C16. C Converse Specifications

Conversion Specifications

Conversion specifications give the programmer a great deal of control over the appearance of output. On the other hand, they can be complicated and hard to read. In fact, describing conversion specifications in complete detail is too arduous a task to tackle this early in the book. Instead, we II just take a briet look at some ot their more important capabilities.

we saw that a conversion specification can include formatting information. In particular, we used % . If to display a float value with one digit after the decimal point. More generally, a conversion specification can have the form %m.pX or %-m.pX. where m and p are integer constants and X is a letter. Both m and p are optional; if p is omitted, the period that separates m and p is also dropped. In the conversion specification %10 . 2f, m is 10. p is 2, and X is f. In the specification %10f. m is 10 and p (along with ihe period) is missing, but in the specification % . 2f, p is 2 and m is missing.

The minimum field width, m, specifies the minimum number of characters to print. If the value to be printed requires fewer than m characters, the value is right- justified within the field. (In other words, extra spaces precede the value.) For example, the specification %4d would display the number 123 as *123. (In this chapter, I'll use • to represent the space character.) If the value to be printed requires more than m characters, the field width automatically expands to the nec­essary size. Thus, the specification %4d would display the number 12345 as 12345  >no digits are lost. Putting a minus sign in front of m causes left justifica­tion; the specification %-4d would display 123 as 123*.

The meaning of the precision. p, isn't as easily described, since it depends on the choice of X, the conversion specify. X indicates which conversion should be applied to the value before it's printed. The most common conversion specifiers for numbers are:

d   > Displays an integer in decimal (base 10) form, p indicates the minimum number of digits to display (extra zeros are added to the beginning of the num­ber if necessary); if p is omitted, it is assumed to have the value 1. (In other words, %d is the same as % . Id.)

e   > Displays a floating-point number in exponential format (scientific nota­tion). p indicates how many digits should appear after the decimal point (the default is 6). If p is 0, the decimal point is not displayed.

f   > Displays a floating-point number in “fixed decimal” format, without an exponent, p has the same meaning as for the e specify.

g   > Displays a floating-point number in either exponential format or fixed decimal format, depending on the number's size, p indicates the maximum number of significant digits (not digits after the decimal point) to be dis­played. Unlike the f conversion, the g conversion won't show trailing zeros. Furthermore, if the value to be printed has no digits after the decimal point, g doesn't display the decimal point.

The g specify is especially useful for displaying numbers whose size can't be predicted when the program is written or that tend to vary widely in size. When used to print a moderately large or moderately small number, the g specify uses fixed decimal format.