subs (Symbolic Math Toolbox)

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Symbolic substitution in a symbolic expression or matrix.

Syntax

R = subs(S) 
R = subs(S,old,new)

Description

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

subs(S,old,new) replaces old with new in the symbolic expression S. old is a symbolic variable or a string representing a variable name. new is a symbolic or numeric variable or expression.

If old and new are cell 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, 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) 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}) 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]