Function reduce() and list of maps

The reduce() function loops on the list of lists that you've provided and the fourth list is one item longer than the others (i.e. the value 29). The function can only work correctly on lists that are the same length. To make it work you'll need to add an additional value 0 to all of the lists that are longer thus:

a!localVariables(
  local!sensor: {
    a!map(data: { 125, 92, 22, 63, 73, 28, 19, 0 }),
    a!map(data: { 35, 65, 92, 22, 63, 73, 28, 0 }),
    a!map(data: { 2, 35, 65, 22, 63, 73, 18, 29 }),
    a!map(data: { 9, 35, 65, 63, 35, 28, 19, 0 })
  },
  reduce(fn!sum, 0, local!sensor.data)
)

Alternatively, you can flatten the list and then simply use fn!sum() thus:

a!localVariables(
  local!sensor: {
    a!map(data: { 125, 92, 22, 63, 73, 28, 19 }),
    a!map(data: { 35, 65, 92, 22, 63, 73, 28 }),
    a!map(data: { 2, 35, 65, 22, 63, 73, 18, 29 }),
    a!map(data: { 9, 35, 65, 63, 35, 28, 19 })
  },
  fn!sum(
    a!flatten(local!sensor.data)
  )
)

Yeah I can't really think of a scenario where you would use reduce with the sum() function. Stewart is correct, but for sum you don't even need to flatten it - just sum(local!sensor.data) will work too!

Yeah I can't really think of a scenario where you would use reduce with the sum() function. Stewart is correct, but for sum you don't even need to flatten it - just sum(local!sensor.data) will work too!

Thanks to all! 

Yes sure, real case doesn't need reduce to perform this operation... but it is an Appian Online Course Exercise (VOF_AdvExpP2_L2_Ex1_Answer)

It was jut to understand the behavior of reduce() in this particular case.