nanotech example - v4.software-carpentry.org · sets and dictionaries nanotech example. how many...

Nanotech Example

Sets and Dictionaries

Nanotech Example

Copyright © Software Carpentry 2010

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.

How many molecules of different kinds can we

make using the atoms in our warehouse?

Sets and Dictionaries Nanotech Example



# Molecular formula file

helium : He 1

water : H 2 O 1

hydrogen : H 2





helium : He 1

water : H 2 O 1

hydrogen : H 2

# Atom inventory file

He 1

H 4

O 3





helium : He 1

water : H 2 O 1

hydrogen : H 2

# Atom inventory file

He 1

H 4

O 3

Now have all the tools we need


Now have all the tools we need

Natural to represent inventory as dictionary



Keys: atomic symbols




Values: number of atoms availableValues: number of atoms available




Values: number of atoms availableValues: number of atoms available

'He'

'H'1


'O'

3

4

Represent individual molecules the same way


Represent individual molecules the same way

water

'O'

'H'

2

water


1

Store formulas as a dictionary of dictionaries



Keys: molecule names




Values: dictionaries of formulasValues: dictionaries of formulas




Values: dictionaries of formulasValues: dictionaries of formulas

'water'

'ammonia'


Number of molecules that can be made is:

minavailable[atom]

required[atom]min required[atom]

atom ∈ formula



minavailable[atom]


atom ∈ formula

If atom not in available, its count is implicitly 0



minavailable[atom]


atom ∈ formula


Store results in yet another dictionary



minavailable[atom]


atom ∈ formula







minavailable[atom]


atom ∈ formula






Values: counts of how many can be made

'''Calculate how many molecules of each type can be made

with the atoms on hand.'''

importimportimportimport sys

ifififif __name__ == '__main__':ifififif __name__ == '__main__':

inventory = read_inventory(sys.argv[1])

formulas = read_formulas(sys.argv[2])

counts = calculate_counts(inventory, formulas)

show_counts(counts)


defdefdefdef read_inventory(filename):

'''Read inventory of available atoms.'''

result = {}

forforforfor line inininin read_lines(filename):

name, count = line.split(' ')name, count = line.split(' ')

result[name] = intintintint(count)

returnreturnreturnreturn result


defdefdefdef read_lines(filename):

'''Read lines from file, stripping out

blank lines and comments.'''

reader = openopenopenopen(filename, 'r')

lines = []lines = []

forforforfor line inininin reader:

line = line.split('#')[0].strip()

ifififif line:

lines.append(line)

reader.close()

returnreturnreturnreturn lines


returnreturnreturnreturn lines

defdefdefdef read_formulas(filename):

'''Read molecular formulas from file.'''

result = {}


name, atoms = line.split(':')

name = name.strip()

atoms = atoms.strip().split(' ')

formula = {}

forforforfor i inininin rangerangerangerange(0, lenlenlenlen(atoms), 2):

formula[atoms[i]] = int(atoms[i+1])



result[name] = formula




result = {}


Storing results

in dictionary


name = name.strip()


formula = {}









result = {}


For each

interesting


name = name.strip()


formula = {}



line in the

input file...







result = {}

forforforfor line inininin read_lines(filename):Separate the


name = name.strip()


formula = {}



Separate the

molecule name

and the formula







result = {}

forforforfor line inininin read_lines(filename):Separate the


name = name.strip()


formula = {}



Separate the

atoms and

their counts







result = {}



name = name.strip()


formula = {}



Loop over pairs

of atoms and

counts







result = {}



name = name.strip()


formula = {}



Store the count





as an integer

with the atomic

symbol as key



result = {}



name = name.strip()


formula = {}



And store the





molecule in

the main

dictionary

defdefdefdef calculate_counts(inventory, formulas):

'''Calculate how many of each molecule

can be made with inventory.'''

counts = {}

forforforfor name inininin formulas:forforforfor name inininin formulas:

counts[name] = dict_divide(inventory,

formulas[name])

returnreturnreturnreturn counts





counts = {}



formulas[name])

returnreturnreturnreturn countsSub-dictionary holding

atom counts for a


particular molecule




counts = {}



formulas[name])


