How to truncate decimals to x places in Swift

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Happy的楠姐 2020-12-04 01:26

In my swift program, I have a really long decimal number (say 17.9384693864596069567) and I want to truncate the decimal to a few decimal places (so I want the

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被撕碎了的回忆 (楼主)

2020-12-04 02:03

Answer for Swift 5.2

I have looked a lot of answers and I always had conversion issues when truncating. According to my maths knowledge, by truncating I understand that if I have 3.1239 and I want 3 decimals then I will have 3.123 with no rounding (!= 3.1234).

Maybe, because of the nature of the process, I was always successful with Doubles but always had issues with Floats.

My approach is to create an extension of BinaryFloatingPoint so I can reuse it for Float, CGFLoat, Double ...

The following extension gets a BinaryFloatingPoint and can return the String or the BinaryFloatingPoint values with a numberOfDecimals given and it handles different type of cases:

extension Numeric where Self: BinaryFloatingPoint {
    
    /// Retruns the string value of the BinaryFloatingPoint. The initiaiser
    var toString: String {
        return String(describing: self)
    }
    
    /// Returns the number of decimals. It will be always greater than 0
    var numberOfDecimals: Int {
        return toString.count - String(Int(self)).count - 1
    }
   
    /// Returns a Number with a certain number of decimals
    /// - Parameters:
    ///   - Parameter numberOfDecimals: Number of decimals to return
    /// - Returns: BinaryFloatingPoint with number of decimals especified
    func with(numberOfDecimals: Int) -> Self {
        let stringValue = string(numberOfDecimals: numberOfDecimals)
        if self is Double {
            return Double(stringValue) as! Self
        } else {
            return Float(stringValue) as! Self
        }
    }
    
    /// Returns a string representation with a number of decimals
    /// - Parameters:
    ///   - Parameter numberOfDecimals: Number of decimals to return
    /// - Returns: String with number of decimals especified
    func string(numberOfDecimals: Int) -> String {
        let selfString = toString
        let selfStringComponents = selfString.components(separatedBy: ".")
        let selfStringIntegerPart = selfStringComponents[0]
        let selfStringDecimalPart = selfStringComponents[1]
        
        if numberOfDecimals == 0 {
            return selfStringIntegerPart
        } else {
            if selfStringDecimalPart.count == numberOfDecimals {
                return [selfStringIntegerPart,
                        selfStringDecimalPart].joined(separator: ".")
            } else {
                if selfStringDecimalPart.count > numberOfDecimals {
                    return [selfStringIntegerPart,
                            String(selfStringDecimalPart.prefix(numberOfDecimals))].joined(separator: ".")
                } else {
                    let difference = numberOfDecimals - selfStringDecimalPart.count
                    let addedCharacters = [Character].init(repeating: "0", count: difference)
                    
                    return [selfStringIntegerPart,
                            selfStringDecimalPart+addedCharacters].joined(separator: ".")
                }
            }
        }
    }
        
}

It might look old school but all my tests are passing:

func test_GivenADecimalNumber_ThenAssertNumberOfDecimalsWanted() {
   //No decimals
    XCTAssertEqual(Float(3).with(numberOfDecimals: 0), 3)
    XCTAssertEqual(Float(3.09).with(numberOfDecimals: 0), 3)
    XCTAssertEqual(Float(3.999).with(numberOfDecimals: 0), 3)
    
    XCTAssertEqual(Double(3).with(numberOfDecimals: 0), 3)
    XCTAssertEqual(Double(3.09).with(numberOfDecimals: 0), 3)
    XCTAssertEqual(Double(3.999).with(numberOfDecimals: 0), 3)
    
    
    //numberOfDecimals == totalNumberOfDecimals
    XCTAssertEqual(Float(3.00).with(numberOfDecimals: 2), 3.00)
    XCTAssertEqual(Float(3.09).with(numberOfDecimals: 2), 3.09)
    XCTAssertEqual(Float(3.01).with(numberOfDecimals: 2), 3.01)
    XCTAssertEqual(Float(3.999).with(numberOfDecimals: 3), 3.999)
    XCTAssertEqual(Float(3.991).with(numberOfDecimals: 3), 3.991)
    
    XCTAssertEqual(Double(3.00).with(numberOfDecimals: 2), 3.00)
    XCTAssertEqual(Double(3.09).with(numberOfDecimals: 2), 3.09)
    XCTAssertEqual(Double(3.01).with(numberOfDecimals: 2), 3.01)
    XCTAssertEqual(Double(3.999).with(numberOfDecimals: 3), 3.999)
    XCTAssertEqual(Double(3.991).with(numberOfDecimals: 3), 3.991)
    
    
    //numberOfDecimals < totalNumberOfDecimals
    XCTAssertEqual(Float(3.00).with(numberOfDecimals: 1), 3.0)
    XCTAssertEqual(Float(3.09).with(numberOfDecimals: 1), 3.0)
    XCTAssertEqual(Float(3.01).with(numberOfDecimals: 1), 3.0)
    XCTAssertEqual(Float(3.999).with(numberOfDecimals: 2), 3.99)
    XCTAssertEqual(Float(3.991).with(numberOfDecimals: 2), 3.99)
    
