我如何正确编写 if - else if 代码

Question

I'm trying to make an if - else if statement writing two comparison operators in the 'else if' line (Int type). maybe is the syntax wrong?

i've tried to remove the parentheses from 'boysAge >= 21' but same results

var boysAge = 21
var message = "The customer is "
if boysAge < 21 {
    message += "underage"
}
else if (boysAge >= 21) && <70 {
    message += "allowed"
}
print(message)

xcode says that '<' is not a prefix unary operator

Answer 1

var boysAge = 21
var message = "The customer is "
if boysAge < 21 {
message += "underage"
}
else if boysAge >= 21 && boysAge < 70 {
  message += "allowed"
}
print(message)

You're missing a space between < and 70 and you need to say what variable to check again, so boysAge < 70 . Also the parentheses are unnecessary.

Answer 2

Your code should work this way :

if boysAge < 21 {
    message += "underage"
}
else if boysAge < 70 {
    message += "allowed"
}

If boysAge is not strictly less than 21, then of course it's greater than or equal to 21 . So you don't need to check it again.

Another way would be :

if boysAge < 21 {
    message += "underage"
}
else if 21..<70 ~= boysAge {
    message += "allowed"
}

Using the ~= operator.

---

This seems to me like a great place to use switch case :

switch boysAge {
case ...20:
    message += "underage"
case 21..<70 :
    message += "allowed"
default:
    break
}

And you coud adjust the ranges to your liking.

Answer 3

The condition (boysAge >= 21) && <70 is not correct. < is a binary operator. That means it needs a left side and a right side. The correct way to write the condition is:

boysAge >= 21 && boysAge < 70

However, note that the boysAge >= 21 part is not necessary because it is already covered in the previous branch.

var boysAge = 21
var message = "The customer is "
if boysAge < 21 {
    message += "underage"
}
else if boysAge < 70 {
    message += "allowed"
}
print(message)