How do I conditionally change the value of a variable based on the value of another variable?

I have two variablesm MARKSV1201 and MARKSVA1201. MARKSVA1201 is only ever relevant if MARKSV1201 is missing, otherwise it will only mess up my analysis.

I try to write a script that sets MARKSVA1201 to '0' whenever a value for MARKSV1201 is recorded

if(!is.na(test$`MARKSV1201     `)){test$`MARKSVA1201    `=0}

This doesn't seem to work however, the program complains that the "condition is >1 and only the first element will be used"

I try using the ifelse-statement instead since I am working with vectors:

ifelse(!is.na(test$`MARKSV1201     `),test$`MARKSVASVA01   `==0,test$`MARKSVASVA01   `==test$`MARKSVASVA01   `)

This seems to work, but I'm only getting a logical vector.

How do I go about changing my actual values conditionally?