php float calculation 2 decimal point

Question 1

Got a math calculation problem.

$a = 34.56

$b = 34.55

$a do some calculation to get this figure

$b is doing rounding to the nearest 0.05 to get this figure

what happens is

$c = $b - $a

supposedly it be -0.01, but I echo out the $c, which shows -0.00988888888888

I try to use number_format($c, 2), but the output is 0.00,

how can I make sure $a and $b is exactly 2 decimals, no hidden number at the back.

in my php knowledge, number_format is only able to format the display, but the value is not really 2 decimal,

I hope I can get help from here. This really frustrated me.

Question 2

Try sprintf("%.2f", $c);

Floating point numbers are represented in IEEE notation based on the powers of 2, so terminating decimal numbers may not be a terminating binary number, that's why you get the trailing digits.

As suggested by Variable Length Coder, if you know the precision you want and it doesn't change (e.g. when you're dealing with money) it might be better to just use fixed point numbers i.e. express the numbers as cents rather than dollars

$a = 3456;

$b = 3455;

$c = $b - $a;

sprintf ("%.2f", $c/100.0);

This way, you won't have any rounding errors if you do a lot of calculations before printing.

Question 3

A float isn't have 0 or 0.00 : those are different string representations of the internal (IEEE754) binary format but the float is the same.

If you want to express your float as "0.00", you need to format it in a string, using number_format :

$numberAsString = number_format($numberAsFloat, 2);

Question 4

You can use the printf method, like so:

System.out.printf("%.2f", val);

In short, the %.2f syntax tells Java to return your variable (val) with 2 decimal places (.2) in decimal representation of a floating-point number (f) from the start of the format specifier (%).

There are other conversion characters you can use besides f:

d: decimal integer
o: octal integer
e: floating-point in scientific notation

Question 5

You aren't doing anything wrong. Floats are notoriously innaccurate. From the docs (In the huge red warning box):

Floating point numbers have limited precision. Although it depends on the system, PHP typically uses the IEEE 754 double precision format, which will give a maximum relative error due to rounding in the order of 1.11e-16. Non elementary arithmetic operations may give larger errors, and, of course, error propagation must be considered when several operations are compounded.

Additionally, rational numbers that are exactly representable as floating point numbers in base 10, like 0.1 or 0.7, do not have an exact representation as floating point numbers in base 2, which is used internally, no matter the size of the mantissa. Hence, they cannot be converted into their internal binary counterparts without a small 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, since the internal representation will be something like 7.9999999999999991118....

So never trust floating number results to the last digit, and do not compare floating point numbers directly for equality. If higher precision is necessary, the arbitrary precision math functions and gmp functions are available.

Question 6

In case input contains a number, there is no need for an external command like bc. You can just use printf:

printf "%.3fn" "$input"

Edit: In case the input is a formula, you should however use bc as in one of the following commands:

printf "%.3fn" $(bc -l <<< "$input")
printf "%.3fn" $(echo "$input" | bc -l)

php float calculation 2 decimal point

Answers

php,floating-point,decimal,rounding