Question

I saw a calendar program written in C which just says 100. instead of 100.00. The program compiled without any issues.

My question is how is this legal. Shouldnt the C compiler not complain that there are no decimals after the .?

Advance thanks for your answers!

Answer 1

Because section 6.4.4.2 of the language standard, “Floating constants” , defines them thusly:

floating-constant:
    decimal-floating-constant
    hexadecimal-floating-constant

decimal-floating-constant:
    fractional-constant exponent-part(opt) floating-suffix(opt)
    digit-sequence exponent-part floating-suffix(opt)

hexadecimal-floating-constant:
    hexadecimal-prefix hexadecimal-fractional-constant
    binary-exponent-part floating-suffixopt
    hexadecimal-prefix hexadecimal-digit-sequence
    binary-exponent-part floating-suffix(opt)

fractional-constant:
    digit-sequence(opt) . digit-sequence
    digit-sequence .

exponent-part:
    e sign(opt) digit-sequence
    E sign(opt) digit-sequence

sign: one of
    + -

digit-sequence:
    digit
    digit-sequence digit

hexadecimal-fractional-constant:
    hexadecimal-digit-sequence(opt) .
    hexadecimal-digit-sequence
    hexadecimal-digit-sequence .

binary-exponent-part:
    p sign(opt) digit-sequence
    P sign(opt) digit-sequence

hexadecimal-digit-sequence:
    hexadecimal-digit
    hexadecimal-digit-sequence hexadecimal-digit

floating-suffix: one of
    f l F L

Bottom line, all of the following would be valid floating point literals meaning “zero”:

0.

.0

0.0

(Your “100.” would be a valid floating-constant, as it is a decimal-floating-constant consisting of a fractional-constant (omitting the optional exponent-part and floating-suffix); it is a digit-sequence followed by a period, which is valid for a fractional-constant as by the second line of that noteable’s definition.)