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"Course 5: Input and output\n",
"==========================\n",
"\n",
"In this worksheet we learn how to read and write files. We will also learn more string manipulation.\n",
"\n",
"Prerequisites (...need to open a terminal...):\n",
"- filesystem (the commands `cd`, `ls`, `pwd` in terminal)\n",
"- relative versus absolute paths\n",
"- special repositories `./` and `../`\n",
"\n",
"The main function to deal with files is the function `open`. When using it you have to specify a mode that can be `'w'` for writing or `'r'` for reading. **When you open a file in writing mode you erase its content!**\n",
"\n",
"Below is an example of a creation of a file in the repository `/tmp/` (that is intended to be used for temporary files)\n",
"```python\n",
"f = open(\"/tmp/hello.txt\", \"w\") # open in write mode\n",
"f.write(\"I am writing to a file\\n\") # write a string in the file\n",
"f.write(\"it is cool!\\n\") # write a string in the file\n",
"f.close() # closing the file\n",
"```\n",
"It is important to close the file (the last step above) since otherwise some data might not have been written in the file (because of buffers). If you have a file on the disc then you can read its content with\n",
"```python\n",
"# opening file in read mode\n",
"g = open(\"/tmp/hello.txt\", \"r\") # open in read mode\n",
"text = g.read() # gettint content as a string\n",
"g.close() # closing the file\n",
"```\n",
"**Exercise:**\n",
"- execute the first set of commands above that creates the file `/tmp/hello.txt`\n",
"- check with nautilus or the terminal that this file does exist on your computer (you can even open it and edit it with gedit)\n",
"- execute the second set of commands above that reads the content of this file `/tmp/hello.txt`\n",
"- print the content of `text`"
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"It is important to know about more string operations to treat data contained in a file. You already learned about concatenation (the `+` operation). We now learn about the \"opposite\" operation. If you have a string `s` the command `s.split(sep)` cut it into pieces along the character `sep`. You can execute the following examples\n",
"```python\n",
"\"how are you doing today?\".split(\" \")\n",
"\"abaabaaababa\".split(\"b\")\n",
"```\n",
"\n",
"**Exercise:**\n",
"- Let `s1` be defined as below\n",
"```python\n",
"s1 = \"Alfred,Eistein\\nAlexandre,Grothendieck\\nBernhard,Riemann\"\n",
"```\n",
"- Print the above string.\n",
"- Using `split` or `splitlines` lines transform `s1` into the list\n",
"```python\n",
"['Alfred,Eistein', 'Alexandre,Grothendieck', 'Bernhard,Riemann']\n",
"```\n",
"- Using `split` transform `s1` into the list\n",
"```python\n",
"[['Alfred', 'Eistein'], ['Alexandre', 'Grothendieck'], ['Bernhard', 'Riemann']]\n",
"```"
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"**Exercise:**\n",
"- Download the file [names.csv](https://raw.githubusercontent.com/videlec/aims-python-rwanda-2016/master/worksheets/names.csv) and save it on your computer. Each line of this file consists in a first name, a name, a country, a birth date separated by a coma as in\n",
"```\n",
"Hammed,Gibbs,Zambia,01/08/1983\n",
"Kais,Poisson,Portugal,09/07/1999\n",
"```\n",
"- Read this file\n",
"- Using the method `split` of strings transform this string into a list of lists of the form\n",
"```python\n",
"[[\"Hammed\", \"Gibbs\", \"Zambia\", \"01\", \"08\", \"1983\"],\n",
" [\"Kais\", \"Poisson\", \"Portugal\", \"09\", \"07\", \"1999\"]]\n",
"```\n",
"- How many persons in the list are from Zambia?\n",
"- How many persons are borned in 1988?\n",
"- Print the list of days and months so that more than one person have their birthday at this date."
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"Saving numpy arrays\n",
"-------------------\n",
"\n",
"The `numpy` library introduced their own format to read arrays in a file. These are the functions `save` and `load` from the `numpy` library.\n",
"\n",
"Recall that in order to use numpy you need to import it and the standard way to do so is\n",
"```python\n",
"import numpy as np\n",
"```\n",
"Once numpy is imported you can save and load as follows\n",
"```python\n",
"a = np.arange(1, 10, 1) # creates an array\n",
"np.save('my_object.npy', a) # saves the array in 'my_object.npy'\n",
"b = np.load('my_object.npy') # loads the object saved and assigned it to b\n",
"```\n",
"**Exercise:**\n",
"- Execute the above code\n",
"- Download the files [array1.npy](https://github.com/videlec/aims-python-rwanda-2016/blob/master/worksheets/array1.npy?raw=true), [array2.npy](https://github.com/videlec/aims-python-rwanda-2016/blob/master/worksheets/array2.npy?raw=true) and [array3.npy](https://github.com/videlec/aims-python-rwanda-2016/blob/master/worksheets/array3.npy?raw=true).\n",
"- For each of the file you downloaded, load them as a numpy array.\n",
"- Each of these arrays was generated as a random sample of a distribution you know. Are you able to find which ones?"
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"Additional remarks\n",
"------------------\n",
"\n",
"You might want to explore the filesystem from Python (instead of using the terminal or nautilus). There are tools for that purpose in the standard Python library (in particular [os.path](https://docs.python.org/3/library/os.path.html) and [pathlib](https://docs.python.org/3/library/pathlib.html)). For example\n",
"```python\n",
"import os\n",
"os.listdir('.') # list the current directory\n",
"os.path.abspath('.') # return the current directory as an absolute path\n",
"```\n",
"\n",
"More information is available in the chapter [File and Directory Access](https://docs.python.org/3/library/filesys.html) of the standard Python library."
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"Copyright (C) 2016 Vincent Delecroix <vincent.delecroix@u-bordeaux.fr>\n",
"\n",
"This work is licensed under a Creative Commons Attribution-NonCommercial 4.0\n",
"International License (CC BY-NC-SA 4.0). You can either read the\n",
"[Creative Commons Deed](https://creativecommons.org/licenses/by-nc-sa/4.0/)\n",
"(Summary) or the [Legal Code](https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode)\n",
"(Full licence)."
]
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