[R] parameterization question

[Dale Garman]

How can I parameterize the function call to aggregate() below with colx (a
character string) in the following code?   That is, replace both if
statements with aaa <- aggregate(colx_something, data=tidy[grpyrs,],sum).
I've tried a couple of ideas for "colx_something" but get an R-error each
time.  Been using R for 5-6 months and want to expand my knowledge.

explain_causes <- function(spread, colx, peakyears) {
    for(i in peakyears) {
        grpyrs <- tidy$year >= (i-spread) & tidy$year <= (i+spread)
        if(colx == "harm")
            aaa <- aggregate(harm ~ EVTYPE, data=tidy[grpyrs,],sum)
        if(colx == "damage")
            aaa <- aggregate(damage ~ EVTYPE, data=tidy[grpyrs,],sum)
        .
        .
        .
    }
}

# "harm" is a column var in tidy dataframe
explain_causes(0,"harm",c(1953,1965,1974,1979,1984,1995,1998,2011))

# "damage" is a column var in tidy dataframe
explain_causes(0,"damage",c(2005,2006))

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