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MATLAB^{®} represents floating-point numbers in either double-precision
or single-precision format. The default is double precision, but you
can make any number single precision with a simple conversion function.

MATLAB constructs
the double-precision (or `double`

) data type according
to IEEE^{®} Standard 754 for double precision. Any value stored as
a `double`

requires 64 bits, formatted as shown in
the table below:

Bits | Usage |
---|---|

| Sign ( |

| Exponent, biased by |

| Fraction |

MATLAB constructs
the single-precision (or `single`

) data type according
to IEEE Standard 754 for single precision. Any value stored as
a `single`

requires 32 bits, formatted as shown in
the table below:

Bits | Usage |
---|---|

| Sign ( |

| Exponent, biased by |

| Fraction |

Because MATLAB stores numbers of type `single`

using
32 bits, they require less memory than numbers of type `double`

,
which use 64 bits. However, because they are stored with fewer bits,
numbers of type `single`

are represented to less
precision than numbers of type `double`

.

Use double-precision to store values greater than approximately
3.4 x 10^{38} or less than approximately -3.4
x 10^{38}. For numbers that lie between these
two limits, you can use either double- or single-precision, but single
requires less memory.

Because the default numeric type
for MATLAB is `double`

, you can create a `double`

with
a simple assignment statement:

x = 25.783;

The `whos`

function shows
that MATLAB has created a 1-by-1 array of type `double`

for
the value you just stored in `x`

:

whos x Name Size Bytes Class x 1x1 8 double

Use `isfloat`

if you just
want to verify that `x`

is a floating-point number.
This function returns logical 1 (`true`

) if the input
is a floating-point number, and logical 0 (`false`

)
otherwise:

isfloat(x) ans = logical 1

You can convert other
numeric data, characters or strings, and logical data to double precision
using the MATLAB function, `double`

.
This example converts a signed integer to double-precision floating
point:

y = int64(-589324077574); % Create a 64-bit integer x = double(y) % Convert to double x = -5.8932e+11

Because MATLAB stores
numeric data as a `double`

by default, you need to
use the `single`

conversion function
to create a single-precision number:

x = single(25.783);

The `whos`

function returns
the attributes of variable `x`

in a structure. The `bytes`

field
of this structure shows that when `x`

is stored as
a single, it requires just 4 bytes compared with the 8 bytes to store
it as a `double`

:

xAttrib = whos('x'); xAttrib.bytes ans = 4

You can convert other
numeric data, characters or strings, and logical data to single precision
using the `single`

function.
This example converts a signed integer to single-precision floating
point:

y = int64(-589324077574); % Create a 64-bit integer x = single(y) % Convert to single x = single -5.8932e+11

This section describes which classes you can use in arithmetic operations with floating-point numbers.

You can perform basic arithmetic operations with `double`

and
any of the following other classes. When one or more operands is an
integer (scalar or array), the `double`

operand must
be a scalar. The result is of type `double`

, except
where noted otherwise:

`single`

— The result is of type`single`

`double`

`int*`

or`uint*`

— The result has the same data type as the integer operand`char`

`logical`

This example performs arithmetic on data of types `char`

and `double`

.
The result is of type `double`

:

c = 'uppercase' - 32; class(c) ans = double char(c) ans = UPPERCASE

You can perform basic arithmetic operations with `single`

and
any of the following other classes. The result is always `single`

:

`single`

`double`

`char`

`logical`

In this example, 7.5 defaults to type `double`

,
and the result is of type `single`

:

x = single([1.32 3.47 5.28]) .* 7.5; class(x) ans = single

For the `double`

and `single`

classes,
there is a largest and smallest number that you can represent with
that type.

The MATLAB functions `realmax`

and `realmin`

return
the maximum and minimum values that you can represent with the `double`

data
type:

str = 'The range for double is:\n\t%g to %g and\n\t %g to %g'; sprintf(str, -realmax, -realmin, realmin, realmax) ans = The range for double is: -1.79769e+308 to -2.22507e-308 and 2.22507e-308 to 1.79769e+308

Numbers larger than `realmax`

or smaller than `-realmax`

are
assigned the values of positive and negative infinity, respectively:

realmax + .0001e+308 ans = Inf -realmax - .0001e+308 ans = -Inf

The MATLAB functions `realmax`

and `realmin`

,
when called with the argument `'single'`

, return
the maximum and minimum values that you can represent with the `single`

data
type:

str = 'The range for single is:\n\t%g to %g and\n\t %g to %g'; sprintf(str, -realmax('single'), -realmin('single'), ... realmin('single'), realmax('single')) ans = The range for single is: -3.40282e+38 to -1.17549e-38 and 1.17549e-38 to 3.40282e+38

Numbers larger than `realmax('single')`

or
smaller than `-realmax('single')`

are assigned the
values of positive and negative infinity, respectively:

realmax('single') + .0001e+038 ans = single Inf -realmax('single') - .0001e+038 ans = single -Inf

If the result of a floating-point arithmetic computation is not as precise as you had expected, it is likely caused by the limitations of your computer's hardware. Probably, your result was a little less exact because the hardware had insufficient bits to represent the result with perfect accuracy; therefore, it truncated the resulting value.

