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rexpressioneval

Evaluate values within expression function

Using the R language, if I want to plug in some real (evaluated) values in the expression and keep other parameters unresolved, what should I do? For example:

> b = 1
> a = 2
> expr = expression(b+a)
> expr
expression(b+a)
> eval(expr)
[1] 3

But what if I want the expr equal to

expression(1+a)

How can I preprocess the variable b?

Thanks so much!

Solution

  • Then you do not need to evaluate, since what you need is an expression. you need to substitute:

    a <- 2
b <- 1
substitute(expression(b + a),list(b=1))
expression(1 + a)