    XCTAssertEqual(Double(3.00).with(numberOfDecimals: 1), 3.0)
    XCTAssertEqual(Double(3.09).with(numberOfDecimals: 1), 3.0)
    XCTAssertEqual(Double(3.01).with(numberOfDecimals: 1), 3.0)
    XCTAssertEqual(Double(3.999).with(numberOfDecimals: 2), 3.99)
    XCTAssertEqual(Double(3.991).with(numberOfDecimals: 2), 3.99)
    
    
    //numberOfDecimals > totalNumberOfDecimals
    XCTAssertEqual(Float(3.00).with(numberOfDecimals: 3), 3.000)
    XCTAssertEqual(Float(3.09).with(numberOfDecimals: 3), 3.090)
    XCTAssertEqual(Float(3.01).with(numberOfDecimals: 3), 3.010)
    XCTAssertEqual(Float(3.999).with(numberOfDecimals: 4), 3.9990)
    XCTAssertEqual(Float(3.991).with(numberOfDecimals: 4), 3.9910)
    
    XCTAssertEqual(Double(3.00).with(numberOfDecimals: 3), 3.000)
    XCTAssertEqual(Double(3.09).with(numberOfDecimals: 3), 3.090)
    XCTAssertEqual(Double(3.01).with(numberOfDecimals: 3), 3.010)
    XCTAssertEqual(Double(3.999).with(numberOfDecimals: 4), 3.9990)
    XCTAssertEqual(Double(3.991).with(numberOfDecimals: 4), 3.9910)
}

func test_GivenADecimal_ThenAssertStringValueWithDecimalsWanted() {
    //No decimals
    XCTAssertEqual(Float(3).string(numberOfDecimals: 0), "3")
    XCTAssertEqual(Float(3.09).string(numberOfDecimals: 0), "3")
    XCTAssertEqual(Float(3.999).string(numberOfDecimals: 0), "3")
    
    XCTAssertEqual(Double(3).string(numberOfDecimals: 0), "3")
    XCTAssertEqual(Double(3.09).string(numberOfDecimals: 0), "3")
    XCTAssertEqual(Double(3.999).string(numberOfDecimals: 0), "3")
    
    
    //numberOfDecimals == totalNumberOfDecimals
    XCTAssertEqual(Float(3.00).string(numberOfDecimals: 2), "3.00")
    XCTAssertEqual(Float(3.09).string(numberOfDecimals: 2), "3.09")
    XCTAssertEqual(Float(3.01).string(numberOfDecimals: 2), "3.01")
    XCTAssertEqual(Float(3.999).string(numberOfDecimals: 3), "3.999")
    XCTAssertEqual(Float(3.991).string(numberOfDecimals: 3), "3.991")
    
    XCTAssertEqual(Double(3.00).string(numberOfDecimals: 2), "3.00")
    XCTAssertEqual(Double(3.09).string(numberOfDecimals: 2), "3.09")
    XCTAssertEqual(Double(3.01).string(numberOfDecimals: 2), "3.01")
    XCTAssertEqual(Double(3.999).string(numberOfDecimals: 3), "3.999")
    XCTAssertEqual(Double(3.991).string(numberOfDecimals: 3), "3.991")
    
    
    //numberOfDecimals < totalNumberOfDecimals
    XCTAssertEqual(Float(3.00).string(numberOfDecimals: 1), "3.0")
    XCTAssertEqual(Float(3.09).string(numberOfDecimals: 1), "3.0")
    XCTAssertEqual(Float(3.01).string(numberOfDecimals: 1), "3.0")
    XCTAssertEqual(Float(3.999).string(numberOfDecimals: 2), "3.99")
    XCTAssertEqual(Float(3.991).string(numberOfDecimals: 2), "3.99")
    
    XCTAssertEqual(Double(3.00).string(numberOfDecimals: 1), "3.0")
    XCTAssertEqual(Double(3.09).string(numberOfDecimals: 1), "3.0")
    XCTAssertEqual(Double(3.01).string(numberOfDecimals: 1), "3.0")
    XCTAssertEqual(Double(3.999).string(numberOfDecimals: 2), "3.99")
    XCTAssertEqual(Double(3.991).string(numberOfDecimals: 2), "3.99")
    
    
    //numberOfDecimals > totalNumberOfDecimals
    XCTAssertEqual(Float(3.00).string(numberOfDecimals: 3), "3.000")
    XCTAssertEqual(Float(3.09).string(numberOfDecimals: 3), "3.090")
    XCTAssertEqual(Float(3.01).string(numberOfDecimals: 3), "3.010")
    XCTAssertEqual(Float(3.999).string(numberOfDecimals: 4), "3.9990")
    XCTAssertEqual(Float(3.991).string(numberOfDecimals: 4), "3.9910")
    
    XCTAssertEqual(Double(3.00).string(numberOfDecimals: 3), "3.000")
    XCTAssertEqual(Double(3.09).string(numberOfDecimals: 3), "3.090")
    XCTAssertEqual(Double(3.01).string(numberOfDecimals: 3), "3.010")
    XCTAssertEqual(Double(3.999).string(numberOfDecimals: 4), "3.9990")
    XCTAssertEqual(Double(3.991).string(numberOfDecimals: 4), "3.9910")
}

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