diff --git a/backend/Calculator.cpp b/backend/Calculator.cpp
index ccb917f..6b4a288 100644
--- a/backend/Calculator.cpp
+++ b/backend/Calculator.cpp
@@ -1,11 +1,27 @@
 #include "Calculator.h"
 
 
+Calculator::Calculator() {
+	std::stringstream *s = new stringstream;
+	this->ip = s;
+	this->ts = new TokenStream(ip);
+	this->parser = new Parser(ts);
+	this->ownsStream = true; // we own the stream and have to delete it
+}
+
 Calculator::Calculator(std::istream *ip) {
 	this->ip = ip;
-	TokenStream *ts = new TokenStream{ip};
-	Parser *ps = new Parser{ts};
-	this->parser = ps;
+	this->ts = new TokenStream{ip};
+	this->parser = new Parser{ts};
+	this->ownsStream = false; // we don't own the stream and can't delete it
+}
+
+Calculator::~Calculator() {
+	if (this->ownsStream) {
+		delete this->ip;
+	}
+	delete this->ts;
+	delete this->parser;
 }
 
 void Calculator::calculate(std::string s) {
diff --git a/backend/Calculator.h b/backend/Calculator.h
index 65f1c87..0dd8315 100644
--- a/backend/Calculator.h
+++ b/backend/Calculator.h
@@ -7,12 +7,18 @@
 #include "Parser.h"
 #include "TokenStream.h"
 
+
 class Calculator {
 private:
 	std::istream *ip;
+	TokenStream *ts;
 	Parser *parser;
+	bool ownsStream;
 public:
+	Calculator();
 	Calculator(std::istream *ip);
+	~Calculator();
+
 	void calculate(std::string s);
 	std::string getMemory();
 };
diff --git a/gui/CalculatorApp.cpp b/gui/CalculatorApp.cpp
index be50501..0a900c8 100644
--- a/gui/CalculatorApp.cpp
+++ b/gui/CalculatorApp.cpp
@@ -6,7 +6,7 @@
 CalculatorApp::CalculatorApp(SDL_Window *win, SDL_Renderer *ren) : App(win, ren) {
 	// widget for memory
 	TextOutput *memory = new TextOutput{
-		win, ren, 30, 30, 600, 400, currentMemory(), 24
+		win, ren, 30, 30, 600, 400, "", 18
 	};
 
 	// widget for input prompt
@@ -15,18 +15,19 @@ CalculatorApp::CalculatorApp(SDL_Window *win, SDL_Renderer *ren) : App(win, ren)
 	};
 
 	// widget for input
-	CalculatorInput *input = new CalculatorInput{win, ren, 300, 450, 600, 500, "", 24, memory};
+	this->calculator = new Calculator{};
+	CalculatorInput *input = new CalculatorInput{
+		win, ren, 300, 450, 600, 500, "", 24, memory, this->calculator
+	};
+
 
 	widgets.push_back(memory);
 	widgets.push_back(prompt);
 	widgets.push_back(input);
 }
 
-// TODO: we don't need this here, CalculatorInput takes care of it
-std::string CalculatorApp::currentMemory() {
-	// TODO: call calculator and get memory
-	// or if it's not the same instance every time, maybe have a separate memory
-	// as member of Calculator App, that will be passed every time a new
-	// Calculator App instance is made
-	return "[m1] 15.2\n[m2] 122\n[m3] 99";
+
+CalculatorApp::~CalculatorApp() {
+	delete this->calculator;
+	// deletion of widgets happens in superclass destructor
 }
diff --git a/gui/CalculatorApp.h b/gui/CalculatorApp.h
index 2d50338..fd84bd7 100644
--- a/gui/CalculatorApp.h
+++ b/gui/CalculatorApp.h
@@ -1,14 +1,21 @@
-#ifndef _CALCULATOR_H_
-#define _CALCULATOR_H_
+#ifndef _CALCULATOR_APP_H_
+#define _CALCULATOR_APP_H_
 
 #include <string>
 
+#include "backend/Calculator.h"
 #include "App.h"
 
+
+class Calculator;
+
+
 class CalculatorApp : public App {
+private:
+	Calculator *calculator;
 public:
 	CalculatorApp(SDL_Window *window, SDL_Renderer *renderer);
-	std::string currentMemory();
+	virtual ~CalculatorApp();
 };
 
 #endif
diff --git a/gui/CalculatorInput.cpp b/gui/CalculatorInput.cpp
index eaca7f2..2969db0 100644
--- a/gui/CalculatorInput.cpp
+++ b/gui/CalculatorInput.cpp
@@ -3,9 +3,10 @@
 
