Automating GIS processes/ FEC 2017 / Exercise 2 - for loops and conditional statements

Exercise 2 for the course Automating GIS processes / FEC 2017.

This exercise is meant to help you to understand for loops and conditional statements in Python. Below you have a series of problems in which you are asked to create new script files and write the code necessary to produce the desired results. After making your changes, you will need to upload these files to GitHub.

General tips

1. Create a general template for your new script files and commit it to GitHub right when you start working on your script(s).

2. Your scripts should also follow the format described earlier in the course with a block comment at the start of the file, inline comments describing how it works and good use of blank lines to make the code easy to read.

3. Look carefully at the requirements for each exercise and be sure to follow them.

4. In case you have forgotten, string variables can be added together. For example,

   >>> a = "Taco "
   >>> b = "time"
   >>> c = a + b
   >>> print(c)
   Taco time

Problem 1 - Batch processing data files with a for loop

This problem is meant to simulate a common problem dealing with data files: batch processing. Batch processing involves using scripts to process many data files, and one common task is generating a list of filenames that will be processed. For this problem you will need to create a new script file animal_file_generator.py that does the following:

1. Creates a new variable basename that contains the text "Animal".

2. Creates a new variable filenames that contains the text "".

3. Iterates over the number range 0-30 and

4. Prints the value in basename with the numbers in the range to the screen as follows:

   Animal_0
   Animal_1
   Animal_2
   ...
   Animal_30
   

   You will most likely need to use the built-in str() function to produce this output. You can learn about how str() works by typing help(str) in an IPython interpreter window.

5. Modifies the value in filenames to contain the output above plus the file extension for a shapefile .shp. You must also include a special escape character \n to indicate a break in the line after each listed shapefile name.

At the end of the script the variable filenames should include a complete list of the different Animal filenames. The output of print(filenames) in your script should be

Animal_0.shp
Animal_1.shp
Animal_2.shp
...
Animal_30.shp

Problem 2 - Working with projections using conditional statements

In Python there is a handy way of asking input from the user using input function such as:

>>> name = input('Please tell me your name:\n')
Please tell me your name:
Henrikki
>>> print("Hello", name)
Hello Henrikki

Your job in this problem is to create a new Python script epsg_finder.py that:

1. Asks the user for an input value by displaying the text "What is the EPSG value of the input shapefile?"

• If the user says 4326, the program prints "EPSG value 4326 corresponds to WGS84" to the screen
• If the user says 3067, the program prints “EPSG value 3067 corresponds to ETRS-TM35FIN" to the screen
• If the user says 2392, the program prints “EPSG value 2392 corresponds to KKJ / Finland zone2" to the screen
• If the user enters any other value, the program should print "There are so many spatial references..I don’t know them all, sorry." to the screen

Problem 3 (EXTRA TASK) - Nested for loops

This is not a compulsory exercise, you can try to solve this problem after you have finished the two first tasks.

In addition to having single for loops that iterate across some variable range, it is possible to nest for loops within one another. Consider the example below:

>>> for char in 'dog':
...     for char2 in 'cat':
...         print (char, char2)
    d c
    d a
    d t
    o c
    o a
    o t
    g c
    g a
    g t

Here, you can see that in the first pass through the first for loop, the value of char is d. Entering the inner (or nested) loop, char2 is set to c. After this, the output is written to the screen and since there are more letters to loop over in the inner for loop, the value of char2 will be updated upon the next pass. The second time through the inner loop, char2 is set to a while char remains d. Like this, the inner loop will run through all of the letters in cat for each time that char is updated in the outer loop. It doesn't take too much imagination to figure out this is a very useful concept.

For this problem you should create a new Python script make_flag.py that does the following:

1. Creates a variable star with text "*" and an empty string variable text. Recall, you can created empty string variables by assigning "" as their value.

2. Uses nested for loops and the variables above to produce the text formation below when print(text) is run at the end of your script.

   *******
   *******
   *******
   

3. Creates a varaiable line with text "-" and an empty string variable flag.

4. Uses nested for loops and the variables above to produce the text formation below when print(flag) is run at the end of your script. Note: You will need to use conditional statements to produce the desired output.

   *******------------
   *******------------
   *******------------
   -------------------
   -------------------