The function in Listing 20 returns a vector of vectors. This is because each document could generate zero to many map entries, and each map entry is represented as a vector.
So in general, we don't have a simple analytical expression for what the pressure is going to do as afunctionof mole fraction a ll the way from zero to one.
所以通常来说，我们不会有，一个简单的解析式，描述压强，作为摩尔分数从零到一，的函数的变化形式。
3
Values of this type can now be allocated using the built-in function new, which returns a pointer to the value in memory with all slots initialized to the zero value.