Substracting previous value in a matrix and assigning it to a new matrix

Hi everyone,

I Would like some help doing the following; substracting the previous value in a cell except the first value.

Lets say we have : a=[1 5 3 6 4] , then i would like the output : b= [1 4 2 3 2].

so in the iteration it would be :

• 1-0 = 1
• 5-1= 4
• 3-6 = 3
• 6-4 = 2

The code works - the only problem is that it does not perform this operation for the first column

a=cell2mat(SortVal)

b=a;

for jj = 2:18

for ii = 2:8

b(jj,ii) = b(jj,ii) - a(jj-1,ii)

end

end

This is a:

a =

1.6766 0.6118 0.3707 0.0771 0.0010 0.2057 0.1219 0.2438

2.2065 0.8679 0.4766 0.0879 0.0017 0.3461 0.1624 0.3007

2.4282 1.0790 0.5340 0.0921 0.0019 0.4416 0.2042 0.3431

2.5295 1.2192 0.5736 0.1017 0.0028 0.4911 0.2265 0.3896

2.6850 1.3342 0.6188 0.1062 0.0032 0.5578 0.2483 0.4327

2.8442 1.3778 0.6719 0.1118 0.0039 0.5993 0.2736 0.4536

3.0451 1.4608 0.7491 0.1133 0.0055 0.6738 0.2959 0.4860

3.1747 1.5049 0.7910 0.1138 0.0062 0.7108 0.3170 0.5029

3.2891 1.5334 0.8172 0.1208 0.0062 0.7247 0.3449 0.5181

3.4065 1.5708 0.8959 0.1238 0.0064 0.7447 0.3641 0.5260

3.4989 1.6455 0.9100 0.1323 0.0064 0.7649 0.3892 0.5480

3.5645 1.6464 0.9129 0.1342 0.0065 0.7828 0.4151 0.5662

3.6503 1.6760 0.9917 0.1364 0.0074 0.7949 0.4171 0.5739

3.7089 1.7567 1.0135 0.1465 0.0077 0.8090 0.4371 0.5834

3.7803 1.7714 1.0432 0.1493 0.0083 0.8243 0.4580 0.5987

3.8278 1.7844 1.0745 0.1540 0.0085 0.8515 0.4608 0.6102

3.8642 1.8361 1.0975 0.1552 0.0114 0.8629 0.4871 0.6260

3.8935 1.8419 1.1220 0.1601 0.0115 0.8665 0.4877 0.6325

and this is my output

b =

1.6766 0.6118 0.3707 0.0771 0.0010 0.2057 0.1219 0.2438

2.2065 0.2560 0.1059 0.0109 0.0007 0.1404 0.0406 0.0569

2.4282 0.2112 0.0574 0.0042 0.0002 0.0955 0.0417 0.0424

2.5295 0.1401 0.0397 0.0096 0.0008 0.0496 0.0224 0.0465

2.6850 0.1150 0.0451 0.0045 0.0004 0.0667 0.0218 0.0431

2.8442 0.0437 0.0532 0.0056 0.0008 0.0415 0.0253 0.0209

3.0451 0.0830 0.0771 0.0014 0.0016 0.0745 0.0222 0.0323

3.1747 0.0441 0.0419 0.0005 0.0007 0.0370 0.0211 0.0170

3.2891 0.0285 0.0262 0.0070 0.0000 0.0139 0.0279 0.0152

3.4065 0.0374 0.0788 0.0030 0.0001 0.0199 0.0192 0.0079

3.4989 0.0747 0.0141 0.0086 0.0000 0.0202 0.0251 0.0219

3.5645 0.0008 0.0029 0.0018 0.0001 0.0179 0.0259 0.0182

3.6503 0.0296 0.0788 0.0022 0.0009 0.0121 0.0020 0.0077

3.7089 0.0808 0.0217 0.0101 0.0003 0.0140 0.0200 0.0095

3.7803 0.0146 0.0297 0.0028 0.0006 0.0153 0.0209 0.0153

3.8278 0.0130 0.0313 0.0048 0.0002 0.0272 0.0028 0.0115

3.8642 0.0517 0.0230 0.0011 0.0029 0.0113 0.0263 0.0158

3.8935 0.0059 0.0244 0.0049 0.0002 0.0036 0.0006 0.0065

The code does it for all the column, but not the first?

