change number of sig figs in a list of numbers

Question

Michael 2011-5-24

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https://ww2.mathworks.cn/matlabcentral/answers/8125-change-number-of-sig-figs-in-a-list-of-numbers

basically i have a list of numbers, all with the same number of significant figures. an example of a number is :

734504.0749692708

I want to make this number the following: 734504.07

But the important thing is that I do not want to round. Also, I want to actually change the value, not simply how it is displayed. Matlab knows the exact value, and only displays 5 sig figs from the total of 16. I want to completely drop the last 8 figures in order to give me the above number. any way to do this?

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Answer 1

Matt Fig 2011-5-24

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在 MATLAB Online 中打开

N = 734504.0749692708
Ns = round(N*100)/100

EDIT Addressing Oleg's concern...

The above keeps two significant decimals. If you instead want to just drop them, then use fix instead. Look at the difference:

N = 7.85999999
Ns1 = round(N*100)/100
Ns2 = fix(N*100)/100
N =
      7.85999999
Ns1 =
      7.86
Ns2 =
      7.85

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Oleg Komarov 2011-5-24

You should change it to fix otherways the following number is rounded:

N = 734504.079;

Matt Fig 2011-5-24

I would think that 734504.08 would be the correct solution in keeping 2 significant decimals?

If it is desired to just drop any digits after the second decimal, then you are correct...

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Answer 2

Andrei Bobrov 2011-5-24

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https://ww2.mathworks.cn/matlabcentral/answers/8125-change-number-of-sig-figs-in-a-list-of-numbers#answer_11193

在 MATLAB Online 中打开

more (EDIT)

 N = 734504.0749692708
 Ns = fix(N*100)/100

more more

"I want to completely drop the last 8 figures in order to give me the above number."(Michael):

pwr = 10^(8-ceil(log10(N)))
Ns = fix(N*pwr)/pwr

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Matt Fig 2011-5-24

ROUNDN is only available with the mapping toolbox, correct?

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Answer 3

Walter Roberson 2011-5-24

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https://ww2.mathworks.cn/matlabcentral/answers/8125-change-number-of-sig-figs-in-a-list-of-numbers#answer_11195

It is not possible to store exactly 734504.07 in MATLAB double .

The algorithms that Matt and Andrei posted can only produce approximations of the number you want. For example, Andrei's solution produces the value 734504.069999999948777258396148681640625 . This is not a bug in Andrei's solution, but rather a limitation in what can be represented in MATLAB without using the Fixed Point Toolbox or the Symbolic Math Toolbox.