function (unbalanced = FALSE)
{
    v.rates <- c(0.58,0.28,0.21,0.07,0.05,0.02,0.01,0.01,
0.5,0.18,0.14,0.11,0,0.01,0.02,0.01,
0.57,0.14,0.1,0,0,0.01,0,0.02,
1,0.42,0.11,0.33,0,0,0,0,
0.67,0.67,0.57,0.25,0.04,0,0,0)
    v.cases <- c(7,11,16,2,7,6,1,1,
7,18,14,3,0,1,6,2,
4,8,11,0,0,0,0,1,
4,5,3,5,0,0,0,0,
2,2,4,1,1,0,0,0)
    row.names <- c("25-29", "30-34", "35-39", "40-44", "45-49")
    col.names <- c("1955-1959", "1960-1964", "1965-1969", "1970-1974",
        "1975-1978","1979-1983","1984-1988","1989-1993")
    rates <- matrix(data = v.rates, nrow = 5, ncol = 8, byrow = TRUE,
        dimnames = list(row.names, col.names))
    cases <- matrix(data = v.cases, nrow = 5, ncol = 8, byrow = TRUE,
        dimnames = list(row.names, col.names))
    index.col <- seq(1, 4)
    if (unbalanced)
        index.col <- seq(1:5)
    return(list(rates = rates[, index.col], response = cases[,
        index.col], dose = cases[, index.col]/rates[, index.col],
        data.format = "AP", age1 = 25, per1 = 1955, coh1 = NULL,
        unit = 5, per.zero = NULL, per.max = NULL, time.adjust = 0))
}

请问这里有NAN的出现，怎么利用循环语句把NAN替换成0呢