Avoiding many if-Statetments in a for-loop

Question

Kilian Helfenbein 2019-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/496122-avoiding-many-if-statetments-in-a-for-loop

编辑： Adam Danz 2019-12-11

Hello everyone :)

I would like to run a for-loop over a vector. What to do with each element in the vector depends on many if-else statements. I simplified it like this:

clear variables;
rng default;
V1 = rand(365,1);
V2 = rand(365,1);
V2 = V2 <= 0.5;
V3 = rand(365,1);
V3 = V3 <= 0.3;
Vresult = zeros(length(V1),1);
tstart = 1;
tend = length(V1);
for t = tstart:tend    
    if V2(t) == 1        
        if V3(t) == 1            
            Vresult(t) = do_stuff(V1(t));            
        else            
            Vresult(t) = do_different_stuff(V1(t));            
        end    
    else        
        if V3(t) == 1            
            Vresult(t) = do_even_different_stuff(V1(t));            
        else            
            Vresult(t) = do_even_more_different_stuff(V1(t))            
        end
    end
end
function [var_out] = do_stuff(var_in)
var_out = var_in * 5;
end
function [var_out] = do_different_stuff(var_in)
var_out = var_in + 3;
end
function [var_out] = do_even_different_stuff(var_in)
var_out = var_in / 4;
end
function [var_out] = do_even_more_different_stuff(var_in)
var_out = var_in - 2;
end

With even more if-else conditions and more calculations per statement it gets really confusing and unreadable. And I feel like I'm writing the same code over and over again as only some parts of the code are different in each statement and other parts stay the same.

How can I make my code more efficient, easier to read and maintainable? Sorry if this should be common knowledge :D

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Kilian Helfenbein 2019-12-11

编辑：Kilian Helfenbein 2019-12-11

Hey Adam,

no, these are just for demo purpose.

In my actual Code I try to simulate a battery system with a PV system, which is integrated into virtual power plant. So there are a lot of variables to watch each step (like state of charge, PV generation, power consumption of the household, grid frequency and so on...).

But the calculations itselfs are pretty simple mostly. With min, max and the usual operators.

Adam Danz 2019-12-11

If those functions can be vectorized, the best approach is illustrated by the answer provided by "Adam". If you need to perform those functions on each value of V1 within a loop, the answer provided by myself ("Adam Danz") is a bit cleaner than nested conditional statements.

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Answer 1

Adam Danz 2019-12-11

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/496122-avoiding-many-if-statetments-in-a-for-loop#answer_405949

I'm assuming the local functions provided are just demo functions. If those functions can be vectorized, it's very likely that you don't need a loop or any conditional statements at all.

Here's a method of performing a function on each element of vector V1 based on the values of V2(n) and V3(n) which are both logical vectors. It uses a switch-case instead of a series of conditional statements. The switch expression must be a scalar or char vector which is why I'm converting the logical values to char where 1 is true and 0 is false.

VAll = [V2,V3]; 
for t = tstart:tend
    switch sprintf('%d %d',VAll(t,:))
        case '1 1'
            Vresult(t) = do_stuff(V1(t));
        case '1 0'
            Vresult(t) = do_different_stuff(V1(t));
        case '0 1'
            Vresult(t) = do_even_different_stuff(V1(t));
        case '0 0'
            Vresult(t) = do_even_more_different_stuff(V1(t)); 
        otherwise
            % This should never happen unless VAll is unexpected
            error('Something is unexpected about the internal variable ''VAll''.')
    end
end

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Steven Lord 2019-12-11

Rather than turning your cases into char vectors I'd probably turn them into numbers. If V2 and V3 can only take on values 0 and 1, treat them like bits in a binary number.

% You may want to assert here to guard against V2 and V3
% taking values other than 0 or 1
VAll = [V2, V3];
v = polyval(VAll, 2);
switch v
    case 0 % V2 = 0, V3 = 0
    case 1 % V2 = 0, V3 = 1
    case 2 % V2 = 1, V3 = 0
    otherwise % V2 = 1, V3 = 1
end

Otherwise treat them like digits in an N-ary number.

Adam Danz 2019-12-11

编辑：Adam Danz 2019-12-11

I like the idea of using numbers rather than char vectors (clever use of polyval!). VAll is an nx2 logical matrix so the switch would need to be

VAll = [V2, V3];
for t = tstart:tend
    switch polyval(VAll(t,:),2) 
        case 0 % V2 = 0, V3 = 0
        case 1 % V2 = 0, V3 = 1
        case 2 % V2 = 1, V3 = 0
        otherwise % V2 = 1, V3 = 1
    end
end

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Answer 2

Adam 2019-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/496122-avoiding-many-if-statetments-in-a-for-loop#answer_405951

编辑：Adam 2019-12-11

Something like this would seem to do the job, off the top of my head, though I may be over-simplifying it...

case1 = V2 && V3;
case2 = V2 && ~V3;
case3 = ~V2 && V3;
case4 = ~V2 && ~V3;
Vresult( case1 ) = do_stuff( V1( case1 ) );
Vresult( case2 ) = do_different_stuff( V1( case2 ) );
Vresult( case3 ) = do_even_different_stuff( V1( case3 ) );
Vresult( case4 ) = do_even_more_different_stuff( V1( case4 ) );