This lab focuses on decisions and gives more techniques for finding errors.

In Lab 3, we started with a program that drew a square to the screen, using turtle graphics: 
import turtle

def main():
    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window

    #Draw a square
    for i in range(4):
        daniel.forward(100)     #Move forward 10 steps
        daniel.right(90)        #Turn 90 degrees to the right

    myWin.exitonclick()         #Close the window when clicked
    
main()
Recall, our turtle, named daniel, moved forward 100 steps, made a 90 degree right turn, and then repeated these actions for a total of 4 times. Let's modify it, so, that it will draw an 8 sided polygon: 
import turtle

def main():
	numSides = 8                #Number of sides of the polygon
    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window

    #Draw a square
    for i in range(numSides):
        daniel.forward(100)     #Move forward 10 steps
        daniel.right(360/numSides)	#Turn 90 degrees to the right

    myWin.exitonclick()         #Close the window when clicked
    
main()
To make it easier to modify, we stored the number of sides in just one place (the variable named numSides) and use it in range statement in the for-loop as well as calculating the amount needed to turn each time.

Run the program to make sure there's no errors. On the graphics window, you should see an octogon (8-sided) figure. How would you make an octogon like this:

Notice that the edges change colors, the first, third, fifth, and seventh edges are red; while the second, fourth, sixth, and eighth edges are green. Since we start counting with 0, we have that the edges are red when the loop index variable i is 0,2,4,6. The edges are green when the loop index variable is 1,3,5,7.

To make the colors change, we need to test for when the loop index variable is even. A way to say that mathematically is i is even when i divided by 2 has no remainder. In python, we can write that as: 

	if i % 2 == 0:
		daniel.color("red")
Let's add that to our program: 
#Blinking turtle for introductory programming lab

import turtle

def main():
    numSides = 8
    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window

    #Draw a square
    for i in range(numSides):
        if i % 2 == 0:
            daniel.color("red")      
        daniel.forward(100)     #Move forward 10 steps
        daniel.right(360/numSides)  #Turn 90 degrees to the right

    myWin.exitonclick()         #Close the window when clicked
    
main()
What does that do? How do we make the color green for when i is odd? Let's add in an else to our if statement: 
	if i % 2 == 0:
		Turn daniel red
	else:
		Turn daniel green
Putting all the pieces together, we get: 
#Blinking turtle for introductory programming lab

import turtle

def main():
    numSides = 8
    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window

    #Draw a square
    for i in range(numSides):
        if i % 2 == 0:
            daniel.color("red")
        else:
            daniel.color("green")        
        daniel.forward(100)     #Move forward 10 steps
        daniel.right(360/numSides)  #Turn 90 degrees to the right

    myWin.exitonclick()         #Close the window when clicked
    
main()
Try the program to make sure that the colors change, depending on the value of i. Next, change your program to make a 10-sided polygon (hint: you only need to change one line).

Finding Errors, II

In Lab 6, we looked at some common syntax errors and how to fix them. Most of those errors were missing punctuation (such at colons, quotes, or plus signs). Let's look at a few more errors that occur when using conditionals, loops, and functions: 

# errors2.py-- modified from Zelle
# recursions.py
#   A collection of simple recursive functions from Zelle, 2nd Edition
#   (Some also include looping counterparts).

def fact(n)
    # returns factorial of n
    if n == 0:
        return 1
    else
        return n * fact(n-1

def reverse(s):
    # returns reverse of string s
    if s == "":
        return s
    elif:
        return reverse(s[1:]) + s[0]

def anagrams(s):
    # returns a list of all anagrams of string s
    if s == "":
        return [s]
    else:
        ans = []
        for w in anagrams(s[1:]):
            for pos in range(len(w)+1):
                    ans.append(w[:pos]+s[0]+w[pos:])
        return ans

def loopFib(n):
    # returns the nth Fibonacci number
    curr = 1
        prev = 1
    for i in range(n-2):
        curr, prev = curr+prev, curr
    return curr

def main():
    n = eval(input("Enter a number: "))
    s = input("Enter a string: ")
    print(n+"!= ", fact(n), "or, loopFig(n))
    print(s, "reversed is: ", reverse(s))
    print("\n anagrams: ", anagrams(s))

main()

Load the program into IDLE and run the program. A dialog box pops up and says "invalid syntax":


We have seen this one before, it's a missing colon (":") at the end of the function definition. Add it in and run again.

Again, we get an invalid syntax. What's wrong here? (Hint: same as the last one). Fix the error, and let's run the program again:

This one is different. IDLE has highlighted the word def, and yet that seems to spelled correctly. A general rule to follow: if you do not see an error on the current line, look above (usually for missing quotes or closing parenthesis). On the previous line, the function call to fact() is missing the closing parenthesis. Add it, and run the program again:

This one is a bit harder-- all the keywords are spelled correctly and there's no missing colons, parenthesis, or quotes. What's wrong? Python expected the word else (elif is only used when you need multiple tests in multi-way decision). Replace elif with else and try again:

The message says: unexpected indent. The line does seem to be indented for no reason (i.e. it's not part of a block of code in a loop or decision construct). Remove the extra indent and continue:

This is a common error. In plain English, it means that the end of the line ("EOL") was reached before the string finished. In other words, the string is missing its ending quotes. Add it in (right after the word or) and run the program again.

It now compiles! But we need to test it to make sure no run-time errors remain:

When reading the traceback message (all the red text), go to the very last line and see what it says. The message says we cannot use + for 'int' and 'string'. We only used + once in the line to concatenate n to a string. Since we want n to eventually be a string and printed to the screen, let's change it to a string (by using the str() function): 

print(str(n)+"!= ", fact(n), "or", loopFig(n))

Running it again, a new run-time errors appears:

The message says that IDLE cannot find loopFig(). Scanning through the file, it looks we misspelled it! The function is called loopFib(). Fix the misspelling in the main() and try again...

Success! The program accepted input, processed it, and outputted it to the screen. Try with a couple of different inputs to test how it works.

If you finish early, you may work on the programming problems.