Filtering and Transforming

Map Transform

Apply a formula to every element of a list, collecting the results in a new list. Here, Celsius readings are converted to Fahrenheit. The replay shows the intermediate result f computed on each iteration before it is appended.

By hand

The Pythonic way

A list comprehension [c * 9 / 5 + 32 for c in celsius] applies the formula to every element in one expression, with no accumulator or append call.

naive.py 
celsius = [0, 20, 37, 100, -10, 25]
fahrenheit = []
for c in celsius:
    f = c * 9 / 5 + 32
    fahrenheit.append(f)
print('RESULT:', fahrenheit)
library.py 
celsius = [0, 20, 37, 100, -10, 25]
fahrenheit = [c * 9 / 5 + 32 for c in celsius]
print('RESULT:', fahrenheit)

RESULT: [32.0, 68.0, 98.6, 212.0, 14.0, 77.0]

Implementation notes

  • The chosen values all produce clean decimal results; no rounding is needed at the print boundary.
  • list(map(lambda c: c * 9 / 5 + 32, celsius)) is an equivalent functional form; the list comprehension is preferred in modern Python for readability.
  • The comprehension runs in its own scope, so neither c nor the intermediate result appears as a traced variable in the library half.