Exercise 9

Added Exercise 9 to repository

Informatik_I/Exercise_9/Task_4/README.org

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+#+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 🇨🇭