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I really have no idea of how to do these questions - in fact I have no idea of how to do any question in the paper - but I have tried to figure out what's going on in the course called Computational Mathematics but the lecturer's notes are honestly useless to someone who doesn't have a strong maths background.

The course also has a high failure.

I'm trying to find materials online but the course isn't focused on just one topic, I even asked the lecturer for a recommended book but he said there isn't one book that covers the whole module, so I'm really stuck. Here a link to the exam paper. Link

Here's the first question from last year's paper:

Question 1.

(i) How many non-unique, non-normalised, numbers can be represented in a floating-point system defined by parameters $\beta, s, m, M$? $\tag*{ [5 Marks]}$

(ii) How many unique, normalised, numbers can be represented in a floating-point system defined by parameters above? Hint: it is proportional in some way to $\beta^{s-1}$ because no number other than zero itself can start with zero. $\tag*{[8 Marks]}$

(iii) Enumerate all the non-negative, non-unique, non-normalised, numbers in the floating-point system defined by parameters $\beta=4, s=2, m=-1, M=1$ $\tag*{[8 Marks] }$

(iv) Convert the numbers enumerated above into a floating-point system with $\beta=10, s=3, m=-1, M=1 .$ Comment on their distribution and some consequences for computation. $ \tag*{[4 Marks ]}$

Please note that I'm not asking for just the solutions but an explanation and probably a link, so that I can have a background knowledge and so that I'll be able to answer similar questions myself. This is not an assignment, I'm just preparing for an exam.

Thank you. :)

Edit:

2 $\quad$ Finite-precision floating point system - FPS

Let $F(\beta, s, m, M)$ be a system where

  • $\beta$ is the base, e.g. $2,4,10,$ or $16$

  • $s$ is the number of significant digits of the mantissa in base $\beta$.

  • $e \in Z$ is an exponent, $m \leq e \leq M$

Each number $x \in\{F\}$ has the structure $$ \pm \, \underbrace{d_{1} d_{2} \ldots d_{s}}_{\text {mantissa }} \times \underbrace{\beta}_{\text {basis }}\,^{\pm e\} \text { exponent }} $$ If $x \neq 0$ then $x$ is normalised if $1 \leq d_{1} \leq \beta-1$ and $0 \leq d_{i} \leq \beta-1, i=2 \ldots s .$ If $x=0$ then $d_{1}=d_{2}=\ldots=d_{s}=0$

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  • $\begingroup$ What do you mean by "a floating-point system defined by parameters $\beta,s,m,M$"? Presumably this is defined in your course, but you have to tell us how! $\endgroup$
    – TonyK
    Apr 17, 2014 at 12:14
  • $\begingroup$ @TonyK Please, see edit. $\endgroup$
    – MosesA
    Apr 17, 2014 at 12:17

1 Answer 1

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What I can help is to provide an analogue using base 10 floating point numbers.

If it is non-normalized, then it has infinitely many non-unique representations. Examples are:-

6.25 = 0.625 * 10………..…… (1)

6.25 = 0.0625*100…………… (2)

6.25 = 625 * 10^(-2)……..… (3)

This is not a ‘healthy’ environment because a number has so many 'looking different' but in fact equivalent representations. In order to ensure the representation of a number is unique, normalization is necessary.

Normalization requires:-

I. All number should start as $0.d_1d_2d_3…d_s$ where the $d_i’s$ are the extracted digits.

II. The leading digit (i.e. $d_1$) must not be zero and other digits have no such a restriction. This is formally stated as $1 \le d_1 \le 10 – 1$ and $0 \le d_i \le 10 – 1$ for $i = 2, … ,s.$ At this stage, only (1) above can meet the requirement.

III. In order to make the so far representation numerically equivalent to the original, it must be compensated by multiplied a suitable exponent. That is, $*10^e$ for some suitable integer e; and e can be 0, + or –.

IV. If the number is 0, then ……

Thus, the normalized representation of $6.25$ is $0.6250000000...00 * 10^1$; the number of 0s appended depends on the size of the ‘container’ or ‘WORD’.

If the size of the WORD and the m and M (as in $m \le e \le M$) are given, one can find the smallest and largest number that this system can hold.

Example in addition of two floating numbers using a simplified representation

$6.25 + 703.94 = 0.625 * 10^1 + 0.70394 *10^3$

$= 0.00625 *10^3 + 0.70394 *10^3$

$= 0.71019 *10^3$

$= 710.19$

Note-1: Add./sub. must be done when the 2 operands are converted to the ‘same level’ first.

Note-2: It is possible that some data are lost due to conversion.

Note-3: The result might exceed the upper/lower limit (i.e. an overflow or underflow).

Example in multiplication of two floating numbers using a simplified representation

$6.25 * 703.94 = (0.625 * 10^1) * (0.70394 *10^3)$

$= (0.625 * 0.70394) *10^{1 + 3}$

$= 0.4399625 *10^4$

Note-4: Comment in Note-3 applies.

Note-5: Truncation may occur.

Further note:- In evaluating an expression via more than one steps, different orders or operations may yield different results. Example, computing the average of a and b by (1) $(a + b)/2$ and by (2) $a + (b – a)/2$ may yield different results due to errors like truncation.

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  • $\begingroup$ Why infinitely many representations if the representation usually must have finite length? $\endgroup$
    – Ruslan
    Apr 17, 2014 at 15:15
  • $\begingroup$ The example (6.25) shown in the introductory part in deed has infinitely many equivalent forms when normalization is not enforced. Varying the exponent can generate that many. Note also that, at that stage, it has not been placed in the WORD of finite length yet. $\endgroup$
    – Mick
    Apr 17, 2014 at 16:37

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