Binary operator ‘*‘ cannot be applied to two ‘Double?‘ operands

When declaring a Double type parameter variable in Swift, the following exception was thrown when performing operations.

Binary operator '*' cannot be applied to two 'Double?' operands

Situation one

Parameter types do not match, parameter types need to be matched

self.max = height / (length * width) // 初始
self.max = height / (length * Double(width)) // 改后

Situation 2

The variable of the initial definition parameter may be empty, add a blank condition, and add the "!" character at the end of the declared parameter

let length = Double(self.lengthText)!
let width = Double(self.weightText)!
let height = Double(self.heightText)!
self.max = height / (length * width)

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