Question

I know that in Java it is often quite handy to do things like that

if(a!=0 && b/a>1){
  ...;
}

Java stops when the first part is already false. R does not do that, producing errors sometimes. So: Is there a possibility to make this code shorter:

if(exists("user_set_variable")){
   if(user_set_variable < 3){
      ...
   }
}

Answer 1

From ?'&&' you can find

& and && indicate logical AND and | and || indicate logical OR.
The shorter form performs elementwise comparisons in much the same
way as arithmetic operators. The longer form evaluates left to right 
examining only the first element of each vector. Evaluation proceeds
only until the result is determined. The longer form is appropriate
for programming control-flow and typically preferred in if clauses.

So probably you’re looking for & instead of &&. See these examples where two conditions are evaluated:

# Case 1
a<- 2
b <- 4

 if(a!=0 &  b/a>1){
   print('Hello World')
 } else{
   print("one or both conditions not met")
 }
[1] "Hello World"

# Case 2
a<- 2
 b <- 1

 if(a!=0 & b/a>1){
   print('Hello World')
 } else{
  print("one or both conditions not met")
 }
[1] "one or both conditions not met"


 # Case 3
 a<- 0
 b <- 1

 if(a!=0 &  b/a>1){
   print('Hello World')
 } else{
  print("one or both conditions not met")
 }
[1] "one or both conditions not met"