# Return true if the elements of the input vector increase monotonically (i.e. each element is larger than the previous). Return false otherwise.

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Ahmed Diaa 2018년 6월 28일
편집: D. Plotnick 2018년 6월 28일
Return true if the elements of the input vector increase monotonically (i.e. each element is larger than the previous). Return false otherwise. this is my code
function tf = mono_increase(x)
% we need to check every number
L=length(x);
% we nust use for loop
% then we must use if statement
if L==1
tf=true; % adding a condition for single element
return %we must close the case for faster operation
end
for ii = 1:(L-1)
if x(ii+1) > x(ii) % we have problems in the -ve numbers
tf= true ;
else
tf=false ;
end
end
it doesn't work for negative numbers like : input x= [ -3 -4 1 2 4] the output must be false but I get true I think the wrong is in the indexing

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### 채택된 답변

Jeffrey Eymer 2018년 6월 28일
for ii = 1:(L-1)
if x(ii+1) > x(ii) % we have problems in the -ve numbers
tf= true ;
else
tf=false ;
end
end
Look at this portion of the code. For each ii in 1:(L-1), this portion of code gets called. This means that the value of tf is being updated 'L' number of times (i.e., when checking every pair of elements).
For example, the last two elements in [ -3 -4 1 2 4] are 2 and 4, which are monotonically increasing. Therefore, on the last iteration the value of tf is set to true and is not affected by the rest of the input.
You should try rewriting your code in a way where the value of tf is only updated to false when an issue is found.
##### 댓글 수: 2표시숨기기 이전 댓글 수: 1
D. Plotnick 2018년 6월 28일
You really want to try to avoid loops, especially for relatively simple operations like this. In this case, you can use a break to stop at the first non-monotonic jump, but the code is way clunkier and a bit slower.
% Set up spaces
x1 = linspace(0,1,1000001).'; % Column it
x2 = rand(1000001,1);
% Using any
tic;
dx1 = diff(x1);
tf1 = ~any(dx1<=0)
dx2 = diff(x2);
tf2 = ~any(dx2<=0)
toc
% Versus loops:
tic
tf1 = true;
tf2 = true;
for ii = 1:length(x1)-1
if x1(ii) >= x1(ii+1)
tf1 = false;
break
end
end
tf1
for ii = 1:length(x2)-1
if x2(ii) >= x2(ii+1)
tf2 = false;
break
end
end
tf2
toc
The any method requires just 2 lines to execute.
Note too that using any you can also return the indices of the bad numbers
function [tf,inds] = ismonotonic(x)
dx = diff(x);
inds = dx<=0;
tf = ~any(inds);
end

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### 추가 답변(1개)

D. Plotnick 2018년 6월 28일
편집: D. Plotnick 2018년 6월 28일
One idea: dx = diff(x); tf = ~any(dx<=0);
Note this will also work on matrices, if you want. E.g. to look at each row
x = {some numbers}; dx = diff(x,2); tf = ~any(dx<=0,2);
Here, tf will return a column of logicals.

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