If vs Switch Case

Question

Mike M 2015-3-11

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/182674-if-vs-switch-case

评论： Guillaume 2015-3-11

I just started coding earlier this year, and I am trying to figure out when to use certain functions. I currently have a series of 9 if/else statements, and I was wondering if it would be more efficient or if there is a reason to use switch/case over my current if else statements. P is a 9x9x9 3D matrix and C[] is a 3x3 matrix

if true
  % code
if (1<= i && i<=3) && (1<= j && j<=3)
              f = ismember(P(j,k,i),C1);
          elseif (1<= i && i<=3) && (4<= j && j<=6)
              f = ismember(P(j,k,i),C2);
          elseif (1<= i && i<=3) && (7<= j && j<=9)
              f = ismember(P(j,k,i),C3);            
          elseif (4<= i && i<=6) && (1<= j && j<=3)
              f = ismember(P(j,k,i),C4);
          elseif (4<= i && i<=6) && (4<= j && j<=6)
              f = ismember(P(j,k,i),C5);
          elseif (4<= i && i<=6) && (7<= j && j<=9)
              f = ismember(P(j,k,i),C6);
          elseif (7<= i && i<=9) && (1<= j && j<=3)
              f = ismember(P(j,k,i),C7);
          elseif (7<= i && i<=9) && (4<= j && j<=6)
              f = ismember(P(j,k,i),C8);
          else
              f = ismember(P(j,k,i),C9);
          end
          if f == 1
              P(j,k,i) = 0;
end

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Answer 1

Guillaume 2015-3-11

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https://ww2.mathworks.cn/matlabcentral/answers/182674-if-vs-switch-case#answer_170977

The only difference between if ... elsif and switch ... case is one of clarity. It shouldn't make much difference in term of performance. switch ... case is less verbose than if ... elseif but can only apply when you're comparing a single variable or expression to a set of possible values.

In your particular, the conditional expression changes all the time so it would be difficult to use a switch.

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Guillaume 2015-3-11

在 MATLAB Online 中打开

However, you could avoid the entire if or switch with:

C = {C1 C2 C3 C4 C5 C6 C7 C8 C9}; %assuming the Cs are vectors
lowerthresh = [1 4 7];
higherthresh = [3 6 9];
cidx = (find(i >= lowerthresh & i <= higherthresh) - 1) * 3 + find(j >= lowerthresh & j <=higherthresh);
%note this assumes that i and j are always between one of these bands
f = ismember(P(j, k, i), C{cidx});