Index exceeds matrix dimensions

clear
clc

count_eight=0;

clear
clc

count_eight=0;

RandStream.setDefaultStream = (RandStream('mt19937ar','seed',sum(100*clock)));

roll=input('Enter number of rolls \n');

for rolling = 1:roll

      dice1 = randi(6);
      dice2 = randi(6);

end

if dice1+dice2==8

      count_eight=count_eight+1;
  end

fprintf('Percentage of times an 8 appeared %.1f ',count_eight(roll*100));

display('End of Program')

roll=input('Enter number of rolls \n');

for rolling = 1:roll

      dice1 = randi(6);
      dice2 = randi(6);

end

if dice1+dice2==8

      count_eight=count_eight+1;
  end

fprintf('Percentage of times an 8 appeared %.1f ',count_eight(roll*100));

display('End of Program')

ERROR MESSAGE:

??? Index exceeds matrix dimensions.

Error in ==> Homework10 at 51
fprintf('Percentage of times an 8 appeared
%.1f ',count_eight(roll*100));

Answer 1

[Image Analyst]

Well at least you made a good attempt at it. but count_eight is a scalar while you try to print it out as an array in your fprintf. Try it this way:

clear;
clc;
count_eight=0;

RandStream.setDefaultStream = (RandStream('mt19937ar','seed',sum(100*clock)));

response = inputdlg('Enter number of rolls \n');
roll = str2double(response{1});

for rolling = 1:roll
    dice1 = randi(6);
    dice2 = randi(6);

  if dice1+dice2==8
    count_eight=count_eight+1;
  end
end

message = sprintf('8 appeared %d times in %d rolls = %.2f%%', ...
  count_eight, roll, count_eight/roll*100);
uiwait(msgbox(message));

Answer 2

[Sven]

It's just a small typo, I think:

fprintf('Percentage of times an 8 appeared %.1f ',count_eight(roll*100));

should be:

fprintf('Percentage of times an 8 appeared in %.1f percent of rolls',count_eight/roll*100);

Let me clarify though: The error is caused by a typo. Note however that I don't think the program does what you intend.

Where you have written:

for rolling = 1:roll
    dice1 = randi(6);
    dice2 = randi(6);
end

if dice1+dice2==8
    count_eight=count_eight+1;
end

Notice that you are only checking if dice one and two add up to 8 after the loop through many dice has finished. Ie, you're only checking once!

I think you want:

for rolling = 1:roll
    dice1 = randi(6);
    dice2 = randi(6);
    if dice1+dice2==8
        count_eight=count_eight+1;
    end
end

There is, however, an even better way of doing that. This way is very "MATLABBY" because it doesn't use loops:

dice1 = randi(6,roll,1);
dice2 = randi(6,roll,1);
count_eight = sum(dice1+dice2 == 8);

Answer 3

[Chris]

Thanks for the advice guys.