if - else if - else 语句和括号

Question

I understand the usual way to write an "if - else if" statement is as follow:

if (2==1) {
  print("1")
} else if (2==2) {
  print("2")
} else {
  print("3")
}

or

if (2==1) {print("1") 
} else if (2==2) {print("2")
} else print("3")

On the contrary, If I write in this way

if (2==1) {
  print("1")
} 
else if (2==2) {
  print("2")
}
else (print("3"))

or this way:

if (2==1) print("1") 
else if (2==2) print("2")
else print("3")

the statement does NOT work. Can you explain me why } must precede else or else if in the same line? Are there any other way of writing the if-else if-else statement in R, especially without brackets?

Answer 1

R reads these commands line by line, so it thinks you're done after executing the expression after the if statement. Remember, you can use if without adding else .

Your third example will work in a function, because the whole function is defined before being executed, so R knows it wasn't done yet (after if() do ).

Answer 2

In R, also we have ifelse() function:

ifelse(1 < 0, "hello", "hi")

Output:

# [1] "hi"

Answer 3

As hrbrmstr has mentioned:

When the initial if is followed by a compound expression (indicated by the {} pair) the parser by default is going to expect the expression followed by else to be compound as well. The only defined use of else is with compound expressions.

In the statement if(cond) cons.expr else alt.expr , the else needs to be put after and in same line with the end `cons.expr' compound.

So if you want to have your code a better look without brackets, apply this way:

if (2==1) print("1") else 
   if (2==2) print("2") else 
      print("3")

Answer 4

ifelse 具有树参数、第一个条件、第二个真结果和最后一个假结果。

y_pred = ifelse(prob_predict > 0.5,1,0)

Answer 5

It's good idea to use braces when there are nested ifs. For example, in

if(n>0)
    if(a>b)
        z=a;
    else
        z=b;

the else goes with inner if not with if(n>0). If that isn't what you want, braces must be used to force proper association:

if(n>0){
    if(a>b)
        z=a;
}
else
    z=b;

For more detail, see a very good complete tutorial: Conditional statements: if-else, else-if and switch in C ! Hope this helps you!