Question

I’m a super beginner to MatLab, and I’m trying to debug a simple script I’m writing. I’m having a weird error while trying to debug my code. Here’s the script:

function [prob] = QuantumHW1(j,k,m)

X = [0 1; 1 0];
Y = [0 -sqrt(-1); sqrt(-1) 0];
Z = [1 0; 0 -1];
H = 1/sqrt(2) * [1 1; 1 -1];
S = [1 0; 0 i];
T = [1 0; 0 exp(sqrt(-1)*pi/4)];

mats = {X,Y,Z,H,S,T};

binJ = dec2bin(j,k);
binM = dec2bin(m,k);

totOps = {};

%Set up all the operators to be used
for p = 1:k
    totOps(p) = mats(mod(p,6));
    if p == 0
        totOps(p) = X;
    end
end

withM = {};

%Dot product with M
for p = 1:k
    p
    binM(p)+1
    totOps(:,1)
    withM(p) = totOps(:,binM(p)+1);
end

rTotal = 0;

%Now take components with respect to J
for p = 1:k
    rTotal = rTotal + [not(binJ(p)),binJ(p)] * withM(p);
end

prob = norm(runningTotal)^2;

disp('The probability to measure j = %d in a k = %d system on input m = %d is %d',j,k,m,prob);
end

When I run the program, I get an Array Index Out of Bounds error on the line withM(p) = totOps(:,binM(p)+1);. I tried making sure the value of p was correct. On the first iteration through the for loop, binM(p) = 0. But when I try to get binM(p) + 1, I get 49. This is super weird.

Any help is much appreciated. I’m beating my head against the wall trying to figure out why this is happening.

Answer 1

Because binM(p) holds the ASCII value of the string ‘0’, not the actual double value 0. And ASCII value of ‘0’ is 48. ‘0’+1 is automatically converted to a double value. You do the rest of the math.