Getting Error While Running an EUnit Test in Erlang?

This Erlang module defines a stack-based calculator that processes Reverse Polish Notation (RPN) expressions. The calc/1 function evaluates mathematical operations such as addition, subtraction, multiplication, and logarithmic functions by using a stack to store intermediate results. The read/1 function converts input strings to either integers or floats for further calculations.

Erlang Code (with Errors):

code-module(calc1).

-export([calc/1]).

calc(L) when is_list(L) -> 
    [Result] = lists:foldl(fun calc/2, [], string:tokens(L," ")),
    Result.

calc("+", [N1,N2|Stack]) -> [N2+N1|Stack];
calc("-", [N1,N2|Stack]) -> [N2-N1|Stack];
calc("*", [N1,N2|Stack]) -> [N2*N1|Stack];
calc("/", [N1,N2|Stack]) -> [N2/N1|Stack];
calc("^", [N1,N2|Stack]) -> [math:pow(N2,N1)|Stack];
calc("ln", [N|Stack])    -> [math:log(N)|Stack];
calc("log10", [N|Stack]) -> [math:log10(N)|Stack];
calc("sum", Stack)   -> [lists:sum(Stack)];
calc("prod", Stack)  -> [lists:foldl(fun erlang:'*'/2, 1, Stack)];
calc(X, [Stack]) -> [read(X)|Stack].

%% read(String()) -> Int() | Float()
read(X) ->
    case string:to_float(X) of
        {error, no_float} -> list_to_integer(X);
        {F,_} -> F
    end.

Fixing Common Errors in Erlang Stack-based Calculator

In Erlang, writing a calculator using stack-based operations is a common exercise. The code presented here demonstrates how to create a basic Reverse Polish Notation (RPN) calculator that can handle arithmetic operations. However, when trying to run the code with an EUnit test, an error arises. Let’s break down the issue and solve it.

The Issue

One of the main issues in the code occurs in this clause:

codecalc(X, [Stack]) -> [read(X)|Stack].

The problem lies in how the stack is being handled. The code attempts to pattern-match a single list element as [Stack], which would expect the entire stack to be a list within another list, not a proper stack. This results in runtime errors because the structure of the stack doesn’t match what is expected.

Corrected Code:

code-module(calc1).

-export([calc/1]).

%% Entry point for calculator, splits input string into tokens and processes them
calc(L) when is_list(L) -> 
    [Result] = lists:foldl(fun calc/2, [], string:tokens(L," ")),
    Result.

%% Handles the basic arithmetic operations
calc("+", [N1, N2 | Stack]) -> [N2 + N1 | Stack];
calc("-", [N1, N2 | Stack]) -> [N2 - N1 | Stack];
calc("*", [N1, N2 | Stack]) -> [N2 * N1 | Stack];
calc("/", [N1, N2 | Stack]) -> [N2 / N1 | Stack];
calc("^", [N1, N2 | Stack]) -> [math:pow(N2, N1) | Stack];

%% Handles advanced math functions
calc("ln", [N | Stack])     -> [math:log(N) | Stack];
calc("log10", [N | Stack])  -> [math:log10(N) | Stack];

%% Handles stack operations for sum and product
calc("sum", Stack)   -> [lists:sum(Stack)];
calc("prod", Stack)  -> [lists:foldl(fun erlang:'*'/2, 1, Stack)];

%% Pushes a number or value onto the stack
calc(X, Stack) -> [read(X) | Stack].

%% Converts strings to integer or float values
read(X) ->
    case string:to_float(X) of
        {error, no_float} -> list_to_integer(X);
        {F, _} -> F
    end.

Has Been Fixed

1. Correct Stack Handling:
   • The line calc(X, [Stack]) -> [read(X)|Stack]. has been corrected to calc(X, Stack) -> [read(X) | Stack]., which ensures that the stack is treated as a list where values can be pushed directly.
2. Error-Free Token Processing:
   • The lists:foldl/3 function is used correctly to apply the calc/2 function to tokens split by string:tokens/2. The operation works by processing each token and updating the stack accordingly.
3. Robust Arithmetic Operations:
   • The calculator now properly handles a variety of basic and advanced operations, from addition to logarithmic functions. The stack is updated at each step by popping off values and pushing the results back.

Testing the Code

This corrected version should now pass any EUnit tests without errors, assuming that proper input is provided. For example:

code1> calc1:calc("3 4 + 5 *").
35

This shows that the stack is being handled correctly, and the operations are processed in the correct order. The final result, 35, is the outcome of (3 + 4) * 5.

Summary

Errors in stack-based calculators can often arise from incorrect pattern matching or mishandling of data structures. By ensuring that the stack is consistently treated as a list, and by pushing and popping values correctly, you can resolve many common issues and get your Erlang calculator working smoothly.