问使用nls跳过数据表计算中的错误

EN

## Example data table
DT <- data.table(
     x = c(1,2,3,4,5,6,7,8,
           1,2,3,4,5,6,7,8,
           1,2,3,4,5,6,7,8),
     y = c(15.4,16,16.4,17.7,20,23,27,35,
           25.4,26,26.4,27.7,30,33,37,45,
           27.4,28,28.4,29.7,32,35,39,47),
groups = c(1,1,1,1,1,1,1,1,
           2,2,2,2,2,2,2,2,
           3,3,3,3,3,3,3,3)
)

## Fit exponential curve using starting values a,b,c for each group 
DT[, c("sigma", "a", "b", "c") := {
    c.0 <- min(y) * 0.5
    model.0 <- lm(log(y - c.0) ~ x, data=.SD)
    start <- list(a=exp(coef(model.0)[1]), b=coef(model.0)[2], c=c.0)
    model <- nls(y ~ a * exp(b * x) + c,
        data=.SD, 
        start=start, 
        control=nls.control(maxiter=500))
    c(.(sigma=summary(model)$sigma), as.list(coef(model)))
},
by=.(groups)]

## Modify data table to ruin nls model for group 2
set(DT, i=16L, j="y", value=3)

## Calculation works for group 1 but stops for group 2 and onwards
DT[, c("sigma", "a", "b", "c") := {
    c.0 <- min(y) * 0.5
    model.0 <- lm(log(y - c.0) ~ x, data=.SD)
    start <- list(a=exp(coef(model.0)[1]), b=coef(model.0)[2], c=c.0)
    model <- nls(y ~ a * exp(b * x) + c,
        data=.SD, 
        start=start, 
        control=nls.control(maxiter=500))
    c(.(sigma=summary(model)$sigma), as.list(coef(model)))
},
by=.(groups)]

## My poor attempt at using a tryCatch just gives NA to every column
DT[, c("sigma","a", "b", "c") := {
    c.0 <- min(y) * 0.5
    model.0 <- lm(log(y - c.0) ~ x, data=.SD)
    start <- list(a=exp(coef(model.0)[1]), b=coef(model.0)[2], c=c.0)
    model <- tryCatch(
        {
            nls(y ~ a * exp(b * x) + c,
                data=.SD, 
                start=start, 
                control=nls.control(maxiter=500))
            c(.(sigma=summary(model)$sigma), as.list(coef(model)))
        },
        error=function(err){
            return(NA_real_)
        }
    )
},
by=.(groups)]