Cell array and strings
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I have initialized a series of vectors named vector1, vector2, ... vector8, and am using the code below.
for k=1:numVecs
myCell{1,k}=sprintf('vector%d',k);
myCell{2,k}=???
end
I want to put, in the second row, kth entry of the cell, the content of variable vectork, where k=1,2, etc. I tried:
myCell{2,k}=(strcat('vector',num2str(k)),1:8),
but MATLAB says "Expression or statement is incorrect--possibly unbalanced (, {, or [." right before the 1. Does anyone have a suggestion? Thanks in advance.
3 个评论
OCDER
2018-6-19
The problem starts right here:
"I have initialized a series of vectors named vector1, vector2, ... vector8"
How did you do this?
J.S.
2018-6-19
Stephen23
2018-6-20
"I have initialized a series of vectors named vector1, vector2, ... vector8"
If you are creating numbered variables then you are doing something wrong. Trying to access numbered variables is one way that beginners force themselves into writing slow, complex, buggy code. Read this to know why:
Indexing is very neat, very simple, very easy to write, very easy to debug. You should be using indexing, because then your code would be simpler, more efficient, and you avoid this entire situation:
vec{1} = ...
vec{2} = ...
vec{3} = ...
Or if the vectors are the same size then put them into one matrix:
mat(:,1) = ...
mat(:,2) = ...
mat(:,3) = ...
In both cases you can trivially and efficiently access the required data using indexing.
回答（1 个）
OCDER
2018-6-19
Take a read here: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval
Your vector1, vector2, etc naming scheme will lead to a lot of headaches and you'll end up recoding everything. See the simple fix below.
Instead of :
vector1 = [1, 4, 5];
vector2 = [4 18 17];
Do this:
vector{1} = [1, 4, 5];
vector{2} = [4 18 17];
To access "vector1", use "vector{1}".
If you want a 2xN cell storing a variable name on row 1, the vector on row 2, then do this:
vecnames = strsplit(sprintf('vector%d;', 1:length(vector)), ';');
C = vertcat(vecnames(1:length(vector)), vector);
C =
2×2 cell array
'vector1' 'vector2'
[1×3 double] [1×3 double]
7 个评论
Jan
2018-6-20
By the way: There is a valuable but undocumented function:
vecnames = sprintfc('vector%d', 1:length(vector))
OCDER
2018-6-20
Nice! This is so much better. Why this is undocumented puzzles me...
OCDER
2018-6-20
Cool, so compose is the official version of sprintfc. But compose is 20% slower as it adds more capabilities over sprintfc. This difference won't matter much for most applications though (micro optimization).
t1 = timeit(@() sprintfc('vector%d', 1:100000), 1);
t2 = timeit(@() compose('vector%d', 1:100000), 1);
t1/t2 =
0.798 %sprintfc is ~20% faster
Jan
2018-6-20
On my R2016b:
v = 1:1e5;
t1 = timeit(@() sprintfc('vector%d', v), 1)
t2 = timeit(@() compose('vector%d', v), 1)
fmt = string('vector%d');
t3 = timeit(@() fmt.compose(v), 1)
t1 = 0.0736
t2 = 0.0717
t3 = 0.0714
OCDER
2018-6-20
t1 = 0.0540
t2 = 0.0702
t3 = 0.0526
Version 2017a. Maybe compose got more lines of codes in 2017a+ ?
Yair Altman
2018-6-29
Note that compose is not a full replacement for sprintfc, although it does cover the most common use-case. Specifically, sprintfc's 3rd [isImaginary] input flag, and its 2nd/3rd output args [errorMsg and isLeft] are not supported by compose.
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2018-6-29
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