How to fill missing values from other variable?

Question

My data set is composed as below.

x    y 
0     
0    0
2    2
     2
     4
2
7    7

I want to merge x and y variable like this

x  
0     
0  
2  
2
4
2
7

That is, I want to fill x variable values with y variable values if x are missing. If there is no missing in both x and y variables two values are always same.

Answer 1

Another variant

x = ifelse(is.na(x), y,x)