问题
I am learning context-free grammar, and I don't understand how to identity expression, factor and term in a programming language like C or C++.
Suppose we have an assignment statement, id := E, where E is any arithmetic expression.
What is a term? What is an expression? and What is a factor in an actual piece of code?
We can have
int i = 3, j = 14
int i = 3 + j * 14;
Thank you very much.
回答1:
The "factor", "term" and "expression" concepts come from math and don't really have to do with programming languages.
Factors are things you multiply:
1*2*(3+4)
Terms are things you add:
1 + 2 + (3*4)
And expressions are for the whole result
1 + 3 * 7
In context-free language parsing you use these distinctions to organize the precedences between operators. So an expression is made of a sum of terms and a term is made of a product of factors and a factor is either a number or a parenthesized subexpression.
回答2:
Your homework probably also has a grammar specification for a (subset of a) programming language like C or C++ , something along the lines of:
Program->Expression|Definition|Declaration
Expression->Expression + Term
Expression->Expression - Term
Expression->Expression*Factor
...
...
...
etc etc.
Then, 3 + j*14 is an expression, 3 is a term (anything connected by a + is either an Expression or a Term according to the grammar above) j and 14 are factors Please note that a grammar above is a very crude imitation of what a grammar of a real programming language might look like.
回答3:
Ok so suppose we have a grammar like this:
Program->(Definitions | lambda)
Definitions->Definitions Definitions
Definitions-> "int" Definition ";" |"int" Definition,Definition ";"
Definition -> Name "=" Expression
Expression-> Term "+" Expression
Expression->Expression "-" Term
Expression->Expression "*" Factor
Term->"3"|"14"
Factor->"3"|"14"
Expression->"3"|"14"
Note that my terminal symbols are in quotes and I omit the part where Name is defined as a combination of letters and numbers and underscores starting with a letter or an underscore :)
So, in your example:
Line 1 int i = 3, j = 14;
Line 2 int i = 3 + j * 14;
i and j are Names. 3, 14 (line 1) and 3 + j*14 (line 2) are are Expressions. Then,on line 2, 3 is a Term, j*14 is an Expression, j is a Factor and 14 is a Factor :)
来源：https://stackoverflow.com/questions/8055605/identity-expression-factor-and-term