# A While loop where the loop clause can receive problematic input

Consider the following highly artificial example:

list = {{1, 2, 3, 4}};
While[Length[list[[1]]] > 3,
If[RandomReal[{0, 1}] < 0.2,
list = {};
,
list = {{1, 2, 3}, {}};
];
];


Some 20% of the time here, you'll see the error:

Part::partw: Part 1 of {} does not exist. >>


How can we avoid this error if we are only allowed to modify the clause in the While loop and must have the Length[list[[1]]] > 3 query somewhere in the clause statement? Can we have an incident where list = {} cause a termination of the loop or ask for this to satisfy the loop clause?

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Perhaps

list = {{1, 2, 3, 4}};
While[list =!= {} && Length[list[[1]]] > 3,
If[RandomReal[{0, 1}] < 0.2, list = {};, list = {{1, 2, 3}, {}};];];

list
(* {{1,2,3},{}} *)


or

ClearAll[list];
list = {{1, 2, 3, 4}};
While[Length[list[[1]]] > 3,
If[RandomReal[{0, 1}] < 0.2, Break[], list = {{1, 2, 3}, {}};];];

list
(* {{1,2,3},{}} *)


or

ClearAll[list];
list = {{1, 2, 3, 4}};
Catch[While[Length[list[[1]]] > 3,
If[RandomReal[{0, 1}] < 0.2, Throw[{}];, list = {{1, 2, 3}, {}};];]];

list
(* {{1,2,3},{}} *)