The document describes how to implement the counting song 'Ten Green Bottles' in Python, covering key programming concepts such as sequences, loops, conditionals, functions, and data structures. It provides step-by-step guidance on coding the song, including handling pluralization and string formatting. By using Python features effectively, the document illustrates how to create a functional program that implements this popular children's song.

1. Coding with Counting Songs:
“Ten Green Bottles”
in Python
Steve Battle

2. counting songs
•A repeated verse, each time with one bottle fewer:
Ten green bottles hanging on the wall
Ten green bottles hanging on the wall
And if one green bottle should accidentally fall,
There'll be nine green bottles hanging on the wall.
•Continue until the number of bottles reaches zero:
One green bottle hanging on the wall,
One green bottle hanging on the wall,
And if that green bottle should accidentally fall,
There'll be no more bottles hanging on the wall.
•See https://en.wikipedia.org/wiki/Ten_Green_Bottles

3. IDLE* and the REPL**
Start IDLE, the Python Development Environment.
Type:
>>> print(“Ten green bottles”)
* Integrated DeveLopment Environment
** Read, Evaluate, Print, Loop

4. programs and strings
•In IDLE, create a new file for our code:
File > New File
•Input the following code:
print(“Ten green bottles, hanging on the wall”)
•Always save your file before running it:
File > Save (Save As: bottles.py)
•Now Run your code:
Run > Run Module (F5)

5. sequence
A program can have a sequence of statements
that are executed in order:
print("Ten green bottles, hanging on the wall")
print("Ten green bottles, hanging on the wall")
The output is simply:
Ten green bottles, hanging on the wall
Ten green bottles, hanging on the wall

6. variables
A variable is like a named box you can put a value into
and use later:
bottles = 10
print(bottles," green bottles, hanging on the wall")
print(bottles," green bottles, hanging on the wall”)
If you’re using Python 2.7 you may have to use:
print str(bottles)+” green bottles, hanging on the wall”
• Variables in Python are untyped.

7. expressions
• Math expressions use the operators:
+ (plus), - (minus), * (times), / (divide), % (quotient)
• Add the following code to the end of your program:
print("And if one green bottle should accidentally fall,")
print("There'll be”, bottles-1,
"green bottles, hanging on the wall.”)
• The multiplicative operators (* / %)
have higher precedence and are evaluated before
the additive operators (+ -)
• Use brackets () to change the order of evaluation

8. loops
We want to repeat the verse 10 times
• The ‘for’ loop defines a loop variable
and loops (iterates) over a range of values
Enter the following in the REPL, what happens?
>>> for i in range(10):
print(i)
>>> for i in range(1,10):
print(i)
Modify your program, indenting the body of the loop:
for bottles in range(10,0,-1):
print(bottles,"green bottles, hanging on the wall,")
print(bottles,"green bottles, hanging on the wall,")
print("And if one green bottle should accidentally fall..")
print("There'll be”, bottles-1,
"green bottles, hanging on the wall.n")

9. functions
Functions let us isolate reusable code,
like the “green bottles” statement.
def greenBottles(b):
print(b,"green bottles, hanging on the wall,")
for bottles in range(10,0,-1):
greenBottles(bottles)
greenBottles(bottles)
print("And if one green bottle should accidentally fall..")
print("There'll be”, bottles-1,"green bottles,
hanging on the wall.n")
• The scope of ‘b’ is limited to the function

10. return values
Proper functions return a value. This lets us reuse
the function on the last line of the verse.
• In this case it returns a string value that is printed
def greenBottles(b):
return str(b)+" green bottles, hanging on the wall"
for bottles in range(10,0,-1):
print(greenBottles(bottles))
print(greenBottles(bottles))
print("And if one green bottle should accidentally fall..")
print("There'll be ", greenBottles(bottles-1))
print()

11. conditionals
How do we handle variation in the verse such as the plurality of
the bottles?
• An ‘if’ statement lets the code select alternatives depending
upon some condition.
• The condition here is b==1 (b ‘equals’ 1)
def greenBottles(b):
if b==1:
return str(b)+" green bottle, hanging on the wall"
else:
return str(b)+" green bottles, hanging on the wall”
And modify the “There’ll be” line to:
print("There'll be ", greenBottles(bottles-1))

12. conditional expressions
What about “one green bottle” changing to “that green
bottle” in the last verse?
• Conditional expressions return a value and are
sometimes a more compact alternative.
for bottles in range(10,0,-1):
print(greenBottles(bottles))
print(greenBottles(bottles))
print("And if " + ("that" if bottles==1 else "one")
+ " green bottle should accidentally fall.”)
print("There'll be ", greenBottles(bottles-1)
print()

13. data structures
We want to use written numerals, “ten”, “nine”,…
• Tuples are sequences of symbols.
numerals = ('no','one','two','three','four','five',
'six','seven','eight','nine','ten')
def bottle(b):
if b==1:
return numerals[b] + " green bottle, hanging on the wall"
else:
return numerals[b] + " green bottles, hanging on the wall"
• Like strings, tuples are immutable.

14. slicing
Finally, we need to properly capitalise every line of the verse.
• We need to be able to slice up a string, converting only the first letter to upper-case.
At the REPL type:
>>> "one".upper()
>>> “one”[0:1]
>>> “one"[0:1].upper()
>>> "one"[0:1].upper() + "one"[1:]
Add the following function:
def caps(s):
return s[:1].upper() + s[1:]
And call the function in the following two lines:
print(caps(greenBottles(bottles)))
print(caps(greenBottles(bottles)))
•Slicing works on all types of sequence.

15. Ten Green Bottles
# Ten Green Bottles
numerals = ('no','one','two','three','four','five','six','seven','eight','nine','ten')
def greenBottles(b):
if b==1:
return numerals[b] + " green bottle, hanging on the wall"
else:
return numerals[b] + " green bottles, hanging on the wall"
def caps(s):
return s[:1].upper() + s[1:]
for bottles in range(10,0,-1):
print(caps(greenBottles(bottles)))
print(caps(greenBottles(bottles)))
print("And if " + ("that" if bottles==1 else "one") + " green bottle
should accidentally fall,")
print("There'll be "+greenBottles(bottles-1)+".")
print()

16. conclusion
While we’ve covered a range of Python features,
the three main features of programs are:
• Sequence
• Loops
• Conditionals
With these, we can write programs that can do
anything (computable).