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

Chapter 2 ■ C – The Basics
While this is usually sufficient for most purposes, there are times when it is useful to be able
to tell C how many print columns to use. For example, if we want to print the value of n in 5 print
columns, we can do this by specifying a field width of 5, as in:
printf("%5d", n);
Instead of the specification %d, we now use %5d. The field width is placed between % and d.
The value of n is printed “in a field width of 5”.
Suppose n is 279; there are 3 digits to print so 3 print columns are needed. Since the field
width is 5, the number 279 is printed with 2 spaces before it, thus: ◊◊279 (◊ denotes a space). We
also say “printed with 2 leading blanks/spaces” and “printed padded on the left with 2 blanks/
spaces.”
A more technical way of saying this is “n is printed right justified in a field width of 5.” “Right
justify” means that the number is placed as far right as possible in the field and spaces added in
front of it to make up the field width. If the number is placed as far left as possible and spaces are
added after it to make up the field width, the number is left justified. For example, 279◊◊ is left
justified in a field width of 5.
The minus sign can be used to specify left justification; %-wd will print a value left justified in a
field width of w. For example, to print an integer value left justified in field width of 5, we use %-5d.
For another example, suppose n is -7 and the field width is 5. Printing n requires two print
columns (one for - and one for 7); since the field width is 5, it is printed with 3 leading spaces,
thus: ◊◊◊-7.
You may ask, what will happen if the field width is too small? Suppose the value to be printed
is 23456 and the field width is 3. Printing this value requires 5 columns, which is greater than the
field width 3. In this case, C ignores the field width and simply prints the value using as many
columns as needed (5, in this example).
In general, suppose the integer value v is printed with the specification %wd where w is an
integer, and suppose n columns are needed to print v. There are two cases to consider:
1. If n is less than w (the field width is bigger), the value is padded on the left
with (w - n) spaces. For example, if w is 7 and v is -345 so that n is 4, the
number is padded on the left with (7-4) = 3 spaces and printed as ◊◊◊-345.
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2. If n is greater than or equal to w (field width is the same or smaller), the
value is printed using n print columns. In this case, the field width is
ignored.
A field width is useful when we want to line up numbers one below the other. Suppose we
have three int variables a, b, and c with values 9876, -3, and 501, respectively. The statements
printf("%d\n", a);
printf("%d\n", b);
printf("%d\n", c);
will print
9876
-3
501
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