MATLAB Answers

Compare two strings based on ASCII dictionary order

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I know the sort builtin function will sort cell arrays of strings in ascii dictionary order. But how may I simply compare two strings to determine which is first (by dictionary order) ? This must be a constituent part of the sort routine, but I cannot find a way to do it. (and sort is builtin so I cannot inspect it).

Accepted Answer

Oleg Komarov
Oleg Komarov on 25 May 2012
[trash,idx] = sort({'abc';'a'})
Then just look at idx(1)
  2 Comments
Brandon Kuczenski
Brandon Kuczenski on 25 May 2012
This plus diff(idx) gives a usable result. it does not detect equal strings, however.

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More Answers (2)

Geoff
Geoff on 25 May 2012
Yeah this is a real failing of the strcmp function in my opinion. That function originated in C, and would tell you whether a string was less, equal or greater than another. But it was probably too non-intuitive for MatLab users because it returned zero if the strings were equal.
Basically, what the C function does is subtract the strings character by character. So you can do that here:
function cmp = cstrcmp( a, b )
% Force the strings to equal length
x = char({a;b});
% Subtract one from the other
d = x(1,:) - x(2,:);
% Remove zero entries
d(~d) = [];
if isempty(d)
cmp = 0;
else
cmp = d(1);
end
end
The output is:
a == b : 0
a > b : positive
a < b : negative
There are probably more efficient ways to do this in MatLab. I just stuck to the easy matrix operations.
  2 Comments
Geoff
Geoff on 25 May 2012
Haha yeah that logic negation of non-logic values is an old C habit. It's technically not very good programming practice (because it's not as readable as "d==0"), but MatLab does document the behaviour. Use at your own peril =)

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Junaid
Junaid on 25 May 2012
Can you give one example. As I understand, you can do it by compare operator.
a = 'abd';
b = 'abc';
a <= b
output is [1 1 0] where 0 indicates that some character in b comes before in by dictionary order.
  2 Comments
Brandon Kuczenski
Brandon Kuczenski on 25 May 2012
Also, the single comparison is insufficient to determine which string comes first.
a='hellob'
b='hellbo'
According to a dictionary test, a>b. In order for me to know that, I would need to do both comparisons:
a>=b
>> a>=b
ans =
1 1 1 1 1 0
>> b>=a
ans =
1 1 1 1 0 1
>>
and then see which one has the earlier zero.

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