15. LAMBDA, MAP, REDUCE, BYROW

LAMBDA(parameter, calculation) is your own small function right inside a formula. By itself it calculates nothing; you pass it to the helper functions:

• MAP(array, LAMBDA(x, …)) applies it to every element and returns an array;
• REDUCE(start, array, LAMBDA(acc, x, …)) folds an array into a single value;
• SCAN is like REDUCE but shows all the intermediate totals (a running total);
• BYROW(array, LAMBDA(row, …)) returns one value per row.

In Excel 365 you can save a LAMBDA in the Name Manager and call it like a normal function: =SalesTax(A2). The practice grid here supports LAMBDA inside formulas and the helper functions.