Big functions: nothing is obviously wrong





counts = {}



formulas[name])


Big functions: nothing is obviously wrong


Small functions: obviously, nothing is wrong

defdefdefdef dict_divide(inventory, molecule):

'''Calculate how much of a single molecule


number = NoneNoneNoneNone

forforforfor atom inininin molecule:forforforfor atom inininin molecule:

required = molecule[atom]

available = inventory.get(atom, 0)

limit = available / required

ifififif (number isisisis None) orororor (limit < number):

number = limit

returnreturnreturnreturn number







forforforfor atom inininin molecule:

Identical format:

keys are atoms,forforforfor atom inininin molecule:





number = limit


keys are atoms,

values are counts












number = limit


Common pattern: None


returnreturnreturnreturn numbermeans "uninitialized",

so initialize the first

time through the loop










number = limit


Common pattern:


returnreturnreturnreturn numberstored value or default










number = limit

returnreturnreturnreturn numberCommon pattern:



find minimum of

calculated values

defdefdefdef show_counts(counts):

'''Show how many of each molecule can be made.'''

counts = invert_dict(counts)

forforforfor key inininin sortedsortedsortedsorted(counts.keys(), reverse=TrueTrueTrueTrue):

forforforfor name inininin sortedsortedsortedsorted(counts[key]):forforforfor name inininin sortedsortedsortedsorted(counts[key]):

printprintprintprint key, name






forforforfor name inininin sortedsortedsortedsorted(counts[key]):forforforfor name inininin sortedsortedsortedsorted(counts[key]):


Reverse to get


greatest first

defdefdefdef invert_dict(src):

'''Invert a dictionary, returning value->set{key}.'''

dst = {}

forforforfor key inininin src:

value = src[key]value = src[key]

ifififif value notnotnotnot inininin dst:

dst[value] = set()set()set()set()

dst[value].add(key)

returnreturnreturnreturn dst


defdefdefdef invert_dict(src):

'''Invert a dictionary, returning value->set{key}.'''

dst = {}

forforforfor key inininin src:

value = src[key]value = src[key]

ifififif value notnotnotnot inininin dst:

dst[value] = set()set()set()set()

dst[value].add(key)

returnreturnreturnreturn dst

Common pattern:


make sure there's a

collection, then add





ifififif key > 0:ifififif key > 0:

forforforfor name inininin sortedsortedsortedsorted(counts[key]):


Go back and only


show molecules that

can actually be made

# inventory-00.txt # formulas-01.txt

helium : He 1

No output, which is correct.


# inventory-01.txt

He 1

# formulas-01.txt

helium : He 1

1 helium


# inventory-02.txt

He 1

H 4

# formulas-01.txt

helium : He 1

1 helium


# inventory-02.txt

He 1

H 4

# formulas-02.txt

helium : He 1

water : H 2 O 1

1 helium


# inventory-02.txt

He 1

H 4

# formulas-03.txt

helium : He 1

water : H 2 O 1

hydrogen : H 2

2 hydrogen

1 helium

hydrogen : H 2


# inventory-03.txt

He 1

H 4

O 3

# formulas-03.txt

helium : He 1

water : H 2 O 1

hydrogen : H 2

2 hydrogen

2 water

1 helium

hydrogen : H 2


# inventory-03.txt

He 1

H 4

O 3

# formulas-03.txt

helium : He 1

water : H 2 O 1

hydrogen : H 2

2 hydrogen

2 water

1 helium

hydrogen : H 2


Looks good...

# inventory-03.txt

He 1

H 4

O 3

# formulas-03.txt

helium : He 1

water : H 2 O 1

hydrogen : H 2

2 hydrogen

2 water

1 helium

hydrogen : H 2


Looks good...

Code is much simpler than it would be using

lists of pairs

June 2010

created by

Greg Wilson

June 2010

Copyright © Software Carpentry 2010

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.

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