Suppose I have a list of values

['small', 'medium', 'large', 'extralarge']

that I would like to map into the numbers [1,2,3,4]

with small corresponding to 1, medium corresponding to 2, large corresponding to 3, and extralarge corresponding to 4.

How can I efficiently do such a functional mapping for a new value x without needing to do something like

if x == "small":
    print(1)
elif x == "medium":
...

CodePudding user response：

Use a dictionary:

sizes = ['small', 'medium', 'large', 'extralarge']
mapping = {size: value for value, size in enumerate(sizes, 1)}

Then for some x,

print(mapping[x])

Which will print the value 1, 2, 3, or 4, based on the value of x being a some size like "small", "medium", etc...

CodePudding user response：

You can use enumerate() to bind each string to an index:

for idx, item in enumerate(data, start=1):
    if x == item:
        print(idx)

CodePudding user response：

You can iterate through them as @BrokenBenchmark suggests. However, you should consider whether a list is the most optimal data structure for your purpose; as @ddejohn suggests, perhaps a dictionary would suit better your purposes as its structure is literally a set of mapped key:value pairs.

CodePudding user response：

If you already have two lists then one way to create dictionary is to use zip:

names = ['small', 'medium', 'large', 'extralarge']
nums = [1,2,3,4]

mapping = dict(zip(names, nums))

If you don't have numbers list and need one number corresponding to the name at the time then you can take advantage of indices. Simple helper function:

names = ['small', 'medium', 'large', 'extralarge']

def get_value(name, list_):
    return list_.index(name)   1

print(get_value('medium', names))    # prints 2

Name lookup in similar fashion could look like:

def get_name(num, list_):
    return list_[num-1]

print(get_name(2, names))    # prints 'medium'

If you append new names to names list then you don't need to update other parts of your code.