Week 3 lab solutions

algorithms/week-03/lab/Week3 Lab.ipynb

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+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Task 1. Code a priority queue\n",
+    "## The elements in the queue are Latin letters (a, b, c, … , z) and the priority is given by which letter comes first in the ABC"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 13,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Regular queue\n",
+      "h a d y r \n",
+      "\n",
+      "Priority queue\n",
+      "y r h d a "
+     ]
+    }
+   ],
+   "source": [
+    "# q = {r, y, d, a, h}  This is the example\n",
+    "q=[]  # We are trying a queue implemented with a list\n",
+    "q.append('r')\n",
+    "q.append('y')\n",
+    "q.append('d')\n",
+    "q.append('a')\n",
+    "q.append('h')\n",
+    "# if we print these values we will see the typical behaviour of a queue: FIFO\n",
+    "#In this case h , a, d, y , r. Remember that the first element to get in was r\n",
+    "print('Regular queue')\n",
+    "while q:\n",
+    "    print(q.pop(),end=\" \")\n",
+    "print('\\n')\n",
+    "#If we want to make a priority queue, where the priority is given by the ABC, we need to sort the queue first. \n",
+    "# This is higly inneficient,and only recommended for small queues. But is what you can build right now with the tools you know. \n",
+    "q.append('r')\n",
+    "q.append('y')\n",
+    "q.append('d')\n",
+    "q.append('a')\n",
+    "q.append('h')\n",
+    "#Sort the queue\n",
+    "q.sort()\n",
+    "# Now pop the elements\n",
+    "print('Priority queue')\n",
+    "while q:\n",
+    "    print(q.pop(),end=\" \")\n",
+    "#The first element to get into q was again r, but the first getting out now is a"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Task2. Code a calculator that uses postfix or Polish notation\n",
+    "Follow this algorithm:\n",
+    "1. Read the input and push numbers onto a stack until you reach an operator.\n",
+    "2. When you see an operator, apply the operator to the two numbers popped from the stack.\n",
+    "3. Push the resulting value back onto the stack.\n",
+    "4. When the input is complete, the value left on the stack is the result."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 19,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "3.0\n"
+     ]
+    }
+   ],
+   "source": [
+    "#define a function to calculate an arithmetic expression in posfix notation\n",
+    "\n",
+    "\n",
+    "def posfixN(inputs):\n",
+    "    stack = []   # Create an empty stack\n",
+    "    for a in inputs:    # Step 1\n",
+    "        if type(a) is int:\n",
+    "            stack.append(a)\n",
+    "            continue\n",
+    "        op1, op2 = stack.pop(), stack.pop() # Step 2\n",
+    "\n",
+    "        if a == '+':\n",
+    "            stack.append(op2 + op1) # Step 3\n",
+    "        elif a == '-':\n",
+    "            stack.append(op2 - op1) # Step 3\n",
+    "        elif a == '*':\n",
+    "            stack.append(op2 * op1) # Step 3\n",
+    "        elif a == '/':\n",
+    "            stack.append(op2 / op1) # Step 3\n",
+    "\n",
+    "    return stack.pop() #Step 4\n",
+    "\n",
+    "# Example 1 2 + 3 * 6 + 2 3 + / \n",
+    "expression = [1, 2, '+', 3, '*', 6, '+', 2, 3, '+', '/' ]\n",
+    "result = posfixN(expression)\n",
+    "print(result)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
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