Buscar elements i índexs a llistes i np.arrays

buscar_elements.ipynb

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 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "# Buscar elements en `list` i `numpy.array` obtenint valors i/o \u00edndexs"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Definim una llista:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "una_llista = [25, 35, 45, 55, 65, 75]"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 1
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## En llistes"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "###Saber si un valor est\u00e0 present"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "if 45 in una_llista:\n",
      "    print \"El 45 est\u00e0 present.\"\n",
      "else:\n",
      "    print \"El 45 no est\u00e0 present.\"\n",
      "\n",
      "# Podem fer la pregunta al rev\u00e9s:\n",
      "if 50 not in una_llista:\n",
      "    print \"El 50 no est\u00e0 present.\"\n",
      "else:\n",
      "    print \"El 50 est\u00e0 present.\""
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "El 45 est\u00e0 present.\n",
        "El 50 no est\u00e0 present.\n"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Buscar valors"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Busquem els elements de llista m\u00e9s grans de 50. Ho fem mitjan\u00e7ant una \"list comprenhension\":"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "elements_grans = [x for x in una_llista if x > 50]\n",
      "print elements_grans"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[55, 65, 75]\n"
       ]
      }
     ],
     "prompt_number": 3
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Equival al seg\u00fcent bucle:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "elements_grans2 = []\n",
      "for x in una_llista:\n",
      "    if x > 50:\n",
      "        elements_grans2.append(x)\n",
      "print elements_grans2\n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[55, 65, 75]\n"
       ]
      }
     ],
     "prompt_number": 4
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Buscar \u00edndex:"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Busquem els \u00edndex de la llista:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "elements_grans_index = [i for i,x in enumerate(una_llista) if x > 50]\n",
      "print elements_grans_index\n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[3, 4, 5]\n"
       ]
      }
     ],
     "prompt_number": 5
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Equival al seg\u00fcent bucle:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "elements_grans_index2 = []\n",
      "for i, x in enumerate(una_llista):\n",
      "    if x > 50:\n",
      "        elements_grans_index2.append(i)\n",
      "print elements_grans_index2\n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[3, 4, 5]\n"
       ]
      }
     ],
     "prompt_number": 6
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "`enumerate(una_llista)` retorna una tupla (index, valor) per cada element de una_llista. "
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Per\u00f2 en cap cas es pot fer aix\u00ed:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print una_llista == 50 # La llista sencera no \u00e9s igual a un nombre!!"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "False\n"
       ]
      }
     ],
     "prompt_number": 7
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print una_llista < 50 # Tampoc t\u00e9 sentit, pot tornar True o False arbitr\u00e0riament."
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "False\n"
       ]
      }
     ],
     "prompt_number": 8
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## En `numpy.array`"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Definim un array de numpy:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "import numpy as np\n",
      "el_meu_array = np.array(una_llista)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 9
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Saber si un element est\u00e0 o no present:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "if 45 in el_meu_array:\n",
      "    print \"El 45 est\u00e0 al array\"\n",
      "else:\n",
      "    print \"El 45 no est\u00e0 al array\"\n",
      "\n",
      "# Podem fer la pregunta al rev\u00e9s:\n",
      "if 50 not in el_meu_array:\n",
      "    print \"El 50 est\u00e0 al array\"\n",
      "else:\n",
      "    print \"El 50 no est\u00e0 al array\"\n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "El 45 est\u00e0 al array\n",
        "El 50 est\u00e0 al array\n"
       ]
      }
     ],
     "prompt_number": 10
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Buscar elements:"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Els array de numpy s\u00f3n m\u00e9s intu\u00eftius: Obtenim un array de numpy amb True/False per cada element"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "el_meu_array == 50"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 11,
       "text": [
        "array([False, False, False, False, False, False], dtype=bool)"
       ]
      }
     ],
     "prompt_number": 11
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "el_meu_array > 50"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 12,
       "text": [
        "array([False, False, False,  True,  True,  True], dtype=bool)"
       ]
      }
     ],
     "prompt_number": 12
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "A m\u00e9s podem indexar amb aquests arrays:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "array_veritat_fals = el_meu_array > 50\n",
      "print el_meu_array[array_veritat_fals]"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[55 65 75]\n"
       ]
      }
     ],
     "prompt_number": 13
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "O combinant les dues expressions anteriors:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print el_meu_array[el_meu_array > 50]"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[55 65 75]\n"
       ]
      }
     ],
     "prompt_number": 14
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Buscar \u00edndexs:"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "La funci\u00f3 `numpy.where` ens demana un array de veritat/fals i ens diu (en la component 0) els \u00edndexs \"veritat\":"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "array_veritat_fals = el_meu_array > 50\n",
      "index_buscats = np.where(array_veritat_fals)[0]\n",
      "print el_meu_array\n",
      "print index_buscats"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[25 35 45 55 65 75]\n",
        "[3 4 5]\n"
       ]
      }
     ],
     "prompt_number": 15
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Ho posem tot junt:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print np.where(el_meu_array > 50)[0]"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "[3 4 5]\n"
       ]
      }
     ],
     "prompt_number": 16
    }
   ],
   "metadata": {}
  }
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}