# Sheldon Ross 10: Example 3.17 Question: In the match problem of Example 2.31 involving n, n > 1, individuals, find the conditional expected number of matches given that the first person did not have a match.

Problem 2.31 – At a party N men throw their hats into the center of a room. The hats are mixed up and each man randomly selects one. Find the expected number of men who select their own hats.

Analytical Solution: As we see from the the problem’s description, 2.31 has been reiterated with a condition. This can be solved by applying the method of the conditional expectations

E[X] = E[E[X|X=x]]

⇒ E[X] = E[X|X1=0] P{X1=0} + E[X|X1=1] P{X1=1}

• E[X] = 1 (from problem 2.31)
• E[X|X1=0] is an unknown
• P{X1=0} = 1-P{X1≠1} = 1 – 1n = n-1n
• E[X|X1=1]
• Since the first one is a match and there are n-1 left, the probability of match of each of them is 1n-1
• The expected number of matches from the set of n-1 would be then equal to one
• Since the first one is already a match, the quantity E[X|X1=1] = 1 + 1 = 2
• P{X1=1} = 1n (from problem 2.31)

Plugging these in the equality above, we get E[X|X1=0] = n-2n-1

Observations:

• There is one significant change of the expected value as compared to scenario 2.31
• In 2.31, the expected value was a constant
• With a condition that the first one is not a match, the probability has become a function of the number of items (or hats)
• This would give us a variable number of hats to make additional observations in the simulation (which is the next section)

Simulations The simulations have been summarized in the chart below. What is being shown in the chart?

• Each bar has 1000 points
• each point is mean value of the number of matches for i iterations
• The number of iterations per point can be seen as a number below the bar
• Reading the first bar as an example
• It has number 10 written below it
• This means that each point is the average of 10 values
• Each value is the count of number of matches in a set of alphabets and it has been made sure that that first element is not a match (which is a requirement from the problem statement)
• Horizontal Bar: If you look closely there is a horizontal bar shown in the chart. This has been kept at 0.96
• Why 0.96? Since the alphabet, has 26 letters, the expected value from theory is 26-226-1 = 0.96 Generalized Algorithm: In the above example, we have looked at a system with a fixed number of units (hats). I have generalized the “Alphabet” algorithm from previous section to accommodate any list. I used the function to pass lists of increasing lengths and to see how the expected value changes, since, E[X|X1=0] varies with n as n-2n-1 . This particular simulation took ~30 minutes for completion.

Reading the Chart: The chart has seven distributions shown and we can read it as follows. Each bar contains 1000 points and each point is a mean value generated from a 1000 selections of varying lengths. The length of the list can be seen from the label at the bottom of each of bar

• for example: The first bar contains a 1000 points and each one of the 1000 points is a mean number of matches from a 1000 selections of length 10. The number 10 can be found at the bottom of the chart as a label
• Also there are 7 horizontal lines showing the theoretical expectations
• As n becomes infinity, theoretically the expectation value becomes 1. This seems to be the case from the simulation too. Comparing with theory. We know that E[X|X1=0] varies with n as n-2n-1 . We can plot this simply as a function to see the behavior of E[X|X1=0] with n. The plot has been done for lengths of lists from 3 to 25. Code

The code is arranged in sequence for each of the charts ——————————————– ——————————————– End of the post 😉

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