Coding with Counting Songs: “Ten Green Bottles”

in PythonSteve Battle

counting songs• A repeated verse, each time with one bottle fewer:

Ten green bottles hanging on the wallTen green bottles hanging on the wallAnd 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

IDLE* and the REPL**Start IDLE, the Python Development Environment.Type:>>> print(“Ten green bottles”)

* Integrated DeveLopment Environment** Read, Evaluate, Print, Loop

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)

sequenceA 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 wallTen green bottles, hanging on the wall

variablesA variable is like a named box you can put a value into and use later:

bottles = 10print(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.

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

loopsWe 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")

functionsFunctions 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

return valuesProper 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()

conditionalsHow 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))

conditional expressionsWhat 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()

data structuresWe 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.

slicingFinally, 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.

Ten Green Bottles# Ten Green Bottlesnumerals = ('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()

conclusionWhile 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).