The mapping table also allows the user to convert source types to target types, or define constant mappings that set constant values to items in the target schema. 映射表还允许用户将源类型转化成目标类型,或定义常量映射来将常量设置为目标架构中的项目。
We then use the def macro to define a constant called big-num-str. 之后,使用def宏来定义一个常量,称为big-num-str。
You then define a constant CONVERSION_STRUCT_SIZE for the size of the whole structure. 随后为整个结构的大小定义常量CONVERSIONSTRUCTSIZE。
If you later decided to change the number of students in each class, you could do so where you define the constant studentsPerClass without having to make a change every place you used that value. 如果你过后决定改变每个班的学生数,你只需在定义常量的地方改动而无需在每个使用此值的地方更改。
The question is raised whether it would not be useful to define the Avogadro constant to 6 significant digits exactly, pending a further improvement of its determination to an uncertainty of < 1 · 10 - 7 N A. 问题起因是严格限定阿伏伽德罗常数6位有效数字是否无用,使得进一步改进测量不确定度至<10-7NA的提议悬而不决。