How to make a double introduction to the string

Question

For example, on a calculator how to make it so that in one line we could enter 3 values, i.e.

5 + 5

but not

 Введите первое число Введите логическое действие +,-,* Введите второе число

And what do you see the restriction on entering n values instead of one?
You accept the entire line, and then break it into separate tokens by spaces.
@ Alexander, if in one line, you can not break at all, but just do eval )
@insolor, I also thought about eval, but decided that it was a bad idea to advise his newbie.

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