iOS 转换器应用程序，度量之间的连接

Question

我想知道是否有人可以帮助我如何在度量之间建立联系，例如我如何计算图片中厘米和英寸之间的关系。这是我的学校项目应用程序，除了那部分，它或多或少已经完成了。所以我正在从 plist 中读取值，并根据我在选择器中的第一列显示第二和第三列。所有单位和措施都是从 plist 中读取的。以能量、距离、体积等为键。

那么在度量之间建立联系的最简单方法是什么？我最简单也是最业余的想法是编写很长的代码，像这样:

if (first column == distance){
    if (second column ==millimeter) {
    case 0: if (third column == millimeter) then result=x*1;
    case 1:if (third column == centimeter) then result=0/100;
      .
      .
      .
    }
    if (second column == centimeter){
    case 0:
    case 1:
      .
      .
      .
    }}

这将导致大约 1000 行不必要的代码。有没有人有更好的主意？提前致谢。

Answer 1

整个事情都可以用数据来完成，没有if，没有switches……只是需要稍微组织一下。请考虑以下事项:

@property (strong, nonatomic) NSArray *distanceIndex;
@property (strong, nonatomic) NSArray *weightIndex;

@property (strong, nonatomic) NSArray *distanceConversion;
@property (strong, nonatomic) NSArray *weightConversion;

- (void)initConversions {

    // we get strings in as our units, so we need to convert these to array indexes
    // make up our own convention, meters=0, inches=1, etc

    self.distanceIndex = @[@"meters", @"inches", @"miles"];
    self.weightIndex = @[@"grams", @"ounces", @"pounds"];

    // you might read these from a file, but in code, just to illustrate...
    // the order follows the order of the index.  so the meters line is first
    // and the conversion factors are meters->meters, meters->inches, meters->miles etc.
    // notice the 1.0's on the diagonal, and that numbers reflected across
    // the diagonal are reciprocals

    self.distanceConversion =
        @[ @[@"meters", @[@1.0, @39.37, @0.000621]],
           @[@"inches", @[@0.254, @1.0, @0.00001578]],
           @[@"miles ", @[@1609.0, @63360.0, @1.0]] ];

   // and so on... same thing for self.weightConversions
}

- (double)convertDistance:(double)value from:(NSString *)fromUnits to:(NSString *)toUnits {

    NSUInteger fromIndex = [self.distanceIndex indexOfObject:fromUnits];
    NSUInteger toIndex = [self.distanceIndex indexOfObject:toUnits];

    // throw an exception if passed units that we don't recognize
    NSAssert(fromIndex != NSNotFound && toIndex != NSNotFound, @"could not find units");

    NSArray *conversionRow = self.distanceConversion[fromIndex][1];
    NSNumber *conversionNumber = conversionRow[toIndex];
    double conversion = [conversionNumber doubleValue];

    return value * conversion;
}
- (double)convertWeight:(double)value from:(NSString *)fromUnits to:(NSString *)toUnits {

    // same idea here, if you have the weight conversion data
    return 0;
}