This calculator takes an input like 1 + 6 * 3 - 2 and produces 17.
> make run or
> echo "1 + 6 * 3 - 2" | make run
The model of this code is similar to the model of a compiler front end because, after all, a calculator is like a very tiny interpreter. It takes a simple mathematical expression (source program) and passes it through a lexical analyzer to produce meaningful tokens. The tokens are then passed through a parser that builds a syntax tree that represents the expression. Finally, there is an evaluater function that traverses the syntax tree to produce a result.
The scanner needs the input expression to be space separated since it is using strsep to break the expression into tokens. It then aliases the tokens with unique integer values.
The parser taken the list of tokens as input and builds a syntax tree. The syntax tree is represented in memory by a linked list of structs that form a binary tree like so:
struct Node {
int value;
struct Node* lchild;
struct Node* rchild;
};The evaluator recursively walks the syntax tree evaluating each node.