[Stephen23]

http://web.archive.org/web/20210204113105/https://www.mathworks.com/matlabcentral/answers/735757-substracting-previous-value-in-a-matrix-and-assigning-it-to-a-new-matrix

Answer 1

[Stephen23]

a = [
    1.6766    0.6118    0.3707    0.0771    0.0010    0.2057    0.1219    0.2438
    2.2065    0.8679    0.4766    0.0879    0.0017    0.3461    0.1624    0.3007
    2.4282    1.0790    0.5340    0.0921    0.0019    0.4416    0.2042    0.3431
    2.5295    1.2192    0.5736    0.1017    0.0028    0.4911    0.2265    0.3896
    2.6850    1.3342    0.6188    0.1062    0.0032    0.5578    0.2483    0.4327
    2.8442    1.3778    0.6719    0.1118    0.0039    0.5993    0.2736    0.4536
    3.0451    1.4608    0.7491    0.1133    0.0055    0.6738    0.2959    0.4860
    3.1747    1.5049    0.7910    0.1138    0.0062    0.7108    0.3170    0.5029
    3.2891    1.5334    0.8172    0.1208    0.0062    0.7247    0.3449    0.5181
    3.4065    1.5708    0.8959    0.1238    0.0064    0.7447    0.3641    0.5260
    3.4989    1.6455    0.9100    0.1323    0.0064    0.7649    0.3892    0.5480
    3.5645    1.6464    0.9129    0.1342    0.0065    0.7828    0.4151    0.5662
    3.6503    1.6760    0.9917    0.1364    0.0074    0.7949    0.4171    0.5739
    3.7089    1.7567    1.0135    0.1465    0.0077    0.8090    0.4371    0.5834
    3.7803    1.7714    1.0432    0.1493    0.0083    0.8243    0.4580    0.5987
    3.8278    1.7844    1.0745    0.1540    0.0085    0.8515    0.4608    0.6102
    3.8642    1.8361    1.0975    0.1552    0.0114    0.8629    0.4871    0.6260
    3.8935    1.8419    1.1220    0.1601    0.0115    0.8665    0.4877    0.6325];
b = diff(a,1,1)

b = 17×8

0.5299 0.2561 0.1059 0.0108 0.0007 0.1404 0.0405 0.0569 0.2217 0.2111 0.0574 0.0042 0.0002 0.0955 0.0418 0.0424 0.1013 0.1402 0.0396 0.0096 0.0009 0.0495 0.0223 0.0465 0.1555 0.1150 0.0452 0.0045 0.0004 0.0667 0.0218 0.0431 0.1592 0.0436 0.0531 0.0056 0.0007 0.0415 0.0253 0.0209 0.2009 0.0830 0.0772 0.0015 0.0016 0.0745 0.0223 0.0324 0.1296 0.0441 0.0419 0.0005 0.0007 0.0370 0.0211 0.0169 0.1144 0.0285 0.0262 0.0070 0 0.0139 0.0279 0.0152 0.1174 0.0374 0.0787 0.0030 0.0002 0.0200 0.0192 0.0079 0.0924 0.0747 0.0141 0.0085 0 0.0202 0.0251 0.0220

[Me Me]

Thanks! but maybe i should have been clear with what i wanted. I want to treat the column as vectors. So the first row of the matrix, needs to be the same:

1.6766 0.6118 0.3707 0.0771 0.0010 0.2057 0.1219 0.2438

2.2065 0.8679 0.4766 0.0879 0.0017 0.3461 0.1624 0.3007

and then the next value needs to be substracted from the previous one so we get:

1.6766 0.6118 0.3707 0.0771 0.0010 0.2057 0.1219 0.2438

0.5299 0.2560 0.1059 0.0109 0.0007 0.1404 0.0406 0.0569

etc .....

I just updated my question, and managed to do it for all columns excepts the first.

[Me Me]

Okay, i think i solved it myself now.

I just feel like its not the most effecient way of doing it at all - So if anyone with more programming experience can help - it would be lovely.

