matlab rand 函數

我們經常看到rand('state',10) ,rand('seed',10)  不知道什麼意思。

關于rand 具體用法 可以輸入help rand 

>> help rand

RAND   Uniformly distributed random numbers.

    RAND(N) is an N-by-N matrix with random entries, chosen from

    a uniform distribution on the interval (0.0,1.0).

    RAND(M,N) and RAND([M,N]) are M-by-N matrices with random entries.

    RAND(M,N,P,...) or RAND([M,N,P,...]) generate random arrays.

    RAND with no arguments is a scalar whose value changes each time it

    is referenced.  RAND(SIZE(A)) is the same size as A.  

    RAND produces pseudo-random numbers.  The sequence of numbers

    generated is determined by the state of the generator.  Since MATLAB

    resets the state at start-up, the sequence of numbers generated will

    be the same unless the state is changed.

    S = RAND('state') is a 35-element vector containing the current state

    of the uniform generator.  RAND('state',S) resets the state to S.

    RAND('state',0) resets the generator to its initial state.

    RAND('state',J), for integer J, resets the generator to its J-th state.

    RAND('state',sum(100*clock)) resets it to a different state each time.

    This generator can generate all the floating point numbers in the 

    closed interval [2^(-53), 1-2^(-53)].  Theoretically, it can generate

    over 2^1492 values before repeating itself.

    MATLAB Version 4.x used random number generators with a single seed.

    RAND('seed',0) and RAND('seed',J) cause the MATLAB 4 generator to be used.

    RAND('seed') returns the current seed of the MATLAB 4 uniform generator.

    RAND('state',J) and RAND('state',S) cause the MATLAB 5 generator to be used.

 看了這一大堆英文可能還不如一個例子講得明白，看下面程式：

>> rand('state',10)
>> a=rand

a =

    0.4493

>> a

a =

    0.4493

>> a=rand

a =

    0.2088

>> rand('state',10) %reset the state to the 10-th state.
>> a=rand

a =

    0.4493 %%從這可以看到又跟開始一樣了。是以每次設定rand('state',10)後獲得的随機數相同。

>> rand('seed',10)
>> a=rand

a =

    0.1291

>> a

a =

    0.1291

>> rand

ans =

    0.6048

>> rand('seed',10)
>> a=rand

a =

    0.1291  %%又是0.1291