How do I index a variable based on the outcome of a logical expression (e.g if x +y = 2 then variable 2 is indexed)

I have an if statement in a for loop which cycles through a row vector, counting the total number of each sum between n elements of both vectors. It is based off of rolling dice so the max value is 6 and the range of the sums can be from 2-12.

For example if the 5th element of array1 was 4 and the 5th element of array2 was 5 then the sum would be 9 and the total number of 'nines' in the sums of the row vectors would increase by 1.

I know how to do this using 11 elseif statements, however how can I do it just using one if/else statement.

In my head it works like this:

• nth element of array 1 and 2 are added together and give x
• use value of x to index corresponding tx variable. so if x = 4, variable t4 would be substituted into the if statement
• continue by adding 1 to the tx value until 300 loops have been completed.

Would there be a way to do it like this?

   if (array1(n) + array2(n)) == x            %Where x = an integer 2-12
           tx = tx+1

Or is this simply not possible and must I use 11 seperate elseif statements for this? I am aware this is a simple problem to have but I am just curious as to whether you can write code to substitute different variables into different places as the code is running. I am fairly new to matlab with just a month or two's experience so do bear with me! Thanks

here is my code currently:

array1 = randi(6,1,300);
array2 = randi(6,1,300);
t2 = 0;
t3 = 0;
t4 = 0;
t5 = 0;
t6 = 0;
t7 = 0;
t8 = 0;
t9 = 0;
t10 = 0;
t11 = 0;
t12 = 0;
for n=1:300
    if (array1(n) + array2(n)) == 2
        t2 = t2+1;
    elseif (array1(n) + array2(n)) == 3
        t3 = t3+1;
    elseif (array1(n) + array2(n)) == 4
        t4 = t4+1;
        ... % ... shows that elseif statements continue until t12=t12+1
    end
end

[Stephen23]

"Would there be a way to do it like this?"

Yes, if you want to force yourself into writing slow, complex, inefficient, insecure, obfuscated, buggy code that is hard to debug:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

The MATLAB approach is very simple: use a vector and indexing. For example, replace all of these seperate scalar values (which are bad data design because you a forcing pseudo-indices into variable names):

t2 = 0;
t3 = 0;
t4 = 0;
t5 = 0;
t6 = 0;
t7 = 0;
t8 = 0;
t9 = 0;
t10 = 0;
t11 = 0;
t12 = 0;

with just one vector:

T = zeros(1,12);

and then use simple, basic, efficient, robust, real indexing.

Answer 1

[Steven Lord]

Here's an example with one die. You can imagine how much more code than this you'd have to use if you wanted to count the frequencies of rolls of a 100-sided die (as an example) using your approach! In my code handling a d100 or d% would require changing one line. You should be able to adapt this to your two die example.

numSides = 6;
rollCounts = zeros(1, numSides);
numRolls = 1e4;
for k = 1:numRolls
    roll = randi(numSides);
    rollCounts(roll) = rollCounts(roll)+1;
end
disp(rollCounts)

        1685        1653        1636        1731        1686        1609

% check that each roll was counted
isequal(sum(rollCounts), numRolls)

ans = logical
   1

Of course there are other ways to do this that don't involve calling randi numRolls times. Using histcounts is one potential solution.