Thanks!

a=cell2mat(SortVal)
[r,c]=size(a)
b=a;
for jj = 2:r
    b(jj) = b(jj) - a(jj-1)
    for ii = 2:c
    b(jj,ii) = b(jj,ii) - a(jj-1,ii)
    end 
end

>> a

a =

    1.6766    0.6118    0.3707    0.0771    0.0010    0.2057    0.1219    0.2438
    2.2065    0.8679    0.4766    0.0879    0.0017    0.3461    0.1624    0.3007
    2.4282    1.0790    0.5340    0.0921    0.0019    0.4416    0.2042    0.3431
    2.5295    1.2192    0.5736    0.1017    0.0028    0.4911    0.2265    0.3896
    2.6850    1.3342    0.6188    0.1062    0.0032    0.5578    0.2483    0.4327
    2.8442    1.3778    0.6719    0.1118    0.0039    0.5993    0.2736    0.4536
    3.0451    1.4608    0.7491    0.1133    0.0055    0.6738    0.2959    0.4860
    3.1747    1.5049    0.7910    0.1138    0.0062    0.7108    0.3170    0.5029
    3.2891    1.5334    0.8172    0.1208    0.0062    0.7247    0.3449    0.5181
    3.4065    1.5708    0.8959    0.1238    0.0064    0.7447    0.3641    0.5260
    3.4989    1.6455    0.9100    0.1323    0.0064    0.7649    0.3892    0.5480
    3.5645    1.6464    0.9129    0.1342    0.0065    0.7828    0.4151    0.5662
    3.6503    1.6760    0.9917    0.1364    0.0074    0.7949    0.4171    0.5739
    3.7089    1.7567    1.0135    0.1465    0.0077    0.8090    0.4371    0.5834
    3.7803    1.7714    1.0432    0.1493    0.0083    0.8243    0.4580    0.5987
    3.8278    1.7844    1.0745    0.1540    0.0085    0.8515    0.4608    0.6102
    3.8642    1.8361    1.0975    0.1552    0.0114    0.8629    0.4871    0.6260
    3.8935    1.8419    1.1220    0.1601    0.0115    0.8665    0.4877    0.6325

and the output:

b =

    1.6766    0.6118    0.3707    0.0771    0.0010    0.2057    0.1219    0.2438
    0.5299    0.2560    0.1059    0.0109    0.0007    0.1404    0.0406    0.0569
    0.2217    0.2112    0.0574    0.0042    0.0002    0.0955    0.0417    0.0424
    0.1014    0.1401    0.0397    0.0096    0.0008    0.0496    0.0224    0.0465
    0.1555    0.1150    0.0451    0.0045    0.0004    0.0667    0.0218    0.0431
    0.1592    0.0437    0.0532    0.0056    0.0008    0.0415    0.0253    0.0209
    0.2009    0.0830    0.0771    0.0014    0.0016    0.0745    0.0222    0.0323
    0.1296    0.0441    0.0419    0.0005    0.0007    0.0370    0.0211    0.0170
    0.1144    0.0285    0.0262    0.0070    0.0000    0.0139    0.0279    0.0152
    0.1175    0.0374    0.0788    0.0030    0.0001    0.0199    0.0192    0.0079
    0.0923    0.0747    0.0141    0.0086    0.0000    0.0202    0.0251    0.0219
    0.0656    0.0008    0.0029    0.0018    0.0001    0.0179    0.0259    0.0182
    0.0859    0.0296    0.0788    0.0022    0.0009    0.0121    0.0020    0.0077
    0.0585    0.0808    0.0217    0.0101    0.0003    0.0140    0.0200    0.0095
    0.0714    0.0146    0.0297    0.0028    0.0006    0.0153    0.0209    0.0153
    0.0475    0.0130    0.0313    0.0048    0.0002    0.0272    0.0028    0.0115
    0.0363    0.0517    0.0230    0.0011    0.0029    0.0113    0.0263    0.0158
    0.0293    0.0059    0.0244    0.0049    0.0002    0.0036    0.0006    0.0065

[Stephen23]

"I just feel like its not the most effecient way of doing it at all"

Using nested loops is not an efficient approach to this task.

"So if anyone with more programming experience can help - it would be lovely"

As my answer shows, using diff is much simpler and more efficient.

Just concatenate the first line on top and you are done:

b = [a(1,:);diff(a,1,1)]