Difference between “precondition” and “assert” in swift?

assert is for sanity checks during testing, whereas precondition is for guarding against things that, if they happen, would mean your program just could not reasonably proceed.

So for example, you might put an assert on some calculation having sensible results (within some bounds, say), to quickly find if you have a bug. But you wouldn’t want to ship with that, since the out-of-bound result might be valid, and not critical so shouldn’t crash your app (suppose you were just using it to display progress in a progress bar).

On the other hand, checking that a subscript on an array is valid when fetching an element is a precondition. There is no reasonable next action for the array object to take when asked for an invalid subscript, since it must return a non-optional value.

Full text from the docs (try option-clicking assert and precondition in Xcode):

Precondition

Check a necessary condition for making forward progress.

Use this function to detect conditions that must prevent the
program from proceeding even in shipping code.

• In playgrounds and -Onone builds (the default for Xcode’s Debug
  configuration): if condition evaluates to false, stop program
  execution in a debuggable state after printing message.

• In -O builds (the default for Xcode’s Release configuration):
  if condition evaluates to false, stop program execution.

• In -Ounchecked builds, condition is not evaluated, but the
  optimizer may assume that it would evaluate to true. Failure
  to satisfy that assumption in -Ounchecked builds is a serious
  programming error.

Assert

Traditional C-style assert with an optional message.

Use this function for internal sanity checks that are active
during testing but do not impact performance of shipping code.
To check for invalid usage in Release builds; see precondition.

• In playgrounds and -Onone builds (the default for Xcode’s Debug
  configuration): if condition evaluates to false, stop program
  execution in a debuggable state after printing message.

• In -O builds (the default for Xcode’s Release configuration),
  condition is not evaluated, and there are no effects.

• In -Ounchecked builds, condition is not evaluated, but the
  optimizer may assume that it would evaluate to true. Failure
  to satisfy that assumption in -Ounchecked builds is a serious
  programming error.