It's a common mistake to make if you forget that I've actually declared that variable up top, but wait a minute, I know any time that I use a variable, I have to declare it.

这是一个通常犯的错误，如果你忘了,之前在前面已经声明了这个变量，等等，我知道在任何时候,我使用一个变量，我必须声明它。

哈佛公开课 - 计算机科学课程节选

Now because among its contents is the prototype for print F, that is why, very simply, I can call print F without having to declare its prototype myself.

现在因为在它的内容中是printf的原型，这就是为什么我可以不声明printf函数的原型,而可以调用它。

哈佛公开课 - 计算机科学课程节选

What's going on underneath the hood is we'll see pictorially X today is that when I declare this X, this variable X here.

底下所做的，我们将看到,当我声明这个X，这个变量。

哈佛公开课 - 计算机科学课程节选