while.

while. T do. B end.
whilst. T do. B end.

The last sentence executed in the T block is tested for a non-zero value in its leading atom. If true, the B block is executed. The T block is then retested, and so on, continuing until the T block tests false. (An empty T block result or an omitted T block tests true.)

whilst. differs from while. only in that it skips test the first time.

break. goes to the end of the while. or whilst. control structure andcontinue. goes to the top.

exp =: 4 : 0 Exponentiation by repeated squaring
 z=.1
 a=.x
 n=.y
 while. n do.
 if. 2|n do. z=.z*a end.
 a=.*:a
 n=.<.-:n
 end.
 z
)
 3 exp 7
2187
 3 ^ 7
2187
 1.1 exp 0
1
 2x exp 128
340282366920938463463374607431768211456