Exercise 3 - `for` loops and conditional statements

The exercise for this week 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. The answers to the questions in this week's exercise should be given by modifying the end of this document in the section titled Answers.

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:

Creates a new variable basename that contains the text "Animal".
Creates a new variable filenames that contains the text "".
Iterates over the number range 0-30 and
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.
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

Questions for Problem 1

What could be some advantages of using a string variable (e.g., filenames) to contain a list of files?
What are some disadvantages of using a string variable for a list of files, in contrast to a Python list variable?
Can you think of any cases where you might use a list of filenames like we generate in this exercise?

Problem 2 - Nested `for` loops

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:

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

Questions for Problem 2

If faced with having to reproduce the character patterns in this exercise with a Python script any way you can, how might you approach this problem differently? For example, let's say you did not need to use the variables star = "*" and line = "-".
Using nested loops is very common for Python scripts that deal with two-dimensional data such as latitude and longitude, or depth versus distance. Can you think of any cases in which you might use a nested loop in a Python script?

Problem 3 - 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:

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

Questions for Problem 3

Did you use any elif statements in your Python code? If so, can you think of any way in which you could rewrite you code without using elif statements? If not, can you think of any way in which you could rewrite your code to include them?
Why did you choose to include or not include elif statements in your code? Did it make more sense to use to use them, or was it easier to write your code without?

General tips

Create a general template for your new script files and commit it to GitHub right when you start working on your script(s).
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.
Look carefully at the requirements for each exercise and be sure to follow them.

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

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

The questions after the exercises do not necessarily have correct or incorrect answers, but they are meant to make you think a bit about how you have written your scripts and how they might be applied to your own data/research/studies.

Python-for-geo-people/Exercise-3

Exercise 3 - for loops and conditional statements

Problem 1 - Batch processing data files with a for loop