 CalculatorInput::CalculatorInput(
 	SDL_Window *w, SDL_Renderer *r, int x1, int y1, int x2, int y2,
-	std::string text, int fontSize, TextOutput *output
+	std::string text, int fontSize, TextOutput *output, Calculator *calculator
 ) : LineInput(w, r, x1, y1, x2, y2, text, fontSize) {
 	this->output = output;
+	this->calculator = calculator;
 }
 
 App * CalculatorInput::handleEvent(SDL_Event event) {
@@ -15,16 +16,9 @@ App * CalculatorInput::handleEvent(SDL_Event event) {
 	}
 
 	if(event.type == SDL_KEYDOWN && event.key.keysym.sym == SDLK_RETURN) {
-		// TODO: call math-tool calculate_single, then call math-tool get_current_memory(),
-		// then assign the result of that to this->output
-		this->output->setText("[m1] 15\n[m2] 27.2");
+		this->calculator->calculate(this->text);
+		this->output->setText(this->calculator->getMemory());
 		this->text = "";
 	}
 	return nextApp; // if it's nullptr, we want to stay in the calculator
 }
-
-
-double CalculatorInput::calculate(std::string text) {
-	// TODO: call the math-tool Calculator here
-	return 14.2;
-}
diff --git a/gui/CalculatorInput.h b/gui/CalculatorInput.h
index 83d6038..2287560 100644
--- a/gui/CalculatorInput.h
+++ b/gui/CalculatorInput.h
@@ -1,20 +1,23 @@
 #ifndef _CALCULATOR_INPUT_H_
 #define _CALCULATOR_INPUT_H_
 
+#include "backend/Calculator.h"
 #include "TextOutput.h"
 #include "LineInput.h"
 
+class Calculator;
+
 class CalculatorInput : public LineInput{
 private:
 	TextOutput *output;
+	Calculator *calculator;
 public:
 	CalculatorInput(
 		SDL_Window *w, SDL_Renderer *r, int x1, int y1, int x2, int y2,
-		std::string text, int fontSize, TextOutput *output
+		std::string text, int fontSize, TextOutput *output, Calculator *c
 	);
 
 	virtual App * handleEvent(SDL_Event event);
-	double calculate(std::string text);
 };
 
 #endif
diff --git a/gui/FunctionPlotterApp.h b/gui/FunctionPlotterApp.h
index 310eccd..ed25e2c 100644
--- a/gui/FunctionPlotterApp.h
+++ b/gui/FunctionPlotterApp.h
@@ -1,5 +1,5 @@
-#ifndef _FUNCTION_PLOTTER_H_
-#define _FUNCTION_PLOTTER_H_
+#ifndef _FUNCTION_PLOTTER_APP_H_
+#define _FUNCTION_PLOTTER_APP_H_
 
 #include "App.h"
 #include "Graph.h"
diff --git a/gui/GeometryApp.h b/gui/GeometryApp.h
index 93abaf4..1c3eaaa 100644
--- a/gui/GeometryApp.h
+++ b/gui/GeometryApp.h
@@ -1,5 +1,5 @@
-#ifndef _GEOMETRY_H_
-#define _GEOMETRY_H_
+#ifndef _GEOMETRY_APP_H_
+#define _GEOMETRY_APP_H_
 
 #include "App.h"
 #include "Graph.h"
diff --git a/gui/render_text.cpp b/gui/render_text.cpp
index 5d5d8cc..37b1d39 100644
--- a/gui/render_text.cpp
+++ b/gui/render_text.cpp
@@ -17,10 +17,11 @@ void render_text(
 	// helper variables
 	int textW = xEnd - xStart;
 	int textH = yEnd - yStart;
+	int lineHeight = fontSize * 1.4; // determine space between lines
 
 	// starting point of message rectangle
 	int startX = xStart;
-	int startY = yStart - 70;
+	int startY = yStart - lineHeight;
 
 	// text has to be converted to istream in order to call getline
 	std::stringstream ss(text);
@@ -28,7 +29,13 @@ void render_text(
 
 	// break at newline and render single line
 	while(getline(ss, textToRender, '\n')) {
-		startY += 70;
+		if (startY >= yEnd) {
+			// reset startY
+			startY = yStart - lineHeight;
+			// clear screen
+			SDL_RenderClear(renderer);
+		}
+		startY += lineHeight;
 		SDL_Surface *textSurface = TTF_RenderText_Solid(
 			font, textToRender.c_str(), color
 		);