Changing the For LOOP conditions

Question

Jason 2019-10-10

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https://ww2.mathworks.cn/matlabcentral/answers/484616-changing-the-for-loop-conditions

评论： Jason 2019-10-10

采纳的回答： Shubham Gupta

Hello.

I have a for loop that starts with image number 1 and ends with the last image n.

for i=1:n

Sometimes, I only need to analyse every other image- starting at image 2, so would require

for i=2:2:n

I would like to use a checkbox to determine the parameters of the loop.

Rather than write out 2 different loops depending on the value of a checkbox, is there a way where i can just use a single for loop with some condition at the start

i.e.

        val=get(handles.checkbox1,'Value)
        if val==1
           use i=1:n 
           else      
           use i=2:2:n 
        end
   

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Answer 1

Shubham Gupta 2019-10-10

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编辑：Shubham Gupta 2019-10-10

One of the way to do this is define the for loop conditions using predefined vectors.

You can write vector to set how the for loop varies. For e.g.

Vec = 1:10
for i = Vec
    printf('%d',i)
end

Now, you just need to set Vec according the checkbox :

val=get(handles.checkbox1,'Value')
Vec1 = 1:n;
Vec2 = 2:2:n;
if val==1
    Vec = Vec1; 
else      
    Vec = Vec2;
end
        
for i = Vec
    % operation
end

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Jason 2019-10-10

Perfect, thankyou

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Answer 2

Stephen23 2019-10-10

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编辑：Stephen23 2019-10-10

If val has a value either 1 or 2:

for k = 1:val:n

If val is a boolean (i.e. 0 or 1) then simply do either of these:

for k = 1:(1+val):n % true->2, false->1
for k = 1:(2-val):n % true->1, false->2

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Stephen23 2019-10-10

编辑：Stephen23 2019-10-10

"But also the starting value needs to be 1 or 2."

That is such a trivially simple change, that I sure that you could have figured that out yourself:

for k = val:val:n

And for the boolean you can also do it quite simply. Using if is a waste of MATLAB, when you can simply set the step size directly. Using intermediate vectors is also pointless.

Jason 2019-10-10