"end-1" as parameter

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Chen Lin
Chen Lin 2016-9-9
回答： Chen Lin 2016-9-9
I have a question on MATLAB Onramp Chapter 5.1 Indexing into Arrays, Task 3:
Under what "real" scenario would I use the "end-1" command to find a value?
In the given example of MATLAB Onramp, the task is to find the value in the 6th row and 3rd column. I understand that I can use the keyword "end" to either find a row or column index to reference of the last element, which is quick and straightforward in a dataset, but why would I use the "end-1" as parameter? Doesn't it make it too complicated to find an "unknown" value?
Any further explanation on this is much appreciated.
  2 个评论
John D'Errico
John D'Errico 2016-9-9
It would help if you provide a link. Without that or a copy of the code/question, it is difficult to know what you are talking about.
Steven Lord
Steven Lord 2016-9-9
I believe that's part of MATLAB Academy.

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采纳的回答

Guillaume
Guillaume 2016-9-9
Well, end-1 is not an unknown location. It's the element before the last. There are plenty of scenarios where you'd want the nth element before the end, so end-n is very useful. e.g.:
  • Implementing a LIFO (Last In First Out) stack. When you want to pop the last element from the stack:
function [stack, element] = pop(stack)
element = stack(end); %get last element
stack = stack(1:end-1); %return the stack without the last element
end
  • When you want to remove the border around an image:
function croppedimage = cropborder(sourceimage, bordersize)
croppedimage = sourceimage(bordersize+1:end-bordersize, bordersize+1:end-bordersize, :);
end
  • split a vector into two:
function [firsthalf, secondhalf] = splitmiddle(vector)
firsthalf = vector(1:ceil(end/2));
secondhalf = vector(ceil(end/2)+1:end);
end
  • etc.

更多回答（2 个）

Steven Lord
Steven Lord 2016-9-9
Let's say I wanted to add each element of a vector to the next element. I could do this with a for loop, or I could use a vectorized approach. But I can't add the last element of the vector with the next element -- there isn't a next element! So I add every element of the vector EXCEPT the last (which I retrieve using the index vector 1:end-1) to the one just after that (which I retrieve using the index vector (1:end-1)+1 or 2:end.)
x = 1:10;
y = x(1:end-1) + x(2:end);
We introduced in release R2016a a function named movsum that you could use instead to compute y, but the indexing approach works back probably to Cleve's original Fortran version of MATLAB.
Chen Lin
Chen Lin 2016-9-9
Hi all, thanks for the quick responses.
And John was right, so is Steven's assumption. The original question came from MATLAB Onramp Chapter 5.1, Task 3 (https://matlabacademy.mathworks.com/R2016a/portal.html?course=gettingstarted#chapter=5&lesson=1&section=1).
It was given: _>> load datafile >> data
data =
3.0000 0.5300 4.0753 NaN
18.0000 1.7800 6.6678 2.1328
19.0000 0.8600 1.5177 3.6852
20.0000 1.6000 3.6375 8.5389
21.0000 3.0000 4.7243 10.1570
23.0000 6.1100 9.0698 2.8739
38.0000 2.5400 5.3002 4.4508_
Task 3: _Info: Note that you can use arithmetic with the keyword end. For example:
>> x = A(end-1,end-2)_
Try creating a scalar variable p that contains the value in the second to last (end-1) row and 3rd column of data.
I didn't understand the meaning of "the second to last" (end-1) row first, but I figured out now. It was easy.
And thanks to Guillaume's practical examples. Perfect gents. Much appreciated!

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