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Until

See Also
- While
- Do
- For
- NestWhile
- TakeWhile
- LengthWhile
- Nest
- Fold
- Select
- Throw
Related Guides
- Looping Constructs
- Procedural Programming
Tech Notes
- Loops and Control Structures
- Repetitive Operations
- See Also
  - While
  - Do
  - For
  - NestWhile
  - TakeWhile
  - LengthWhile
  - Nest
  - Fold
  - Select
  - Throw
- Related Guides
  - Looping Constructs
  - Procedural Programming
- Tech Notes
  - Loops and Control Structures
  - Repetitive Operations

Until

Until[test,body]

evaluates body and then test, repetitively, until test first gives True.

Details

Until[test] evaluates test repeatedly, effectively with a null body.
If Break[] is generated in the evaluation of body, the Until loop exits.
Continue[] exits the evaluation of body and continues the loop.
Unless an explicit Return is used, the value returned by Until is Null.
Note that in an example like i=0;Until[i>10,tot+=f[i];i++], the roles of ; and , are reversed relative to C-like programming languages.

Examples

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Basic Examples (1)

Print and increment n until the condition n>3 is satisfied:

Wolfram Language code: n = 1;Until[n > 3, Print[n];n++]

Scope (2)

Break breaks out of the Until loop:

Wolfram Language code: n = 1;Until[False, If[n > 10, Break[]];n++]

Wolfram Language code: n

Evaluate and echo random integers from 0 to 100 until finding an integer divisible by 7:

Wolfram Language code: Until[Divisible[Echo[RandomInteger[100]], 7]]

Applications (2)

Compute the GCD of two numbers:

Wolfram Language code:

{a, b} = {27, 6};
Until[b == 0, {a, b} = {b, Mod[a, b]}];
a

Count the number of iterations in a fixed-point calculation:

Wolfram Language code:

x = 1.0;i = 0;
Until[Cos[x] == x, x = Cos[x];i++];{x, i}

Properties & Relations (3)

Until[test,body] is effectively equivalent to Until[body;test]:

Wolfram Language code: n = 1;Until[n > 3, Echo[n++]];n

Wolfram Language code: n = 1;Until[Echo[n++];n > 3];n

Until[test,body] evaluates body before test:

Wolfram Language code: Until[True, Print["body"]]

While[test,body] evaluates test before body:

Wolfram Language code: While[False, Print["body"]]

Until and While require opposite tests to reproduce similar behavior:

Wolfram Language code: i = 0;Until[i > 3, Print[i];i++]

Wolfram Language code: i = 0;While[Not[i > 3], Print[i];i++]

However, the test is evaluated one more time in the While loop:

Wolfram Language code: i = 0;Until[Print[i];i > 3, i++]

Wolfram Language code: i = 0;While[Print[i];Not[i > 3], i++]

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