The min and max function return incorrect result!

Hi, I generate a feature matrix from wavelet dicomposion of an EEG signal, for example with 2x16 double dimensions. But when I want to see max and min for all columns, for some columns, it return 0!!

here is my matrix:

Features = [1227752731.63331, 18338294.3655838, 72667987.4176357, 2710006.87295031, 2623292.81882001, 16791624.0266024, 570620.052920511, 206222.333358212, 1001033.71805223, 16658.1448182973, 43506.1703317381, 143319.583017336, 21259.4058920161, 33646.8754811107, 87870.4592040121, 10139.3064159190
    ; 373181415.803611, 2184323.98283064, 7124198.60955997, 168359.098887688, 287423.644267531, 1448871.84718216, 42715.7953000498, 30445.2160607579, 100263.257495253, 5559.10902863085, 9485.91790552470, 74748.0071948079, 2814.63617073193, 4522.57176240473, 10081.3836180636, 1715.21296944122];

% As you can see, my 16th column values are:
Features(:,16)

ans =

   1.0e+04 *

    1.0139
    0.1715
    
% But if I type max(Features), it gets me 0.0000 for column 16th and some other columns as well
max(Features)

ans =

   1.0e+09 *

  Columns 1 through 10

    1.2278    0.0183    0.0727    0.0027    0.0026    0.0168    0.0006    0.0002    0.0010    0.0000

  Columns 11 through 16

    0.0000    0.0001    0.0000    0.0000    0.0001    0.0000
    
% But if I  type max(Features(:,16)
max(Features(:,16))

ans =

   1.0139e+04
   

So I was wundering what would this happen?!!

Note: I didn't use max or min keywords in any part of my code. I check that with which('max', '-all') command.

Tnx in advance.

Answer 1

[Stephen23]

The default display format short displays arrays with five significant figure precision and a common multiplier for all values in the array (the multiplier is the order of the largest magnitude value). The common multiplier seems to confuse you, so lets have a look at your data. The largest value is 1227752731.63, which has an order of magnitude 9. So all numbers will display with that magnitude and five digits precision, we can visualize this easily:

...00001227752731  largest value
...****98765*****  order of digits to be shown for ALL numbers in that array

Now lets see when we use only those digits for the value 10139:

...****98765*****  order of digits to be shown for ALL numbers in that array
...00000000010139

The first five digits, starting from order 9, are 0, 0, 0, 0, 0. The digits marked * are not shown, they are beyond the precision for the specific format that you are using. Note also the implicit infinite leading zeros (I didn't manage to write them all in on this forum, so they are a bit truncated). Note also how this applies to all values in that array, e.g. the fifth element in the first row is displayed as 0.0026, and now it should be clear why:

...****98765*****  order of digits to be shown for ALL numbers in that array
...00000002623292

If you want to use a different display format, then change it. Probably shortE would give you what you expect:

>> format short E
>> max(Features,[],1)
ans =
  Columns 1 through 8
 1227752731.63   18338294.37   72667987.42    2710006.87    2623292.82   16791624.03     570620.05     206222.33
  Columns 9 through 16
    1001033.72      16658.14      43506.17     143319.58      21259.41      33646.88      87870.46      10139.31

Note this does NOT have a common multiplier at the top, now every value has its own multipler.

[MYB]

Wow! tnx man. I fully understand it. You helped a lot. Thank you very much.

Answer 2

[Rik]

You didn't notice the exponent:

max(Features)
ans =
   1.0e+09 *

If you want to control how your data shows up in the command window you should use fprintf.

[MYB]

Actually, I noticed that. With 1.0e+09 *, the last column should be this 1715.21296944122 not 0.0000 !!!

Tnx for fprintf. I checked that and it is useful. But I guess matlab should print result of max(Features) just like max(Features(:,16)).

I don't understand why in max(Features) it prints zero!!!

[Stephen23]

"he last column should be this 0.171521296944122"

No, that is incorrect. You didn't notice the exponent 1.0e+04

"but I guess matlab should print result of max(Features)"

It does.

The value 1.0139e4 printed to five significant figures starting with order 9 figure is 0.0000

[Walter Roberson]

Give the command

format long g

and display the data again.

[MYB]

@Stephen Cobeldick

I'm sorry. I don't fully understand. Could you please explain how "The value 1.0139e4 printed to five significant figures starting with order 9 figure is 0.0000" in more detail?!

Thank you very much.

[MYB]

@Water Roberson

Tnx. It shows better now.

[Stephen23]

"Could you please explain how "The value 1.0139e4 printed to five significant figures starting with order 9 figure is 0.0000" in more detail?!"

Lets use a simple example with 654321 and 9.

Now lets print their values using a common multiplier (the order of the largest value) and three significant figures. First we find the three digits at the required orders:

  654321
  000009
  ^^^    % three significant digits

So the output is clearly 6.54e5 and 0.00e5. Then you move the common multiplier and display it at the top:

1e5*
6.54   0.00