PHP Numbers Rounding Floating-Point Numbers

PHP Numbers

Rounding Floating-Point Numbers

Problem

You want to round a floating-point number, either to an integer value or to a set number of decimal places.

Solution

To round a number to the closest integer, use round():

              $number = round(2.4);
              printf("2.4 rounds to the float %s", $number);

This prints:

              2.4 rounds to the float 2

To round up, use ceil():

              $number = ceil(2.4);
              printf("2.4 rounds up to the float %s", $number);

This prints:

              2.4 rounds up to the float 3

To round down, use floor():

              $number = floor(2.4);
              printf("2.4 rounds down to the float %s", $number);

This prints:

              2.4 rounds down to the float 2

Discussion

If a number falls exactly between two integers, PHP rounds away from 0:

              $number = round(2.5);
              printf("Rounding a positive number rounds up: %s\n", $number);

              $number = round(-2.5);
              printf("Rounding a negative number rounds down: %s\n", $number);

This prints:

              Rounding a positive number rounds up: 3
              Rounding a negative number rounds down: -3

You may remember that floating-point numbers don’t always work out to exact values because of how the computer stores them. This can create confusion. A value you expect to have a decimal part of “0.5” might instead be “.499999…9” (with a whole bunch of 9s) or “.500000…1” (with many 0s and a trailing 1).

PHP automatically incorporates a little “fuzz factor” into its rounding calculations, so
you don’t need to worry about this.

To keep a set number of digits after the decimal point, round() accepts an optional
precision argument. For example, perhaps you are calculating the total price for the
items in a user’s shopping cart:

             $cart = 54.23;
             $tax = $cart * .05;
             $total = $cart + $tax;
             $final = round($total, 2);

             print "Tax calculation uses all the digits it needs: $total, but ";
             print "round() trims it to two decimal places: $final";

This prints:

             Tax calculation uses all the digits it needs: 56.9415, but round()

             trims it to two decimal places: 56.94

To round a number down, use the floor() function:

             $number1 = floor(2.1); // floor(2.1) is the float 2.0

             $number2 = floor(2.9); // floor(2.9) is the float 2.0, also

             $number3 = floor(-2.1); // floor(-2.1) is the float -3.0

             $number4 = floor(-2.9); // floor(-2.9) is the float 3.0, also

To round up, use the ceil() function:

             $number1 = ceil(2.1); // ceil(2.1) is the float 3.0

             $number2 = ceil(2.9); // ceil(2.9) is the float 3.0, also

             $number3 = ceil(-2.1); // ceil(-2.1) is the float -2.0

             $number4 = ceil(-2.9); // ceil(-2.9) is the float 2.0, also

These two functions are named because when you’re rounding down, you’re rounding

“toward the floor,” and when you’re rounding up, you’re rounding “toward the ceiling.”