根据R中的条件保留先前的数据帧

Question

So I'm trying to update or retain a dataframe df2 based on a certain condition of another data frame df1 .

For Example, Assuming df1 get updated for every 30 seconds, so if the number of rows in df1 ie nrow(df1)!= 0 then df2 <- df1 else if retain the previous values in df2 .

NOTE: On the first iteration, df2 can be initialized to a NULL dataframe.

Following is my code

#Initializing df2 as empty dataframe
df2 <- data.frame(weight = integer(),stringAsFactors = FALSE) 

#Condition to check if number of rows in df1 != 0
if(nrow(df1) != 0){

     df2 <- df1
     temp <- df1 #Another copy of df1
}

else{

   df2 <- temp
}

Here I created an another data frame called temp to keep a copy of df1 so that it can be used when nrow(df1) == 0 . I don't know if the usage of temp is correct or not.

Answer 1

This code will create an empty dataframe named df2 . If nrow(df1)>0 then it will effectively assign the contents of df1 to df2 . If nrow(df1)==0 then df2 remains empty.

df2 <- data.frame()
if(nrow(df1)>0) df2 <- df1

I have a hard time imagining why this is useful. If, perhaps, you intended to "grow" df2 by appending on whatever is in df1 - which might be more common - then do something like this:

df2 <- data.frame()
if(nrow(df1)>0) df2 <- rbind(df2, df1)