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: Building a Basic Calculator UI

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A step-by-step tutorial on creating a functional calculator application, using uCalc's expression parser to handle the evaluation logic.

Remarks

💡 Project: Building a Basic Calculator UI

This project brings together the core concepts of the uCalc expression parser to build a familiar, practical application: a basic calculator. You'll see how to handle user input from a simple UI and let uCalc do all the heavy lifting of parsing and calculation with a single function call.

This tutorial is UI-framework-agnostic. Whether you're using WinForms, WPF, Qt, or a web framework, the core logic for interfacing with uCalc remains the same.

Prerequisites

  • Familiarity with the EvalStr method.
  • Basic knowledge of your chosen UI framework for creating buttons and a text display.

🖥️ Step 1: The User Interface

Every calculator needs a basic set of components:

  1. A Display: A read-only text box to show the current expression and the final result. Let's call it display_text.
  2. Number Buttons: Buttons for 0 through 9.
  3. Operator Buttons: Buttons for +, -, *, /.
  4. Action Buttons:
    • An Equals (=) button to trigger the calculation.
    • A Clear (C) button to reset the display.

The layout would look something like a standard pocket calculator.

🧠 Step 2: The Core Logic - Handling Button Clicks

The logic behind the UI is surprisingly simple. We'll manage a single string variable that represents the content of the calculator's display. The example below will simulate this process.

Number and Operator Buttons (0-9, +, -, *, /)

When a user clicks any of these buttons, the action is the same: append the button's character to the display_text string.

  • If the display shows "123", pressing + changes it to "123+".
  • If the display shows "123+", pressing 4 changes it to "123+4".

The Clear Button (C)

When the 'C' button is clicked, simply clear the display_text string to an empty string ("").

✨ Step 3: The Magic - The Equals Button (=)

This is where uCalc shines. When the user presses the = button, all you need to do is:

  1. Get the complete expression string from your display_text.
  2. Pass this string directly to the EvalStr method.
  3. Take the result from EvalStr and set it as the new content of display_text.

That's it! uCalc handles everything else:

  • It correctly parses the order of operations.
  • It performs the calculations.
  • If the user entered an invalid expression (like "5 * + 3"), EvalStr will return a descriptive error message (e.g., "Syntax error"), which you can display directly to the user.

💡 Why uCalc? (Comparative Analysis)

How would you do this without uCalc? You would need to build a complete parsing engine yourself. This typically involves complex algorithms like:

  • Shunting-yard algorithm: To convert the infix expression (5 + 2) into a postfix (Reverse Polish Notation) expression (5 2 +).
  • Postfix Evaluation: To evaluate the RPN expression using a stack.
  • State Management: To handle operator precedence, parentheses, and error states.

This can be hundreds of lines of complex, error-prone code. uCalc reduces this entire complex problem to a single line: result = uc.EvalStr(expression).

🚀 Step 4: Putting It All Together

The following example simulates the complete logic of the calculator's "brain". It shows how a string is built up from simulated button presses and then evaluated by uCalc.

Examples

Simulates the core logic of a calculator, showing how user input is built into an expression string and evaluated.
				
					using uCalcSoftware;

var uc = new uCalc();
string display_text = "";

// Simulate user pressing buttons: 1, 2, 3, +, 4, 5, 6
display_text = display_text + "123";
Console.WriteLine($"Display: {display_text}");

display_text = display_text + "+";
Console.WriteLine($"Display: {display_text}");

display_text = display_text + "456";
Console.WriteLine($"Display: {display_text}");

// Simulate pressing '='. This is where uCalc does the work.
var result = uc.EvalStr(display_text);
display_text = result;
Console.WriteLine($"Result: {display_text}");

Console.WriteLine("");

// Simulate a new calculation with an error
display_text = "5 * / 3";
Console.WriteLine($"Display: {display_text}");
result = uc.EvalStr(display_text);
display_text = result;
Console.WriteLine($"Result: {display_text}");

Console.WriteLine("");

// Simulate clearing the display
display_text = "C"; // Let's say 'C' is a special command
if (display_text == "C") {
   display_text = "";
}
Console.WriteLine($"Display after clear: '{display_text}'");
				
			
Display: 123
Display: 123+
Display: 123+456
Result: 579

Display: 5 * / 3
Result: Syntax error

Display after clear: ''
				
					#include <iostream>
#include "uCalc.h"

using namespace std;
using namespace uCalcSoftware;

int main() {
   uCalc uc;
   string display_text = "";

   // Simulate user pressing buttons: 1, 2, 3, +, 4, 5, 6
   display_text = display_text + "123";
   cout << "Display: " << display_text << endl;

   display_text = display_text + "+";
   cout << "Display: " << display_text << endl;

   display_text = display_text + "456";
   cout << "Display: " << display_text << endl;

   // Simulate pressing '='. This is where uCalc does the work.
   auto result = uc.EvalStr(display_text);
   display_text = result;
   cout << "Result: " << display_text << endl;

   cout << "" << endl;

   // Simulate a new calculation with an error
   display_text = "5 * / 3";
   cout << "Display: " << display_text << endl;
   result = uc.EvalStr(display_text);
   display_text = result;
   cout << "Result: " << display_text << endl;

   cout << "" << endl;

   // Simulate clearing the display
   display_text = "C"; // Let's say 'C' is a special command
   if (display_text == "C") {
      display_text = "";
   }
   cout << "Display after clear: '" << display_text << "'" << endl;
}
				
			
Display: 123
Display: 123+
Display: 123+456
Result: 579

Display: 5 * / 3
Result: Syntax error

Display after clear: ''
				
					Imports System
Imports uCalcSoftware
Public Module Program
   Public Sub Main()
      Dim uc As New uCalc()
      Dim display_text As String = ""
      
      '// Simulate user pressing buttons: 1, 2, 3, +, 4, 5, 6
      display_text = display_text + "123"
      Console.WriteLine($"Display: {display_text}")
      
      display_text = display_text + "+"
      Console.WriteLine($"Display: {display_text}")
      
      display_text = display_text + "456"
      Console.WriteLine($"Display: {display_text}")
      
      '// Simulate pressing '='. This is where uCalc does the work.
      Dim result = uc.EvalStr(display_text)
      display_text = result
      Console.WriteLine($"Result: {display_text}")
      
      Console.WriteLine("")
      
      '// Simulate a new calculation with an error
      display_text = "5 * / 3"
      Console.WriteLine($"Display: {display_text}")
      result = uc.EvalStr(display_text)
      display_text = result
      Console.WriteLine($"Result: {display_text}")
      
      Console.WriteLine("")
      
      '// Simulate clearing the display
      display_text = "C" '// Let's say 'C' is a special command
      If display_text = "C" Then
         display_text = ""
      End If
      Console.WriteLine($"Display after clear: '{display_text}'")
   End Sub
End Module
				
			
Display: 123
Display: 123+
Display: 123+456
Result: 579

Display: 5 * / 3
Result: Syntax error

Display after clear: ''
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