diff --git a/Informatik_I/Exercise_9/Task_1/README.org b/Informatik_I/Exercise_9/Task_1/README.org
new file mode 100644
index 0000000..92fac2f
--- /dev/null
+++ b/Informatik_I/Exercise_9/Task_1/README.org
@@ -0,0 +1,175 @@
+#+title: Task 1: Complex Numbers
+
+#+author: JirR02
+* Task
+:PROPERTIES:
+:CUSTOM_ID: task
+:END:
+Create your own data type for complex numbers that can be used as a
+drop-in replacement for double. The definition of the data type and the
+forward declarations of operators must be in file =complex.h=, while the
+implementation of operators must be in file =complex.cpp=.
+
+1. Define a struct named =Complex= that represents a complex number in
+   the cartesian form. As data type use floating point numbers with
+   =double= precision.
+
+2. Implement input and output =(>>, <<)= operators for reading and
+   writing of complex numbers.
+
+A complex number \((x pm yi)\) is represented in the format =[x,y]=,
+where =x= and =y= follows the format of floating point numbers with
+=double= precision.
+
+E.g., =[-2,5]= represents \((-2 + 5i)\) . *Important: No spaces*.
+
+*Note*: You can assume that your input value is a well-formatted complex
+number.
+
+3. [@3] Implement arithmetic operators for addition, subtraction,
+   multiplication, and division \((+,-,*,/)\) of complex numbers. For
+   the division, you may assume that the squared modulus of the divisor
+   is not zero, even after rounding it to a floating point number.
+   Finally, you do not need to treat the case of division by zero.
+
+4. Implement the negation operator \((-)\) of complex numbers.
+
+5. Implement comparison operators for equality and inequality
+   \((==,!=)\) of complex numbers.
+
+*Hint*: recall from Exercise 5 how to correctly compare floating point
+numbers.
+
+* Solution
+:PROPERTIES:
+:CUSTOM_ID: solution
+:END:
+** =complex.h=
+:PROPERTIES:
+:CUSTOM_ID: complex.h
+:END:
+#+begin_src cpp
+#pragma once
+
+#include <cmath>
+#include <iostream>
+
+struct Complex {
+  double a;
+  double b;
+};
+
+// arithmetic operators
+Complex operator+(const Complex &cn1, const Complex &cn2);
+Complex operator-(const Complex &cn1, const Complex &cn2);
+Complex operator*(const Complex &cn1, const Complex &cn2);
+Complex operator/(const Complex &cn1, const Complex &cn2);
+
+// negation operator
+
+Complex operator-(const Complex &cn1);
+
+// equality and inequality operators
+
+bool operator==(const Complex &cn1, const Complex &cn2);
+bool operator!=(const Complex &cn1, const Complex &cn2);
+
+// reads a complex number from input
+std::istream &operator>>(std::istream &in, Complex &a);
+std::ostream &operator<<(std::ostream &out, const Complex &a);
+#+end_src
+
+** =complex.cpp=
+:PROPERTIES:
+:CUSTOM_ID: complex.cpp
+:END:
+#+begin_src cpp
+#include "complex.h"
+#include <cstdlib>
+
+std::istream &operator>>(std::istream &in, Complex &a) {
+  char open;
+  char comma;
+  char close;
+  double aa;
+  double ab;
+  in >> open;
+  if (open == '[') {
+    in >> aa >> comma >> ab >> close;
+    if (comma == ',' && close == ']') {
+      a.a = aa;
+      a.b = ab;
+    } else {
+      std::cerr << "Invalid Input!";
+  }
+  } else {
+    std::cerr << "Invalid Input!";
+  }
+  return in;
+}
+
+std::ostream &operator<<(std::ostream &out, const Complex &a) {
+  return out << "[" << a.a << "," << a.b << "]";
+}
+
+Complex operator+(const Complex &cn1, const Complex &cn2) {
+  Complex res;
+  double a = cn1.a + cn2.a;
+  double b = cn1.b + cn2.b;
+  res.a = a;
+  res.b = b;
+  return res;
+}
+
+Complex operator-(const Complex &cn1, const Complex &cn2) {
+  Complex res;
+  double a = cn1.a - cn2.a;
+  double b = cn1.b - cn2.b;
+  res.a = a;
+  res.b = b;
+  return res;
+}
+
+Complex operator*(const Complex &cn1, const Complex &cn2) {
+  Complex res;
+  double a = (cn1.a * cn2.a) - (cn1.b * cn2.b);
