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GNU Emacs Lisp Reference Manual

3.5 Numeric Conversions

To convert an integer to floating point, use the function float.

Function: float number

This returns number converted to floating point. If number is already a floating point number, float returns it unchanged.

There are four functions to convert floating point numbers to integers; they differ in how they round. These functions accept integer arguments also, and return such arguments unchanged.

Function: truncate number

This returns number, converted to an integer by rounding towards zero.

(truncate 1.2) => 1 (truncate 1.7) => 1 (truncate -1.2) => -1 (truncate -1.7) => -1

Function: floor number &optional divisor

This returns number, converted to an integer by rounding downward (towards negative infinity).

If divisor is specified, floor divides number by divisor and then converts to an integer; this uses the kind of division operation that corresponds to mod, rounding downward. An arith-error results if divisor is 0.

(floor 1.2) => 1 (floor 1.7) => 1 (floor -1.2) => -2 (floor -1.7) => -2 (floor 5.99 3) => 1

Function: ceiling number

This returns number, converted to an integer by rounding upward (towards positive infinity).

(ceiling 1.2) => 2 (ceiling 1.7) => 2 (ceiling -1.2) => -1 (ceiling -1.7) => -1

Function: round number

This returns number, converted to an integer by rounding towards the nearest integer. Rounding a value equidistant between two integers may choose the integer closer to zero, or it may prefer an even integer, depending on your machine.

(round 1.2) => 1 (round 1.7) => 2 (round -1.2) => -1 (round -1.7) => -2