Loops and iterations

Envision relies first and foremost on its relational algebra features to iterate over large collections of data: every operation in Envision is applied to all the lines of a table. Such vectorized operations are well-optimized both in terms of speed and memory usage, and should be used whenever possible.

For cases when a computation cannot be expressed using the vectorized operations alone, Envision provides several advanced iteration features:

• for blocks are the standard way of iterating over all lines of a table, and executing a block of code for every line. Not all operations are allowed inside the block.
• each blocks are a more complex alternative to for blocks that can take advantage of the relations between tables. Not all operations are allowed inside the block.
• loop blocks have little to no limitations on what operations are allowed inside the block, but are limited to 10 iterations.

The design of Envision enforces a constraint on all forms of iteration: any loop, no matter how complex, must have a maximum number of iterations. This excludes loops that run until a condition becomes false (the while loops of many languages).

Other language features, such as montercarlo and autodiff, also rely on iteration to execute their body several times.