How to check a solved Sudoku puzzle

When you solve a Sudoku puzzle, you should *always* verify your solution is correct.  That sounds simple, right?  You would proofread an email before hitting the send button, but . . . how exactly do you verify a Sudoku puzzle?

There are many Sudoku solver websites-- so you could, in theory, enter the puzzle manually into the site, have it solve the puzzle and then compare your solution with the solution generated by the Sudoku solver site.  But honestly, it's such a simple process that using a solver website is overkill.  If you can count to nine eighteen times in a row, you can check a Sudoku puzzle.

1. Focusing on the first row alone, find and count the numbers from 1 to 9.  If you are missing any numbers, you have an error (either a blank or a duplicated number) in this row.  You should mark errors for future reference to be corrected.
2. Repeat the process used in step 1 for all the remaining rows.
3. Focusing on the first column alone, find and count the numbers from 1 to 9.  If you are missing any numbers, you have an error in this row.
4. Repeat step 3 for the remainder of the columns.
5. If you have checked all the columns and rows for the numbers 1 through 9, and no numbers are missing, then your solution is valid.

Note: You must check both the rows AND the columns.  It is possible for all rows to be correct, and still have a problem in one or more columns-- or vice versa.