Today's lab will focus on random numbers, indefinite loops, and finding errors.

Software tools needed: web browser and Python IDLE programming environment with the pandas, numpy, and folium package installed.

In-class Quiz

During lab, there is a quiz. The password to access the quiz will be given during lab. To complete the quiz, log on to Blackboard (see Lab 1 for details on using Blackboard).

Random Range

Python has a built-in library for generating random numbers. To use it, you include at the top of your file:
import random
The random library includes a function that's similar to range, called randrange. As with range, you can specify the starting, stopping, and step values, and the function randrange chooses a number at random in that range. Some examples: Let's use that last example to have our turtle make a random walk:

Notice that our turtle turns a degrees, where a is chosen at random between 0 and 359 degrees. What if your turtle was in a city and had to stay on a grid of streets (and not ramble through buildings)? How can you change the randrange() to choose only from the numbers: 0,90,180,270 (submit your answer as Problem #10).

Indefinite Loops

We have been using for-loops to repeat tasks a fixed number of times (often called a definite loop). There is another type of loop that repeats while a condition holds (called a indefinite loop). The most common is a while-loop.

while condition:
While the condition is true, the block of commands nested under the while statement are repeated.

For example, let's have a turtles continue their random walk as long as their x and y values are within 50 of the starting point (to keep them from wandering off the screen):

Indefinite loops are useful for simulations (like our simple random walk above) and checking input. For example, the following code fragment:

age = int(input('Please enter age: '))
while age < 0:
    print('Entered a negative number.')
    age = int(input('Please enter age: '))
print('You entered your age as:', age)
will ask the user for their age, and continue asking until the number they entered is non-negative (example in pythonTutor).

Finding Errors

Finding, and fixing errors, in your programs is a very useful skill. Let's look at a program with lots of errors and work through how to identify the issues and fix them. If you cloned the repo above, you will have a copy of on your computer (you can also download from the webpage). When loaded into IDLE, it does not run:
# is based on from Chapter 5 of the Zelle textbook
#     Converts day month and year numbers into two date formats

def main()
    # get the day month and year
    day, month year = eval(input("Please enter day, month, and year numbers: ")

    date1 = str(month)"/"+str(day)+"/"+str(year)

    months = ["January", "February", "March", "April", 
              "May", "June", "July", "August", 
              "September", "October", "November", "December"]
    monthStr = months[-1]
    date2 = monthStr+" " + str(day) + ",  + str(year)

    print("The date is" date1, "or", date2+".")

Instead, a dialog box pops up and says "invalid syntax":

The red line indicates where the intepreter has found an error. Can you tell what it is? Syntax is another word for grammar, so, it most likely missing `punctuation' or a misspelling of some sort. We have spelled def correctly and have the right number of parenthesis, so, what else is missing?

The answer is after the parenthesis on a function definition, a colon is required. Add that in:

    def main():
and try to run the program again.

Again, we get a dialog box:

Instead of the whole line being highlighted, only the word year is. The Python intepreter was not expecting year and says there is a grammatical mistake. Since year does not include any grammatical constructs, we need to look before the message to see where the error is. Do you see it?

The answer is lists of variables need commas in between them to distinguish one from the next. Add the comma in:

    day, month, year = ...
and try to run the program again.

Once more we get a dialog box:

It has highlighted the first item, date1 on the line. That is a name and looks fine. So, as above, let's look before the highlighted error to see if there's a problem. The line above it is:

    day, month, year = eval(input("Please enter day, month, and year numbers: ")
It did not highlight this line, so, the problem must be at the end. Do you see it?

The answer is we are missing a closing parenthesis. The line has two left parenthesis but only one right parenthesis. Add the right parenthesis in:

    ... and year numbers: "))
and try to run the program again.

Again, we get a dialog box:

The intepreter does not understand the second " on the line. Why? What is this line doing? It's constructing a string and storing it in the variable date1. How do you build a string out of smaller strings?

The answer is to put smaller strings together (called concatenation) we need to use the plus sign (+). The line is missing a plus sign right before the quotes. Add the plus sign in:

    date1 = str(month) + "/" ...
and try to run the program again.

Again, we get a dialog box, but this one has a different message:

EOL means "End of the line", so, the message says that the end of the line was reached before you finished defining the string. How can you fix this?

The answer is to end the string, using quotation marks. The line is missing a quotation mark at the very end. Add the quotation mark :

    ...+ ",  + str(year)"
and try to run the program again.