+  double b = (cn1.a * cn2.b) + (cn1.b * cn2.a);
+  res.a = a;
+  res.b = b;
+  return res;
+}
+
+Complex operator/(const Complex &cn1, const Complex &cn2) {
+  Complex res;
+  double a = (cn1.a * cn2.a + cn1.b * cn2.b) / (cn2.a * cn2.a + cn2.b * cn2.b);
+  double b = (cn1.b * cn2.a - cn1.a * cn2.b) / (cn2.a * cn2.a + cn2.b * cn2.b);
+  res.a = a;
+  res.b = b;
+  return res;
+}
+
+Complex operator-(const Complex &cn1) {
+  Complex res;
+  double a = -cn1.a;
+  double b = -cn1.b;
+  res.a = a;
+  res.b = b;
+  return res;
+}
+
+bool operator==(const Complex &cn1, const Complex &cn2) {
+  double epsilon = 0.1;
+  return (std::abs(cn1.a - cn2.a) < epsilon) &&
+  (std::abs(cn1.b - cn2.b) < epsilon);
+}
+
+bool operator!=(const Complex &cn1, const Complex &cn2) {
+  double epsilon = 0.1;
+  return (std::abs(cn1.a - cn2.a) > epsilon) ||
+  (std::abs(cn1.b - cn2.b) > epsilon);
+}
+#+end_src
+
+--------------
+
+Made by JirR02 in Switzerland 🇨🇭
diff --git a/Informatik_I/Exercise_9/Task_2/README.org b/Informatik_I/Exercise_9/Task_2/README.org
new file mode 100644
index 0000000..e9f861b
--- /dev/null
+++ b/Informatik_I/Exercise_9/Task_2/README.org
@@ -0,0 +1,96 @@
+#+title: Task 2: Understanding struct & classes
+
+#+author: JirR02
+/This task is a text based task. You do not need to write any
+program/C++ file: the answer should be written in main.md (and might
+include code fragments if questions ask for them)./
+
+* Task
+:PROPERTIES:
+:CUSTOM_ID: task
+:END:
+Consider the following definitions:
+
+1. 
+
+#+begin_src cpp
+struct A {
+int a;
+double b;
+int c;
+};
+
+A str = {1, 1.5, 2};
+std::vector<int> vec = {1, 1, 3};
+int& a = vec[0];
+#+end_src
+
+For each of the provided expressions state their /C++ type/ and /value/:
+
+1. =str.a * str.b=
+2. =str.b == vec[1]=
+3. =str.a * str.b / str.c=
+4. =vec[str.a] / str.c=
+5. =a / 2 - str.b=
+
+2. [@2] 
+
+#+begin_src cpp
+class B {
+public:
+B(): vec(128) {
+for (int i = 0; i < 128; ++i) {
+vec[i] = 0;
+}
+}
+    
+// PRE: ...
+// POST: ...
+void add(const char c) {
+++vec[c];
+}
+    
+// PRE: ...
+// POST: ...
+int get(const char c) const {
+return vec[c];
+}
+
+private:
+std::vector<int> vec;
+};
+#+end_src
+
+Determine PRE- and POST-conditions for the methods add and get. As these
+methods are members functions of class B, PRE- and POST-conditions
+should be derived taking the public interface of the class into account.
+
+* Solution
+:PROPERTIES:
+:CUSTOM_ID: solution
+:END:
+#+begin_src markdown
+
+1. `plain|`
+
+1. Type: `cpp|double` / Value: `cpp|1.5`
+2. Type: `cpp|bool` / Value: `cpp|true`
+3. Type: `cpp|double` / Value: `cpp|0.75`
+4. Type: `cpp|int` / Value: `cpp|0`
+5. Type: `cpp|double` / Value: `cpp|-1.5`
+
+2. ```c++
+// PRE: 0 <= c < 128
+// POST: Increments the value of the vector at position c
+void add(const char c);
+
+// PRE: 0 <= c < 128
+// POST: Gets the value of the vector at position c
+int get(const char c) const;
+#+end_src
+
+```
+
+--------------
+
+Made by JirR02 in Switzerland 🇨🇭
diff --git a/Informatik_I/Exercise_9/Task_3/README.org b/Informatik_I/Exercise_9/Task_3/README.org
new file mode 100644
index 0000000..dcfd746
--- /dev/null
+++ b/Informatik_I/Exercise_9/Task_3/README.org
@@ -0,0 +1,62 @@
+#+title: Task 3: Averager
+
+#+author: JirR02
+* Task
+:PROPERTIES:
+:CUSTOM_ID: task
+:END:
+Write a class =Averager= that computes averages of given values of type
+=double=.
+
+Initially, an instance of class =Averager= does not contain any value.
+The class =Averager= must provide the following functionality:
+
+#+begin_src cpp
+// POST: Adds a value to the current average calculation.
+void add_value(double value);
+
+// POST: Returns the average of all added values,
+//       or zero, if no value has been added.
+double get_average();
+
+// POST: Removes all values from the current average calculation.
