If语句-条件长度大于1

我有两列( df $ Z 和 df $ A )

我基本上要说:如果df $ Z小于5，则用 NA 填充 df $ A ，如果没有，则保留 df $一个.我已经尝试了这些方法，但是不确定哪里出了问题或错误消息表示什么.

I basically want to say: if df$Z is less than 5, then fill df$A with an NA and if not then leave df$A alone. I've tried these things but am not sure where I'm going wrong or what the error message means.

if(df$X<5){df$A <- NA}

错误:

如果if(df $ X< 5){:条件的长度> 1，并且仅使用第一个元素

In if (df$X < 5) { : the condition has length > 1 and only the first element will be used

我也尝试做更多类似的事情.

I also tried to do something more like this.

for(i in dfX){
if(df$X<5){
df$A <- "NA"
 }
}