subs（）函数（）

摘自matlab

Utilities for obsolete MUTOOLS commands.

subs 既是目录也是函数。

subs Symbolic substitution.
subs(S,OLD,NEW) replaces OLD with NEW in the symbolic expression S.
OLD is a symbolic variable, a string representing a variable name, or
a string (quoted) expression. NEW is a symbolic or numeric variable
or expression.

subs(S,VALUES), where VALUES is a STRUCT, replaces the symbolic
variables in S which are field names in VALUES by the corresponding
entries of the struct.

subs(S) replaces all the variables in the symbolic expression S with
values obtained from the calling function, or the MATLAB workspace.

subs(S,NEW) replaces the free symbolic variable in S with NEW.

If OLD and NEW are vectors or arrays of the same size, each element
of OLD is replaced by the corresponding element of NEW.  If S and OLD
are scalars and NEW is an array or cell array, the scalars are expanded
to produce an array result.  If NEW is a cell array of numeric matrices,
the substitutions are performed elementwise (i.e., subs(x*y,{x,y},{A,B})
returns A.*B when A and B are numeric).

If subs(S,OLD,NEW) does not change S, then subs(S,NEW,OLD) is tried.
This provides backwards compatibility with previous versions and
eliminates the need to remember the order of the arguments.
subs(S,OLD,NEW,0) does not switch the arguments if S does not change.

Examples:
  Single input:
    Suppose a = 980 and C1 = 3 exist in the workspace.
    The statement
       y = dsolve('Dy = -a*y')
    produces
       y = exp(-a*t)*C1
    Then the statement
       subs(y)
    produces
       ans = 3*exp(-980*t)

  Single Substitution:
    subs(a+b,a,4) returns 4+b.

  Multiple Substitutions:
    subs(cos(a)+sin(b),{a,b},[sym('alpha'),2]) or
    subs(cos(a)+sin(b),{a,b},{sym('alpha'),2}) returns
    cos(alpha)+sin(2)

  Scalar Expansion Case:
    subs(exp(a*t),'a',-magic(2)) returns

    [   exp(-t), exp(-3*t)]
    [ exp(-4*t), exp(-2*t)]

  Multiple Scalar Expansion:
    subs(x*y,{x,y},{[0 1;-1 0],[1 -1;-2 1]}) returns
    [  0, -1]
    [  2,  0]

用法（翻译）

• 用于过时的MUTOOLS命令的实用程序。
  subs既是目录也是函数。
  subs 用符号替换。
  subs(S,OLD,NEW)：用NEW来带替S中的OLD。OLD是一个符号变量，代表了一个变量的名字或者是表达式。NEW是一个数字或者符号变量或者是表达式。

• subs(S,VALUES)：其中value是STRUCT，用STRUCT的相应条目替换S中的符号变量，S是值中的字段名。
  subs(S)：用从调用函数或MATLAB工作区中获得的值替换符号表达式S中的所有变量。