Let’s start by considering one number from the list at a time.

Base Case

When we have received just one number, this number is both the maximum and the minimum. In the initial state, you have max holding the maximum, and min holding the minimum. The invariant is the following:

1. The variable max holds the maximum of all the numbers considered so far
2. The variable min holds the minimum of all the numbers considered so far

The algorithm is depicted by the following flowchart and pseudocode:

print "Enter a Number:"
input X
MAX = X
MIN = X

The Next Number

Then, our program will enter a loop to read 10 more numbers from the user. So, we’ll need to perform the following process during each iteration of the loop:

1. compare the value in max with this new number and update the max value if the new number is greater. In this way, the invariant is preserved.
2. compare the value in min with this new number and update the min value if the new number is smaller. In this way, the invariant is preserved.

This part of the program is depicted by the following flowchart and pseudocode:

loop I from 1 to 10
    print "Enter a Number:"
    input X
    if X > MAX
        MAX = X
    end if
    if X < MIN
        MIN = X
    end if
end loop

After you’ve considered the second number, you end up in the same situation at the beginning: you have a maximum and a minimum value of the numbers input by the user so far. You have found an invariant if you verify that the preconditions before executing an iteration of the loops are the same as the conditions at the end of the loop, known as postconditions:

• Loop preconditions:
  1. The variable max holds the maximum value among all the numbers considered so far
  2. The variable min holds the minimum value among all the numbers considered so far
  3. The new input considered is a number
• Loop postconditions:
  1. The variable max holds the maximum value among all the numbers considered so far
  2. The variable min holds the minimum value among all the numbers considered so far
• Loop invariant:
  1. The variable max holds the maximum value among all the numbers considered so far
  2. The variable min holds the minimum value among all the numbers considered so far

You can then generalize the solution for the nth input: when you consider the nth number, compare it with the values in max and min, updating them if necessary. In each step, we can show that the invariant holds.

A full flowchart of this program can be found by clicking the following link:

Min Max Flowchart

It is helpful to have this diagram available in a second browser tab for review on the next few pages.