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

Chapter 6 ■ Characters
The program reads one character at a time from the file and prints it on the screen using
putchar. It does this until \n is read, indicating that the entire line has been read. On exit from the
while loop, it uses putchar('\n') to terminate the line on the screen.
Be careful, though. This program assumes that the line of data is terminated by an end-of-line
character, \n (generated when you press “Enter” or “Return”). However, if the line is not terminated
by \n, the program will ‘hang’—it will be caught in a loop from which it cannot get out (we say it will
be caught in an infinite loop). Why?
Because the while condition ((ch = getc(in)) != '\n') will never become false (this
happens when ch is '\n') since there is no \n to be read. But, as discussed before, when we reach
the end-of-file, the value returned by getchar, and now also by getc, is the symbolic constant EOF
defined in stdio.h. Knowing this, we could easily fix our problem by testing for \n and EOF in the
while condition, thus:
while ((ch = getc(in)) != '\n' && ch != EOF)
Even if \n is not present, getc(in) will return EOF when the end of the file is reached, and the
condition ch != EOF would be false, causing an exit from the loop.
6.10 Write Characters to a File
Suppose we want to write characters to a file (output.txt, say). As always, we must declare a file
pointer (out, say) and associate it with the file using
FILE * out = fopen("output.txt", "w");
If ch is a char variable, we can write the value of ch to the file with
fprintf(out, "%c", ch);
C also provides the function putc (put a character) to do the same job. To write the value of ch to
the file associated with out, we must write:
putc(ch, out);
Note that the file pointer is the second argument to putc.
6.10.1 Echo Input, Number Lines
Let us expand the example on the previous page to read data from a file and write back the same
data (echo the data) to the screen with the lines numbered starting from 1.
The program would read the data from the file and write it to the screen, thus:
1. First line of data
2. Second line of data
etc.
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