[R] help in substitute function

[Ronaldo Reis Junior]

Hi,

I try to use a substitute function to generalise a equation.

I have this:

expression(1+2*pred+3*lat)

I need to define the equation in function of x1 and x2 variables. Also I try 
to define who is x1 and x2. I try this:

X1 <- "pred"
X2 <- "lat"

Now I need substitute pred and lat in equation by x1 and x2 defined on X1 and 
X2 above, I try this way:

> 
substitute(expression(1+2*pred+3*lat),list(as.name(X1)=as.name(x1),as.name(X2)=as.name(x2))
Error: unexpected '=' 
in "substitute(expression(1+2*pred+3*lat),list(as.name(X1)="

My expected result is:

expression(1 + 2 * x1 + 3 * x2)

Any help?
Thanks
Ronaldo
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