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Asked: 2026-05-27T12:53:12+00:00

I am new to randomized algorithms, and learning it myself by reading books. I

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I am new to randomized algorithms, and learning it myself by reading books. I am reading a book Data structures and Algorithm Analysis by Mark Allen Wessis
.

Suppose we only need to flip a coin; thus, we must generate a 0 or 1
randomly. One way to do this is to examine the system clock. The clock
might record time as an integer that counts the number of seconds
since January 1, 1970 (atleast on Unix System). We could then use the
lowest bit. The problem is that this does not work well if a sequence
of random numbers is needed. One second is a long time, and the clock
might not change at all while the program is running. Even if the time
were recorded in units of microseconds, if the program were running by
itself the sequence of numbers that would be generated would be far
from random, since the time between calls to the generator would be
essentially identical on every program invocation. We see, then, that
what is really needed is a sequence of random numbers. These numbers
should appear independent. If a coin is flipped and heads appears,
the next coin flip should still be equally likely to come up heads or
tails.

Following are question on above text snippet.

  1. In above text snippet ” for count number of seconds we could use lowest bit”, author is mentioning that this does not work as one second is a long time,
    and clock might not change at all”, my question is that why one second is long time and clock will change every second, and in what context author is mentioning
    that clock does not change? Request to help to understand with simple example.

  2. How author is mentioning that even for microseconds we don’t get sequence of random numbers?

Thanks!

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1 Answer

  1. Editorial Team

    1. Computers are fast. I’m over simplifying, but if your clock speed is measured in GHz, it can do billions of operations in 1 second. Relatively speaking, 1 second is an eternity, so it is possible it does not change.

    2. If your program is doing regular operation, it is not guaranteed to sample the clock at a random time. Therefore, you don’t get a random number.

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