+void reset();
+#+end_src
+
+The declaration and implementation of class Averager must be split
+between header file (=averager.h=, containing declaration) and
+implementation file (=averager.cpp=, containing implementation).
+
+/Note: The above functions have already been declared in averager.h for
+you. You only need to provide function definitions in/ =averager.cpp=.
+
+* Solution
+:PROPERTIES:
+:CUSTOM_ID: solution
+:END:
+#+begin_src cpp
+#include "averager.h"
+
+void Averager::add_value(double value) {
+  sum += value;
+  ++count;
+}
+
+double Averager::get_average() {
+  double res;
+  if (count == 0)
+    res = 0;
+  else
+  res = sum / count;
+  return res;
+}
+
+void Averager::reset() {
+  sum = 0;
+  count = 0;
+}
+#+end_src
+
+--------------
+
+Made by JirR02 in Switzerland 🇨🇭
diff --git a/Informatik_I/Exercise_9/Task_4/README.org b/Informatik_I/Exercise_9/Task_4/README.org
new file mode 100644
index 0000000..707eedb
--- /dev/null
+++ b/Informatik_I/Exercise_9/Task_4/README.org
@@ -0,0 +1,111 @@
+#+title: Task 4: [Hidden tests][Exam 2019.02 RW] Number Pairs with Sum
+
+#+author: JirR02
+* Number of pairs with given sum
+:PROPERTIES:
+:CUSTOM_ID: number-of-pairs-with-given-sum
+:END:
+In this exercise, you are going to use iterators to count the number of
+pairs in a vector which sum up to a given value.
+
+More formally, given a vector \(v\) of length \(n\) and a value \(s\),
+we are interested in the number of pairs \(i\),\(j\) for which the
+following conditions hold:
+
+1. \(0 leq i < j < n\)
+2. \(v\[i\] + v\[j\] = s\)
+
+* Task
+:PROPERTIES:
+:CUSTOM_ID: task
+:END:
+The function =pairs_with_sum= declared in =pair_sum.h= computes the
+number of pairs which sum up to a given value. Your task is implementing
+this function in =pair_sum.cpp=.
+
+* Input & Output
+:PROPERTIES:
+:CUSTOM_ID: input-output
+:END:
+The program first reads a list of integers (separated by spaces) from
+=std::cin=. The first integer specifies the value \(s\). The other
+integers specify the vector \(s\).
+
+The program then calls the function
+=pairs_with_sum(s, v.begin(), v.end())= to compute the number of pairs
+that sum up to \(s\).
+
+Finally, the result is printed to the console.
+
+*** Example:
+:PROPERTIES:
+:CUSTOM_ID: example
+:END:
+- sum = 8, vector = 1 7 2 6 6
+- pairs with sum(1,7), (2,6), (2,6)
+- \(Rightarrow\) number pairs = 3.
+
+#+begin_src shell
+8 1 7 2 6 6
+number of pairs with sum 8: 3
+#+end_src
+
+*Note*: This was a task in the 2019.02 RW exam but has since been
+adapted slightly: better input handling (no need to press CTRL+D/CTRL+Z
+to end vector), description, file structure, and master solution.
+
+*Note*: You can safely assume that the sum of two values in the vector
+does not cause an overflow.
+
+* Hidden test cases
+:PROPERTIES:
+:CUSTOM_ID: hidden-test-cases
+:END:
+This exercise has some hidden test cases. You will see whether these
+tests passed or failed, but you will not see the test input or the
+expected output.
+
+** Persistent input
+:PROPERTIES:
+:CUSTOM_ID: persistent-input
+:END:
+To help you in testing your code, you can enter your custom test input
+in input.txt. When running the program, you can let CodeExpert read the
+input from the file.
+
+Write some program input in input.txt. Run the program as usual. Type f
+(no spaces) in the terminal window and press Enter. The program will use
+the contents of input.txt as input. Using this feature is entirely
+optional and does not affect your score.
+
+* Solution
+:PROPERTIES:
+:CUSTOM_ID: solution
+:END:
+#+begin_src cpp
+#include "pair_sum.h"
+#include <climits>
+
+// PRE:  for any two indices i and j, v[i] + v[j] ≤ INT_MAX
+// POST: returns the number of indices (i,j), i<j of a vector v
+//       that corresponds to the given iterator range such that
+//       v[i] + v[j] == sum.
+int pairs_with_sum(int sum, iterator begin, iterator end) {
+  int count = 0;
+  int size = end - begin;
+
+  for (int i = 0; i < size; ++i) {
+    for (int j = i + 1; j < size; ++j) {
+      iterator cur_begin = begin + i;
+      iterator cur_end = begin + j;
+      if (*cur_begin + *cur_end == sum)
+  ++count;
+    }
+  }
+
+  return count;
+#+end_src
+
+--------------
+
+Made by JirR02 in Switzerland 🇨🇭