Scala阶乘函数返回结果存疑：调用fact(4)得24而非预期12

Hey Tyler, let's walk through exactly why fact(4) returns 24 instead of the 12 you expected—your recursive function is working correctly, you just missed a step in the recursive breakdown!

First, let's recap your function definition (which is perfectly valid for calculating factorials):

def fact(n:Int):Int = if (n<2) 1 else n*fact(n-1)

Let's expand the recursive calls step by step for fact(4):

• When you call fact(4), since 4 is not less than 2, it evaluates to 4 * fact(3)
• Next, fact(3) evaluates to 3 * fact(2) (3 is also not <2)
• Then fact(2) evaluates to 2 * fact(1) (2 is not <2)
• Finally, fact(1) hits the base case: since 1 < 2, it returns 1

Now let's compute the results working backwards:

• fact(2) = 2 * 1 = 2
• fact(3) = 3 * 2 = 6
• fact(4) = 4 * 6 = 24

Your confusion came from assuming fact(3) equals 3, but remember—fact(3) doesn't just return 3, it returns 3 multiplied by the result of fact(2), which itself is 2 multiplied by fact(1). That's the magic (and sometimes tricky part) of recursion: each call depends on the next smaller call until we hit the base case.

If you want to verify this further, try printing intermediate values by modifying your function slightly:

def fact(n:Int):Int = {
  println(s"Calculating fact($n)")
  if (n<2) 1 else n*fact(n-1)
}

Running fact(4) will show you each step of the recursion, making it even clearer how the values build up to 24.

内容的提问来源于stack exchange，提问作者Tyler