uCalc API Version: 2.1.3-preview.2 Released: 6/16/2026

Warning

uCalc API Preview Release Notice:The documentation describes the intended behavior of the API. The current preview build contains incomplete features, unoptimized performance, and is subject to breaking changes.

Project: An Interactive Command-Line Shell (REPL)

Product: 

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A step-by-step tutorial for building a command-line calculator (REPL) that dynamically evaluates user input using the uCalc expression parser.

Remarks

💻 Project: Building an Interactive Command-Line Shell (REPL)

This project demonstrates one of the most classic and powerful use cases for an expression parser: building a Read-Eval-Print Loop (REPL). A REPL is an interactive, command-line environment that takes single user inputs, evaluates them, and returns the result. You've seen this in environments like the Python interactive shell or your web browser's developer console.

Our goal is to build a simple but fully functional command-line calculator that can process mathematical formulas, execute functions, and handle errors gracefully, all powered by a simple loop and a single uCalc method.

Prerequisites

  • Familiarity with the EvalStr method.

⚙️ The Core Logic: A 4-Step Loop

Every REPL follows the same fundamental cycle:

  1. Read: Get a line of input from the user.
  2. Eval: Evaluate the input string.
  3. Print: Display the result.
  4. Loop: Repeat the process.

Let's break down how we'll implement this with uCalc.

Step 1: The "Read" Phase

This step involves prompting the user and reading a line of text from the console. In our example, we will simulate this by reading from a pre-defined array of strings to ensure the example is self-contained.

Step 2: The "Eval" Phase - The Magic of EvalStr

This is where the uCalc engine does all the heavy lifting. The raw string provided by the user is passed directly to the EvalStr method.

result = uc.EvalStr(userInput);

This single method call is powerful enough for our entire REPL because:

  • It can parse and evaluate any valid expression, including arithmetic, function calls, and string operations.
  • It automatically handles any data type and returns the result as a string, ready for display.
  • Most importantly, it is safe. If the user enters an invalid expression (like "1 +"), it does not throw an exception but instead returns the error message as a string (e.g., "Syntax error").

Step 3: The "Print" Phase

This is the simplest step. We take the string returned by EvalStr—whether it's a valid result or an error message—and print it directly to the console.

Step 4: The Loop and Exit Condition

We wrap this logic in a do...loop that runs continuously. To make it a usable tool, we add a simple check for an exit command (like "exit" or "quit") to break the loop and end the program.

💡 Why uCalc? (Comparative Analysis)

How would you build this REPL without uCalc? You would need to build a complete parsing and evaluation engine from scratch. This is a non-trivial computer science problem that typically involves:

  1. Lexical Analysis (Tokenization): Writing code to split the input string "5 * (10 + 2)" into a stream of tokens: 5, *, (, 10, +, 2, ).
  2. Syntactic Analysis (Parsing): Implementing an algorithm like the Shunting-yard algorithm to convert the infix token stream into a postfix (Reverse Polish Notation) representation that respects operator precedence: 5, 10, 2, +, *.
  3. Evaluation: Writing code to evaluate the postfix expression using a stack.
  4. Error Handling: Building robust error detection and reporting for every stage.

This would involve hundreds of lines of complex, error-prone code. uCalc encapsulates this entire pipeline into a single, highly-optimized, and safe method call: EvalStr.

Now let's see the complete implementation in action.

Examples

A complete, working command-line REPL that reads user input, evaluates it with uCalc, and prints the result or any error messages.
				
					using uCalcSoftware;

var uc = new uCalc();
// This example simulates a full REPL session by iterating through a series of inputs.
Console.WriteLine("uCalc Interactive Shell (simulated session)");
Console.WriteLine("Type 'exit' or 'quit' to end.");
Console.WriteLine("");

string[] inputs = {
   "10 * (5 + 3)",
   "UCase('hello world')",
   "1 / 0",
   "1 +",
   "exit"
};

var index = 0;

do {
   Console.Write("> ");

   // Simulate reading from the console by assigning inputs sequentially.
   // In a real application, this would read the input interactively from the console.
   var input = inputs[index];

   Console.WriteLine(input);

   // 1. Check for an exit command
   if (input == "exit" || input == "quit") {
      Console.WriteLine("Exiting.");
      return ;
   }

   // 2. Evaluate the input and 3. Print the result
   Console.WriteLine(uc.EvalStr(input));
   Console.WriteLine("");

   index = index + 1;
} while (true);
				
			
uCalc Interactive Shell (simulated session)
Type 'exit' or 'quit' to end.

> 10 * (5 + 3)
80

> UCase('hello world')
HELLO WORLD

> 1 / 0
inf

> 1 +
Syntax error

> exit
Exiting.
				
					#include <iostream>
#include "uCalc.h"

using namespace std;
using namespace uCalcSoftware;

int main() {
   uCalc uc;
   // This example simulates a full REPL session by iterating through a series of inputs.
   cout << "uCalc Interactive Shell (simulated session)" << endl;
   cout << "Type 'exit' or 'quit' to end." << endl;
   cout << "" << endl;

   vector<string> inputs = {
      "10 * (5 + 3)",
      "UCase('hello world')",
      "1 / 0",
      "1 +",
      "exit"
   };

   auto index = 0;

   do {
      cout << "> ";

      // Simulate reading from the console by assigning inputs sequentially.
      // In a real application, this would read the input interactively from the console.
      auto input = inputs[index];

      cout << input << endl;

      // 1. Check for an exit command
      if (input == "exit" || input == "quit") {
         cout << "Exiting." << endl;
         return 0;
      }

      // 2. Evaluate the input and 3. Print the result
      cout << uc.EvalStr(input) << endl;
      cout << "" << endl;

      index = index + 1;
   } while (true);
}
				
			
uCalc Interactive Shell (simulated session)
Type 'exit' or 'quit' to end.

> 10 * (5 + 3)
80

> UCase('hello world')
HELLO WORLD

> 1 / 0
inf

> 1 +
Syntax error

> exit
Exiting.
				
					Imports System
Imports uCalcSoftware
Public Module Program
   Public Sub Main()
      Dim uc As New uCalc()
      '// This example simulates a full REPL session by iterating through a series of inputs.
      Console.WriteLine("uCalc Interactive Shell (simulated session)")
      Console.WriteLine("Type 'exit' or 'quit' to end.")
      Console.WriteLine("")
      
      Dim inputs() As String = {
      "10 * (5 + 3)",
      "UCase('hello world')",
      "1 / 0",
      "1 +",
      "exit"
      }
      
      Dim index = 0
      
      Do
         Console.Write("> ")
         
         '// Simulate reading from the console by assigning inputs sequentially.
         '// In a real application, this would read the input interactively from the console.
         Dim input = inputs(index)
         
         Console.WriteLine(input)
         
         '// 1. Check for an exit command
         If input = "exit" Or input = "quit" Then
            Console.WriteLine("Exiting.")
            return
         End If
         
         '// 2. Evaluate the input and 3. Print the result
         Console.WriteLine(uc.EvalStr(input))
         Console.WriteLine("")
         
         index = index + 1
      Loop While true
   End Sub
End Module
				
			
uCalc Interactive Shell (simulated session)
Type 'exit' or 'quit' to end.

> 10 * (5 + 3)
80

> UCase('hello world')
HELLO WORLD

> 1 / 0
inf

> 1 +
Syntax error

> exit
Exiting.
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