Substitute

Substitute[ expr, a, b ] replaces all occurrences of an expression with another expression.

Usages

Substitute[ expr, a, b ] replaces every occurrence of a in expr with b. The result is not calculated; use Simplify for that.

Example	in	out
Replace `x^2` with `p`.	`Substitute[ x^4 + x^2 + 3x^2 + 6, x^2, p ]`	`x^4+p+3p+6`
Simplify the result.	`Simplify[ Substitute[ x^4 + x^2 + 3x^2 + 6, x^2, p ] ]`	$4p+x^{4}+6$
Substitute a value for a variable.	`Substitute[ x^2 + 2x + 1, x, 3 ]`	`3^2+2*3+1`
Substitute an expression; brackets are added where needed.	`Substitute[ x^2 + 2x + 1, x, a+1 ]`	`(a+1)^2+2(a+1)+1`
Replace a function.	`Substitute[ Sin[x] + Cos[x], Sin, Cos ]`	`Cos[x]+Cos[x]`
Replace elements of a list.	`Substitute[ {a, b, a}, a, c ]`	`{c, b, c}`
Replace a parameter in a mapping.	`f := x -> x^2 + a r := Substitute[f, a, 3]`	`r = x -> x^2+3`

Options

If set to True, variables in b with the same name as the variable of a mapping in expr are linked to that mapping variable. Default: False.

Syntax
`Substitute[ expr, a, b, SubstituteMappingVariables=>True ]`

Details

Whole sub-expressions: a is only replaced where it occurs as a complete part of the expression. x+y is not replaced in x+y+1, because that sum has the terms x, y and 1: Substitute[ x+y+1, x+y, z ] gives x+y+1. The order of terms and factors does not matter: Substitute[ 2x, x*2, z ] gives z.
Mapping variables are not replaced: in f := x -> x^2 + a, Substitute[f, x, 3] does not change f. Use FuncApplication or f[3] to evaluate a mapping.

Invalid input	Result
`Substitute[ x^2, x ]`	Left unevaluated: `Substitute` takes three arguments.
`Substitute[ x^2, x, 3, Foo=>1 ]`	Error: Invalid associative argument for Substitute: Foo