Because there are only a finite number of double-precision numbers,
you cannot represent all numbers in double-precision storage. On any
computer, there is a small gap between each double-precision number
and the next larger double-precision number. You can determine the
size of this gap, which limits the precision of your results, using
the `eps`

function. For example,
to find the distance between `5`

and the next larger
double-precision number, enter

format long eps(5) ans = 8.881784197001252e-16

This tells you that there are no double-precision numbers between
5 and `5 + eps(5)`

.
If a double-precision computation returns the answer 5, the result
is only accurate to within `eps(5)`

.

The value of `eps(x)`

depends on `x`

.
This example shows that, as `x`

gets larger, so does `eps(x)`

:

eps(50) ans = 7.105427357601002e-15

If you enter `eps`

with no input argument, MATLAB returns
the value of `eps(1)`

, the distance from `1`

to
the next larger double-precision number.

Similarly, there are gaps between any two single-precision numbers.
If `x`

has type `single`

, `eps(x)`

returns
the distance between `x`

and the next larger single-precision
number`.`

For example,

x = single(5); eps(x)

returns

ans = single 4.7684e-07

Note that this result is larger than `eps(5)`

.
Because there are fewer single-precision numbers than double-precision
numbers, the gaps between the single-precision numbers are larger
than the gaps between double-precision numbers. This means that results
in single-precision arithmetic are less precise than in double-precision
arithmetic.

For a number `x`

of type `double`

, `eps(single(x))`

gives
you an upper bound for the amount that `x`

is rounded
when you convert it from `double`

to `single`

.
For example, when you convert the double-precision number `3.14`

to `single`

,
it is rounded by

double(single(3.14) - 3.14) ans = 1.0490e-07

The amount that `3.14`

is rounded is less than

eps(single(3.14)) ans = single 2.3842e-07

Almost all operations in MATLAB are performed in double-precision arithmetic conforming to the IEEE standard 754. Because computers only represent numbers to a finite precision (double precision calls for 52 mantissa bits), computations sometimes yield mathematically nonintuitive results. It is important to note that these results are not bugs in MATLAB.

Use the following examples to help you identify these cases:

The decimal number `4/3`

is not exactly representable
as a binary fraction. For this reason, the following calculation does
not give zero, but rather reveals the quantity `eps`

.

e = 1 - 3*(4/3 - 1) e = 2.2204e-16

Similarly, `0.1`

is not exactly representable
as a binary number. Thus, you get the following nonintuitive behavior:

a = 0.0; for i = 1:10 a = a + 0.1; end a == 1 ans = logical 0

Note that the order of operations can matter in the computation:

b = 1e-16 + 1 - 1e-16; c = 1e-16 - 1e-16 + 1; b == c ans = logical 0

There are gaps between floating-point numbers. As the numbers get larger, so do the gaps, as evidenced by:

(2^53 + 1) - 2^53 ans = 0

Since `pi`

is not really π, it is not
surprising that `sin(pi)`

is not exactly zero:

sin(pi) ans = 1.224646799147353e-16

When subtractions are performed with nearly equal operands, sometimes cancellation can occur unexpectedly. The following is an example of a cancellation caused by swamping (loss of precision that makes the addition insignificant).

sqrt(1e-16 + 1) - 1 ans = 0

Some functions in MATLAB, such as `expm1`

and `log1p`

,
may be used to compensate for the effects of catastrophic cancellation.

Round-off, cancellation, and other traits of floating-point
arithmetic combine to produce startling computations when solving
the problems of linear algebra. MATLAB warns that the following
matrix `A`

is ill-conditioned, and therefore the
system `Ax = b`

may be sensitive to small perturbations:

A = diag([2 eps]); b = [2; eps]; y = A\b; Warning: Matrix is close to singular or badly scaled. Results may be inaccurate. RCOND = 1.110223e-16.

These are only a few of the examples showing how IEEE floating-point arithmetic affects computations in MATLAB. Note that all computations performed in IEEE 754 arithmetic are affected, this includes applications written in C or FORTRAN, as well as MATLAB.

See Floating-Point Functions for a list of functions most commonly used with floating-point numbers in MATLAB.

The following references provide more information about floating-point arithmetic.

[1] Moler, Cleve, "Floating Points," *MATLAB News
and Notes*, Fall, 1996. A PDF version is available on the
MathWorks Web site at http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf

[2] Moler, Cleve, *Numerical Computing
with MATLAB*, S.I.A.M. A PDF version is available
on the MathWorks Web site at http://www.mathworks.com/moler/.

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