Grading is the assignment of an ordinal number to each item of an array, with all numbers distinct, and with the earliest of equal values being assigned a lower ordinal number. Grading can be used to produce the permutation that will put an array in order. Since sorting is a primitive operation in J, grading is more commonly used to put a companion array into an order derived from another array. For example, a table of addresses may be put into an order derived from a table of last names.
m0=: /: | Grade y up |
m1=: \: | Grade y down |
d2=: /:@i. | Grade y up according to key x |
d3=: \:@i. | Grade y down according to key x |
y=:'leap','note',:'file' (] ; m0 ; (m0 { ]) ; m1 ; (m1 { ])) y +--------------------------+ ¦leap¦2 0 1¦file¦1 0 2¦note¦ ¦note¦ ¦leap¦ ¦leap¦ ¦file¦ ¦note¦ ¦file¦ +--------------------------+ x=: ' abcdefghijklmnopqrstuvwxyz' [ y=: 'senator' x (d2 ; (d2{]) ; d3 ; (d3{])) y +-------------------------------------------+ ¦3 1 2 5 6 0 4¦aenorst¦4 0 6 5 2 1 3¦tsronea¦ +-------------------------------------------+