Question

I have a function that takes a list of floats and returns the same list of floats except they have exactly 4 digits after the decimal, as strings

Currently it is simply

for i in range(len(floats)):
   floats[i] = "%.4f" %floats[i]

Which does the job.

But can I do this using the map function (since http://wiki.python.org/moin/PythonSpeed/PerformanceTips says using map is faster)

I call this function several thousand times and from the profile results it is one of the functions that takes up more time.

Answer 1

map is only faster if you use built-in functions (or at least it was when I read about it once ;)). You could use list comprehension though:

pattern = "%.4f"
floats = [pattern % i for i in floats]

Note that map and list comprehension will create a new list, whereas the for loop won’t. This might be relevant.

Depending on the rest of your application, if possible, you should format the numbers when you add them to the list.