Chapter 4 Selection Structures: if and switch statements
Condition are expressions that evaluate to either true or false
In C, we use 0 and 1 to represent.
We use && to represent ‘and’, || to represent ‘or’ （并集和交集的思想）
At the same time, we could use ! to represent ‘not’ for example, ! = is not equal.
All the symbols have the priority of the procedure, so it is often useful to use the brackets
日后回顾：复习没有任何帮助，多敲作业的代码就好！