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# 浮點數

指定浮點形態 (DOUBLES) 的變數可用以下句法 :

`$a = 1.234; $a = 1.2e3;`

The size of a floating point number is platform-dependent, although a maximum of ~1.8e308 with a precision of roughly 14 decimal digits is a common value (that's 64 bit IEEE format).

切記!
It is quite usual that simple decimal fractions like 0.1 or 0.7 cannot be converted into their internal binary counterparts without a little loss of precision. This can lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7 instead of the expected 8 as the result of the internal representation really being something like 7.9999999999.... This is related to the fact that it is impossible to exactly express some fractions in decimal notation with a finite number of digits. For instance, 1/3 in decimal form becomes 0.3333333. . .. So never trust floating number results to the last digit and never compare floating point numbers for equality. If you really need higher precision, you should use the arbitrary precision math functions or gmp functions instead.

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