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Stack and Undefined Behavior [TOC]
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In the lecture, an array was used to implement a simple stack that can store a
maximum of 5 integer values. You can find the program code (File: `stack.abc`)
at the end of the task.

Tasks
=====

- Change the instructions in `main` so that two push operations are executed
  first, followed by three pop operations. Confirm that the program compiles
  but encounters a runtime error during execution.
- Modify the instructions in `main` so that six `push` operations are executed
  first, followed by three `pop` operations. Again, confirm that the error is
  detected at runtime.
- Comment out the assertions in the functions `push` and `pop`, and repeat the
  previous two subtasks. Whether an error occurs or the program seems to
  execute correctly cannot be predicted. However, it is highly likely that an
  error will occur if you modify the instructions in `main` so that a pop
  operation is followed by a push operation.
- Remove the comments for the assertions in `push` and `pop`.


---- CODE(file=session05/page04/stack.abc, type=abc, linenumbers) --------------
@ <stdio.hdr>

global
    stack: array[5] of int,
    stackSize: int = 0;

fn push(val: int)
{
    assert(stackSize < sizeof(stack) / sizeof(stack[0]));
    stack[stackSize++] = val;
}

fn pop(): int
{
    assert(stackSize > 0);
    return stack[--stackSize];
}

fn main()
{
    push(3);
    push(4);
    pop();
}
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