gcd - Greatest common divisor - MATLAB (original) (raw)

Syntax

Description

[G](#btrjwl2-1-G) = gcd([A,B](#btrjwl2-1-AB)) returns the greatest common divisors of the elements of A and B. The elements in G are always nonnegative, and gcd(0,0) returns 0. This syntax supports inputs of any numeric type.

example

[[G](#btrjwl2-1-G),[U,V](#btrjwl2-1-UV)] = gcd([A,B](#btrjwl2-1-AB)) also returns the Bézout coefficients, U and V, which satisfy: A.*U + B.*V = G. The Bézout coefficients are useful for solving Diophantine equations. This syntax supports double, single, and signed integer inputs.

example

Examples

collapse all

A = [-5 17; 10 0]; B = [-15 3; 100 0]; G = gcd(A,B)

gcd returns positive values, even when the inputs are negative.

A = uint16([255 511 15]); B = uint16([15 127 1023]); G = gcd(A,B)

G = 1×3 uint16 row vector

15 1 3

Solve the Diophantine equation, 30x+56y=8 for x and y.

Find the greatest common divisor and a pair of Bézout coefficients for 30 and 56.

u and v satisfy the Bézout's identity, (30*u) + (56*v) = g.

Rewrite Bézout's identity so that it looks more like the original equation. Do this by multiplying by 4. Use == to verify that both sides of the equation are equal.

(30u4) + (56v4) == g*4

Calculate the values of x and y that solve the problem.

Input Arguments

collapse all

Input values, specified as scalars, vectors, or arrays of real integer values. A and B can be any numeric type, and they can be of different types within certain limitations:

If A or B is of type single, then the other can be of type single or double.
If A or B belongs to an integer class, then the other must belong to the same class or it must be a double scalar value.

A and B must be the same size or one must be a scalar.

Example: [20 -3 13],[10 6 7]

Example: int16([100 -30 200]),int16([20 15 9])

Example: int16([100 -30 200]),20

Output Arguments

collapse all

Greatest common divisor, returned as an array of real nonnegative integer values. G is the same size as A and B, and the values in G are always real and nonnegative. G is returned as the same type as A and B. If A and B are of different types, then G is returned as the nondouble type.

Bézout coefficients, returned as arrays of real integer values that satisfy the equation, A.*U + B.*V = G. The data type of U and V is the same type as that of A and B. If A and B are of different types, then U and V are returned as the nondouble type.

Algorithms

g = gcd(A,B) is calculated using the Euclidean algorithm.[1]

[g,u,v] = gcd(A,B) is calculated using the extended Euclidean algorithm.[1]

References

[1] Knuth, D. “Algorithms A and X.” The Art of Computer Programming, Vol. 2, Section 4.5.2. Reading, MA: Addison-Wesley, 1973.

Extended Capabilities

Version History

Introduced before R2006a