Mathematica Tutor

Part 5: What Mathematica remembers

1. Create a new cell and type
   x=15
   Then press the RETURN key. This gives you a new line within the cell and does not evaluate the current line. Now type
   7.3/2.5
   Then press SHIFT-RETURN to evaluate both lines.
   
   
2. Go back into the cell, delete the line
   x=15
   and press SHIFT-RETURN. In a new cell, enter
   x
   Notice that Mathematica still remembers that x has been assigned the value 15 -- even though that line no longer appears in the worksheet.

We have come to the second important fact about Mathematica. (The difference between RETURN and SHIFT-RETURN was the first.) Mathematica remembers what has been entered in the order it was entered.

• It doesn't matter what is currently in the worksheet, nor the order things appear in that worksheet. You may have assigned a value, erased the assigning command line, and forgotten about it. But Mathematica remembers. If you use x later, assuming it is an unassigned variable, strange things will happen.
  
  
• If variables are assigned new values, formulas using those variables acquire new values as well.

Finally a third important fact about Mathematica: If you save a Mathematica file without removing the output, then reload it later, it will look the same as when you saved it. But Mathematica will not remember any of the displayed commands. All of those commands must be reentered before you are again in the same state as when you saved and closed the file.