Home/ Questions/Q 8058595
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-05T09:24:29+00:00

I’m working on assignment 2 of CS193P – Stanford’s IOS programming course. One thing

  • 0

I’m working on assignment 2 of CS193P – Stanford’s IOS programming course. One thing I was wondering about is how the calculatorBrain is supposed to be able to accept and run a stored program, a program being an array or stack of operands and operations.

So let’s say we want to perform the following calculation: 2, 3, 4, +, *

If you typed this into the calulator, the following would happen:

2 3 4 get pushed onto the stack one at a time, and runProgram called for each one, which simply pops the number off the stack and returns it’s value which gets pushed onto the stack.
You press +, and runProgram pops this and sees it has to add the top 2 items which it does and pushes the result onto the stack which now contains 2, 7. You press * and the stack now contains 14.

But I can’t see how you can pass an array containing (2, 3, 4, +, *) to the brain (the instructor says later you can just pass a program to the runProgram class method and get the result, without having to instantiate a brain object), as runProgram would first try to execute the top operand i.e. * and to do this it would take the next two objects off the stack and try to multiply them and to push the result back onto the stack. These 2 objects are “+” and “4” which won’t work.

Now the instructor has been doing this a lot longer than I have, as I assume that I’m missing somethings, but I’m not sure what.

Any ideas?

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    OK – the answer is that the way runProgram gets the next item off the stack is by recursively calling a popOperandOffStack (pOOS) method.

    So when it gets passed a program consisting of 2 3 4 + *, it starts by popping * off the stack. It then has to pop the next two operands off the stack. So it calls pOOS, which firstly returns a ‘+’, so it then calls pOOS again (twice) which gets ‘4’ & ‘3’ respectively, which are added to get 7 which is pushed back onto the stack (which now contains 2 7) and also returned as the result of calling pOOS. So when it called pOOS for the first operand of the ‘*’ operation it didn’t get ‘+’, it actually got ‘7’. The second call to pOOS (for the second operand of the *) gets ‘2’ which it then happily multiplies to get 14.

    I did try looking up recursion in my IT dictionary, but it just said ‘See recursion’.

    • 0