In short, I have a list of expressions that I want to apply to each row of a dataframe. This is very similar to this question, but there is a subtle difference in that I do not have a list of *functions*, but have a list of *expressions*. Here's what I am attempting:

I have a dataframe of equation terms that I pass to `paste`

to form text strings of expressions:

```
# build a test dataframe
a <- c(10, 11, 12)
b <- c(0.5, 06, 0.7)
c <- c(300, 400, 500)
d <- c(1000, 2000, NA)
p <- c(1.32, 1.37, 1.43)
m <- c(0.45, .55, .65)
params <- data.frame(a, b, c, d, p, m)
# create vector of expressions
ff.txt <- paste(params$a, "*2 + ",
params$b, "*3 + ",
params$c, "*4 + ",
params$d, "*5 * ",
"x * ",
params$p, " * ",
params$m,
sep = "")
```

This will return a vector of expressions stored as text, such as "10*2 + 0.5*3 + 300*4 + 1000*5 * x * 1.32 * 0.45".

I then wrote a function to convert the text expression to an R expression using `parse`

:

```
funs.list <- lapply(funs.txt, function(x) parse(file = "", text = x))
```

... where `funs.txt`

is the vector of expressions stored as text.

Now, this is where I'm having difficulty. Imagine `funs.list`

contains two expressions. If I do something like:

```
lapply(funs.list, eval, envir = list(x=c(7, 60)))
```

... I get back a 2x2 matrix, where I only want the answers are on the diagonal. (Ultimately I'm going to have about 1,000 expressions that will be called repeatedly as part of an optimization, so do not want the additional computational effort.)

The other approach I've tried is:

```
mapply(do.call, funs.list, lapply(params, list))
```

... but I get an error message from `do.call`

stating `'what' must be a character string or a function`

.

Any suggestions on where to go from here?

Answer:

If I understand correctly you want to evaluate the first expression with the first value of x, the second with the second etc.

You could do:

```
mapply(function(ex, x) eval(ex, envir = list(x = x)), funs.list[1:2], c(7, 60))
```

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