这是一个简单的递归函数。递归意味着它一遍又一遍地调用自身,直到满足结束递归的条件。如果条件永远不满足,你最终会得到无限递归,这不是一件好事:)
由于您已经了解该函数的用途,因此以下是其内部工作原理的详细信息:
// Saves the absolute value (removes the negative sign) of num
let positive = abs(num)
// Returns -1 if num is 0 or negative
guard nth > 0 else { return -1 }
// Returns 0 if the absolute value of num is 0 (can't be negative)
guard positive > 0 else { return 0 } // Could be guard positive == 0
// nth is a counter that is decremented with every recursion.
// positive % 10 returns the remainder of positive / 10
// For example 23 % 10 = 3
// In this line it always returns a number from 0 - 9 IF nth <= 0
guard nth > 1 else { return positive % 10 }
// If none of the above conditions are true, calls itself using
// the current absolute value divided by 10, decreasing nth.
// nth serves to target a different digit in the original number
return findDigit(positive / 10, nth - 1)
让我们逐步运行一个示例:
findDigit(3454, 3)
num = 3454, positive = 3454, nth = 3
-> return findDigit(3454 / 10, 3 - 1)
num = 345, positive = 345, nth = 2 // 345, not 345.4: integer type
-> return findDigit(345 / 10, 2 - 1)
num = 35, positive = 35, nth = 1
-> return 35 % 10
-> return 5