Transforming and grouping collections

The idea

map transforms every element into a corresponding result. compactMap additionally discards nil transformation results, making it useful for parsing only valid values. reduce combines a sequence into one accumulated result starting from an explicit initial value. The chosen operation should communicate the required result shape rather than simply minimize the number of lines in the implementation.

Grouping and counting add another contract: which keys are present and in what order the results are displayed. A dictionary is useful storage, but sorting its keys explicitly is clearer when an alphabetical report is required. Preserve duplicates when they represent repeated observations. A count or total of zero should not automatically remove a group if the requirement distinguishes observed groups from missing groups.

let texts=["4","bad","2"]
let values=texts.compactMap{Int($0)}
print(values)
print(values.reduce(0,+))

[4, 2]
6

The invalid parse yields nil and is discarded; reduction sums the two successful values from a zero starting point.