Implement a ternary search

#1

i need a function that implement a ternary search algorithm in MATLAB? any one could help ?

#2

Is there something that confuses you in the description of the ternary search algorithm?

#3

no i do understand it but my problem is the syntax it self since i never used matlab, i thought if someone could help
thats my code

function [numComparisons, returnindex ] = ternarySearch( array, target, firstIndex , lastIndex)
numComparisons = 0;
lastIndex = length(array);
firstIndex = 1;

mid1 = firstIndex + (lastIndex - 1);
mid2 = lastIndex - (lastIndex - 1);
xmid1 = 0;
xmid2 = 0;

if(array(mid1)== target)
xmid1 = mid1;

elseif(array(mid2) == target)
xmid2 = mid2;
elseif(target < array(mid1))

ternarySearch(mid2+1,lastIndex, target);

else

ternarySearch(mid1+1,mid2-1,target);

end
end

#4

been a while since I’ve used matlab, but it’s definitely harder to read unformatted

If you put the code between triple backticks (a line of three of these: `, then the code, then a line with 3 of those symbols again) it’ll be much easier for us to read

#5

sorry about that
the error is that any search target always 0 which means that algorithm isnt working

function [numComparisons, returnindex ] = ternarySearch( array, target, firstIndex , lastIndex)
numComparisons = 0;
lastIndex = length(array);
firstIndex = 1;

mid1 = firstIndex + (lastIndex - 1) / 3;
mid2 = lastIndex - (lastIndex - 1)/ 3;
xmid1 = 0;
xmid2 = 0;

if(array(mid1)== target)
    xmid1 = mid1;
    
elseif(array(mid2) == target)
    xmid2 = mid2;
elseif(target > array(mid1))

    ternarySearch(mid2+1,lastIndex, target);
elseif(target < array(mid1))
    ternarySearch(firstIndex,mid1 - 1, target);
else
  
    ternarySearch(mid1+1,mid2-1,target);
end
end
#6

This function you’ve defined is a little odd, you’ve defined a function with 4 parameters, then only used 3 of them later on

You also declare two output parameters then don’t seem to assign to them, is that intentional? (probably not, I’m guessing)

Edit: you do assign to one of them, numComparisons, so the function returns what you’ve set it to, which is 0

Edit2: forgot to mention, you set the values of firstIndex and lastIndex immediately inside the function, you don’t want to do that, you should just get them from the function parameters

#7

as i mention it’s only the syntax that is a bit weird for me, would you help me to rewrite it please?
the only param i need is the number of comparisons so i can use it in a graph later on

#8

The syntax looks almost fine, not got matlab installed to check for definitely, it’s the actual code that’s wrong

To return values from a function you assign to your declared output parameters, which you’ve not been doing

You’ve declared a function, ternarySearch which takes in 4 parameters, array, target, firstIndex and lastIndex

You immediately wipe out the values of firstIndex and lastIndex (don’t do this if you’re intending to do the recursive calls)

You then call this function (wrongly) with only 3 parameters, e.g. ternarySearch(mid2+1,lastIndex, target); You’re missing something here

Also you’re not returning anything useful

To return useful values from a function, you assign to the variables you’ve declared as output parameters (in this case numComparisons and returnIndex)

Don’t forget to get those values out of the ternarySearch calls you make inside ternarySearch too, e.g. [numComparions, returnIndex] = ternarySearch(array, target, newLeftIndex, newRightIndex); or whatever you name the new indices to search with

#9