fprintf into a stored variable
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Instead of using fprintf I want to store all the i, xe, and f(xe) into a vector instead. That way I can call them later. Also I had another problem where I wanted to use another 'i' later on in the code. But it only allowed me to use 1. How can you use two different 'i's. So they don't get mixed up.
f = @(x) x^2-6;
xl = 0;
xu = 10;
xe = (xu+xl)/(2);
acc = 0.00001;
while abs(f(xe))> acc
i = i + 1;
fprintf('%d)\t%f\tf(x)=%f\n',i,xe,f(xe));
if (f(xe)*f(xl)) < 0
xu = xe;
else
xl = xe;
end
xe = (xu+xl)/2;
end
采纳的回答
per isakson
2016-2-13
编辑：per isakson
2016-2-13
Add
M = zeros( 0, 2 );
before the loop and replace
fprintf('%d)\t%f\tf(x)=%f\n',i,xe,f(xe));
by
M( end+1, 1:2 ) = [ xe, f(xe) ];
You'll get a warning about "Growing ..." , but if speed is acceptable ignore it.
 
"fprintf into a stored variable"   If understood literally, replace
fprintf('%d)\t%f\tf(x)=%f\n',i,xe,f(xe));
by
C{end+1,1} = sprintf('%d)\t%f\tf(x)=%f\n',i,xe,f(xe));
and add C=cell(0,1) before the loop
10 个评论
WhatIsMatlab-
2016-2-13
if speed is acceptable ignore it.
Speed will decrease the longer the loop goes on (more and more elements need to be copied to a new memory location). If the loop converges fast enough, then indeed speed should not be an issue.
To stop the warning from appearing altogether:
M( end+1, 1:2 ) = [ xe, f(xe) ]; %#ok<AGROW>
per isakson
2016-2-13
编辑：per isakson
2016-2-13
Welcome point.
On the FEX there is a solution and good discussion on the problem of pre-allocation when the final size is not known: Incremental growth of an array, revisited, by John D'Errico
WhatIsMatlab-
2016-2-14
per isakson
2016-2-14
编辑：per isakson
2016-2-14
- "How do I stop it from using the same 'i' for each while loop"   Do you mean the same i in the first and second while loop? In the code you show i is incremented but not used.
- Did you inspect the values of m and s after running the code? These values are they not the ones you expect?
WhatIsMatlab-
2016-2-14
编辑：WhatIsMatlab-
2016-2-14
Guillaume
2016-2-14
Well, set i to 0 before the 2nd loop, or use a different variable.
WhatIsMatlab-
2016-2-14
per isakson
2016-2-15
- "but I got an error"   we cannot help if you don't show what you did and what error Matlab reported.
- "a better way of achieving this"   length(s) and length(m) tell the number of iterations in the two loops.
per isakson
2016-2-15
This example illustrates
- two ways to count the number of loop iterations
- two ways to store result in the loop
N = 1e6;
M1 = zeros( 0 );
ii = 0;
M2 = nan( N, 1 ); % Pre-allocate
while sqrt(ii) < sqrt(N) - 10
ii = ii + 1;
M1( end+1, 1 ) = ii;
M2(ii) = ii;
end
M2(isnan(M2)) = [];
disp( all( M2 == M1 ) )
disp( ii )
disp( length(M1) )
disp( length(M2) )
outputs
1
980100
980100
980100
where "1" shows that M1 and M2 are identical.
[Run and Time], i.e. the profiler, shows that "M1" is slower.
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