how do i substract the first value from the last one on an array taking out zero values?

Good morning people,

How could I substract the first value minus the last one of just the non zero values of an array?

How could I substract 20.08794 (first non zero value) to the last non zero value (23.57)?

Thanks in advance,

Have a good one

0
0
0
20.0879453799688
20.2939815063105
20.5010823257263
20.7087372901217
20.9165579913015
21.1228352488585
21.3295563001918
21.5354478396482
21.7403423949296
21.9443776718216
22.1482301195365
22.3520614637941
22.5558741325109
22.7596657087283
22.9635408028690
23.1674804705021
23.3714846914593
23.5754962282169
0
0
0
0

Answer 1

[Jan]

Get the corresponding indices:

first  = find(X, 1, 'first');
last   = find(X, 1, 'last');
Result = X(last) - X(first)

Answer 2

[Ameer Hamza]

If all the non-zero values are in increasing order, the try this

M(M~=0) = M(M~=0) - min(M(M~=0));

Result

>> M = [
    0
    0
    0
    20.0879453799688
    20.2939815063105
    20.5010823257263
    20.7087372901217
    20.9165579913015
    21.1228352488585
    21.3295563001918
    21.5354478396482
    21.7403423949296
    21.9443776718216
    22.1482301195365
    22.3520614637941
    22.5558741325109
    22.7596657087283
    22.9635408028690
    23.1674804705021
    23.3714846914593
    23.5754962282169
    0
    0
    0
    0];
>> M(M~=0) = M(M~=0) - min(M(M~=0));
>> M
M =
         0
         0
         0
         0
    0.2060
    0.4131
    0.6208
    0.8286
    1.0349
    1.2416
    1.4475
    1.6524
    1.8564
    2.0603
    2.2641
    2.4679
    2.6717
    2.8756
    3.0795
    3.2835
    3.4876
         0
         0
         0
         0