
important for output files since, the way some compilers operate, this is the only way to ensure
that all output is sent to the file. (For instance, they send output to a temporary buffer in memory;
only when the buffer is full is it sent to the file. If you do not close the file, some output may be left
in the buffer and never sent to the file.) We close the file with fclose, as follows:
fclose(out);
There is one argument, the file pointer (not the name of the file). This statement breaks the
association of the file pointer out with the file output.txt. If we need to, we could now link the
identifier out with another file (payroll.txt, say) using:
out = fopen("payroll.txt", "w");
Note that we do not repeat the FILE * part of the declaration, since out has already been
declared as FILE *. Subsequent fprintf(out, ...) statements will send output to the file
payroll.txt.
For an example, we rewrite Program P5.6 as Program P5.7 by adding the fopen and fprintf
statements. The only difference is that P5.6 sends its output to the screen while P5.7 sends its
output to the file output.txt.
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