Cannot find a solution to K&R exercise 4-6

In K&R we have managed to create an RPN.

The exercise now is to:

Add commands for handling variables, (It's easy to provide twenty-six variables with single letter names.) Add a variable for the most recently printed value.

So this is meant to act somewhat like the Python interpreter, where we can do:

>>>5
>>>_ (where _ prints 5)
>>>_ + 5 (which prints 10)

or A = 5 _ + A (which p开发者_运维百科rints 10)

and so on, but I'm not so sure about how I want to go about it in C. I just feel stumped.

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Create a 26th variable. Any time you print something, write that value into the 26th variable. When they use _ (or whatever name you choose) read from that variable.

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This is first step of building a command line calculator I guess.

Parse the input string for operands and operator. Map the operator to a enum

enum operator { TYPE_ADD, TYPE_SUBTRACT,TYPE_MAX);

Call function to compute result

  int    calculate(int i_op1, int i_op2, operator e_operator)
           {
              /*Use switch case to calculate result*/
           }

Save this result into a variable. In input in the string is equal to "_" then use this as first input to the function calculate.