Question

 
I had a discussion with a friend about cutting vegetables on the Sabbath. My friend argued that it is forbidden to cut vegetables on Friday night after the Friday night meal if the intention is to eat them during the daytime Sabbath meal. I, on the other hand, think that this is permitted, as long as it does not involve prohibited labors such as “selecting” or “grinding.”
For example: if I cut a cucumber into large pieces on Friday night in order to eat it at the Sabbath meal, then as I understand it there is no prohibition of selecting or grinding here.
I would be glad to know: who is right in this case—me or my friend?
In addition, I have a question about cutting watermelon:
Is cutting watermelon on the Sabbath considered “selecting,” and therefore does it require eating it immediately?
Is it permitted to cut watermelon on Friday night and leave it to be eaten on the Sabbath day?
There may be a difference between different ways of cutting the watermelon:

• If you cut it into triangles with the rind on, I do not see a problem of “selecting” here. Am I right?
• On the other hand, with cutting it into cubes without the rind, I am unsure. On the one hand, maybe some flesh will remain attached to the rind, in which case perhaps it would not be selecting—or perhaps because I made a separation, this would indeed be selecting.