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

Chapter 6 ■ Characters
To get around this problem, we must delay printing the line number until we are sure that
there is at least one character on the line. We will use an int variable writeLineNo, initially
set to 1. If we have a character to print and writeLineNo is 1, the line number is printed and
writeLineNo is set to 0. When writeLineNo is 0, all that happens is that the character just read is
printed.
When \n is printed to end a line of output, writeLineNo is set to 1. If it turns out that there is
a character to print on the next line, the line number will be printed first since writeLineNo is 1.
If there are no more characters to print, nothing further is printed; in particular, the line number
is not printed.
Program P6.12 contains all the details. When run, it will number the lines without printing an
extra line number.
Program P6.12
//This program prints the data from a file numbering the lines
#include <stdio.h>
int main() {
char ch;
FILE *in = fopen("input.txt", "r");
int lineNo = 0, writeLineNo = 1;
while ((ch = getc(in)) != EOF) {
if (writeLineNo) {
printf("%2d. ", ++lineNo);
writeLineNo = 0;
}
putchar(ch);
if (ch == '\n') writeLineNo = 1;
}
fclose(in);
}
We wrote the if condition as follows:
if (writeLineNo)...
If writeLineNo is 1 the condition evaluates to 1 and is, therefore, true; if it is 0, the condition is
false. We could also have written the condition as
if (writeLineNo == 1)...
In the statement
printf("%d. ", ++lineNo);
the expression ++lineNo means that lineNo is incremented first before being printed. By
comparison, if we had used lineNo++, then lineNo would be printed first and then incremented.
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