Intermediate R

calculate_max <- function(values){
    max_value <- NA

    for (value in values){
        number <- as.numeric(value)

        if (is.na(number)){
            warning("Ignoring '", value,
                    "' as it is not a number")
        } else {
            if (is.na(max_value) || number > max_value){
                max_value = number
            }
        }
    }

    if (is.na(max_value)){
        stop("You can only find the maximum of numeric values")
    }

    return(max_value)
}

> calculate_max(c("cat", "dog", 5))
[1] "5"
Warning messages:
1: In calculate_max(c("cat", "dog", 5)) : NAs introduced by coercion
2: In calculate_max(c("cat", "dog", 5)) :
  Ignoring 'cat' as it is not a number
3: In calculate_max(c("cat", "dog", 5)) : NAs introduced by coercion
4: In calculate_max(c("cat", "dog", 5)) :
  Ignoring 'dog' as it is not a number

> calculate_max(c("cat", "dog"))
Error in calculate_max(c("cat", "dog")) : 
  You can only find the maximum of numeric values
In addition: Warning messages:
1: In calculate_max(c("cat", "dog")) : NAs introduced by coercion
2: In calculate_max(c("cat", "dog")) :
  Ignoring 'cat' as it is not a number
3: In calculate_max(c("cat", "dog")) : NAs introduced by coercion
4: In calculate_max(c("cat", "dog")) :
  Ignoring 'dog' as it is not a number