如何将NULL文本字符串转换为数字？

Question

I have input data from SQL which has fields that are the string "NULL", such that

unique(file$x) 

returns

Levels: 1 10 14 8 NULL

I would like any of the results that are "NULL" to become "0" and then to convert using as.numeric().

I have tried:

i <- 1
for(i <= nrow(file) {
    if(file$x[i] == "NULL") {
        file$x[i] <- 0
    }
i <- i + 1
}

However, I get the result now that the NULL has simply turned into and I am still unable to convert it to the number 0.

Answer 1

If you're reading this in with read.csv , set the option na.strings='NULL' . This will import file$x as numeric , instead of factor , with numeric NA s objects in place of the NULL strings. Then to replace NA s with 0 s:

file$x[is.na(file$x)] <- 0

Answer 2

In your SQL Query add: ISNULL(myNullableField, 0)

This makes sure that - if your value is NULL - it is converted to 0

it is a select query I presume?

Answer 3

在R中：

file$x[file$x=="NULL"] <- 0

Answer 4

Although you should fix this problem at source (ie SQL), here is one way to do it.

convert the factors to characters, then replace NA by 0

I am not sure if this is right way to do for very big data though.

> x <- as.factor(c('1', '10', '14', '8', 'NULL'))
> y <- as.numeric(as.character(x))
Warning message:
NAs introduced by coercion 
> y[is.na(y)] <- 0
> y
[1]  1 10 14  8  0