Our familiar dialog box returns:

We have seen this type of error before. How do you fix it?

The answer is lists of arguments need commas in between them to distinguish one from the next. Add the comma in:

    ... date is", date1 ...
and try to run the program again.

It runs! Now let's make sure it works. Type in at the prompt:

Please enter day, month, and year numbers: 31, 12, 2014
Uh oh, instead of output, we get the following messages:
Traceback (most recent call last):
  File "/Users/stjohn/public_html/teaching/cmp/cmp230/f14/", line 18, in 
  File "/Users/stjohn/public_html/teaching/cmp/cmp230/f14/", line 13, in main
    monthStr = months[month+1]
IndexError: list index out of range

When you see messages like this, go to the very last line:

IndexError: list index out of range
It says that the index for our list is out of range. An index is the item of the list that we're accessing. For example, months[1] has index 1 and will give us February. The range of the index for a list is 0 to one less than the length of the list. In the case of months, the range is [0,1,2,...,11]. What went wrong when we entered 12 for our month?

The answer is we used month+1 = 12 + 1 = 13 as the index:

    monthStr = months[month+1]
which is out of range. What do we want instead? Instead of adding 1, we should subtract 1. Change it in the program:
    monthStr = months[month-1]
and try to run the program again.

It still runs, but does it work? Let's try the same input again:

Please enter day, month, and year numbers: 31, 12, 2014
The date is 12/31/14 or December 31,  + str(year).

Something odd is happening at the end-- str(year) does not look right. Let's look at the print statement:

date2 = monthStr+" " + str(day) + ",  + str(year)"
The intepreter is treating ", + str(year)" as a string (instead of evaluating str(year)), so, we must have put the quotation mark in the wrong place before. Let's move it:
date2 = monthStr+" " + str(day) + ","  + str(year)
and try to run the program again.

Success! But try a few other inputs, just to make sure. It is always good to try cases that are near the `boundary' of what's allowed, since those are the places we are most likely to make mistakes:

Please enter day, month, and year numbers: 1,1,2015
The date is 1/1/2015 or January 1,2015.

Please enter day, month, and year numbers: 1, 2, 2003
The date is 2/1/03 or February 1,2003.

Please enter day, month, and year numbers: 4, 7, 1976
The date is 7/4/1976 or July 4,1976.

We have removed all the errors, and the program now runs correctly!

More on Command Line Scripts

In Lab 6, we wrote a simple script that prints: Hello, World. We can write scripts that take the shell commands we have learned and store them in a file to be used later. In Lab 3, we introduced the shell, or command line, commands to create new directories (folders) and how to list the contents of those folders (and expanded on this with relative paths in Lab 4 and absolute paths in Lab 5).

It's a bit archaic, but we can create the file with the vi editor. It dates back to the early days of the Unix operating system. It has the advantage that it's integrated into the Unix operating system and guaranteed to be there. It is worth trying at least once (so if you're in a bind and need to edit Unix files remotely, you can), but if you hate it (which is likely), use the graphical gEdit (you can find it via the search icon on the left hand menu bar).

Let's create a simple shell script with vi:

  1. Open a terminal window.
  2. Type in the terminal window: vi setupProject
  3. To add text to the file, begin by typing i (the letter i) which stands for "insert"
  4. Type the lines:
    #Your name here
    #October 2017
    #Shell script:  creates directories for project
    mkdir projectFiles
    cd projectFiles
    mkdir source
    mkdir data
    mkdir results
  5. Click on the ESC (escape key-- usually on the upper row of keys) which escapes from insert mode.
  6. Type: :wq which stands for "write my file" and "quit" (the ":" is necessary-- it tells vi that you want to use the menu commands).
  7. This brings us back to the terminal window. Type ls to see a listing of the directory, which should include the file, setupProject, you just created.
  8. Next, we'll change the permissions on the file, so that we can run it directly, by just typing its name:
    chmod +x setupProject
    (changes the "mode" to be executable).
  9. To run your shell script, you can now type its name at the terminal:
  10. Check to see that your script works, type ls to see the new directories your script you created.

Troubles with vi? It's not intuitive-- here's more on vi:

If you hate vi, just edit using gEdit (the Linux version of TextEdit) or IDLE.

When done, see the Programming Problem List.

What's Next?

If you finish the lab early, now is a great time to get a head start on the programming problems due early next week. There's instructors to help you, and you already have Python up and running. The Programming Problem List has problem descriptions, suggested reading, and due dates next to each problem.