Unless and Until in programming languages

Question

What is the difference between

if (false == <condition>)

and

if (!<condition>)

and

unless (<condition>)

What is the difference between

while (false == <condition>)

and

while (!<condition>)

and

until (<condition>)

Why isn't unless and until in all programming languages?

Thank you for your help.

Because for at least one programming language the language designers chose to not include it. — mkrieger1, Sep 29 '22 at 18:35
What these examples mean depends on the particular programming language you are referring to. — mkrieger1, Sep 29 '22 at 18:36
@mkrieger1: So, for example, Perl, but Perl does not have a boolean data type, and I can't think of another language. Please try to name one. :) — user.regular, Sep 29 '22 at 18:41
@mkrieger1: So what is the difference between them in Python? — user.regular, Sep 29 '22 at 18:43
See e.g. https://stackoverflow.com/questions/56257554/difference-between-if-not-and-if-false. But Python doesn't have unless and until. — mkrieger1, Sep 29 '22 at 18:53
@mkrieger1: And is there any other language besides Perl that has unless and until? — user.regular, Sep 29 '22 at 18:56

score 0 · Accepted Answer · answered Sep 29 '22 at 18:44

if (false == <condition>)

and

if (!<condition>)

and

unless (<condition>)

Logically, they are all equivalent.

Likewise:

while (false == <condition>)

and

while (!<condition>)

and

until (<condition>)

Logically they are equivalent. Except syntactically, until() might be used at the end of a loop and while() at the beginning of a loop.

Although logically they are equivalent, some programming languages provide them (unless, until) for readability.

And you might want to take a look of this to trace back their first usage in programming language.

1 Answers1