# Expressions combining variables

For an example of an Expression combining two (or more) variables, use the <ins>Clyde environmental</ins> sheet but copy it (**Tools>Duplicate**), which is always a good idea when experimenting! The aim is to create a new variable (column) which is the C:N ratio, so first **Edit>Insert>Column**, which will be placed to the left of the current cursor position – here this might logically be on the %C column. \[Incidentally, remember that the new **Edit>Undo** will step back any **Edit** operations like this which change the current data matrix, rather than creating a new sheet (where a new sheet is created it can always be deleted if incorrect, and the process repeated). And note that **Edit>Undo** is local to the currently active matrix, so will undo the last such operation on <u>this</u> sheet, irrespective of whether similar operations have been performed since, on other sheets in the workspace – they will not be wound back by **Edit>Undo** if their sheet is not the active one\]. The new column is labelled (V10) because of its position in the matrix, and the calculation we do to create the C:N ratio is to be put in this column, so it needs to be highlighted. Take **Pre-treatment>Transform(individual)** and delete the V from the Expression box (that refers only to values in the new column, which we shall not be using – they are all zero of course). Then, under Pick, take (Type•Variable) & (Item: <ins>%C</ins>)>**Pick**) and follow by (Type•Variable) & (Item: <ins>%N</ins>)>**Pick**), which creates the two variables we need in the Expression box. Manually insert the divide symbol (/) between them, to give the (Expression:<ins> VAR(“%C”)/VAR(“%N”)</ins>), and **OK** now gives a new sheet with the added C:N ratio variable V10. (Even if you chose to tick the (✓Rename variables) box, the new name will still be clumsy and it would be better to change it to <ins>C:N</ins> using **Edit>Labels>Variables)**. 

[![ScreenshotPage218a.png](https://learninghub.primer-e.com/uploads/images/gallery/2024-09/scaled-1680-/screenshotpage218a.png)](https://learninghub.primer-e.com/uploads/images/gallery/2024-09/screenshotpage218a.png)

An alternative, e.g. if you just intend to take this new variable back into the earlier transformed sheet, is not to insert a new blank column, instead just highlighting the <ins>%C</ins> column (which will now be V in the Expression box), and **Pre-treatment>Transform(individual)** with (Expression: <ins>V/VAR(“%N”)</ins>). In the new sheet, this will have overwritten the <ins>%C</ins> row with the C:N ratio. Either way, you can now put the new C:N variable back into the transformed sheet simply by highlighting it, then **Select>Highlighted** and **Tools>Merge** it with the transformed sheet, taking the defaults.