What must be true to use binary search?

To effectively use binary search, the list must be sorted. This is fundamental because binary search works by repeatedly dividing the search interval in half. When you start with a sorted list, you can make informed decisions about which half of the list to continue searching in based on the comparison between the target value and the middle element of the current search interval. If the middle element is greater than the target, the search can be confined to the left half, and if it's less, the right half can be explored. This method relies entirely on the ordering of the elements, which is why a sorted list is an absolute requirement.

Without a sorted list, the assumptions about the relative positioning of the elements break down, rendering the binary search algorithm ineffective and potentially leading to incorrect results. Therefore, ensuring that the list is sorted is crucial before implementing a binary search.