Index exceeds matrix dimensions

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Chris
Chris 2011-11-6
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));

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采纳的回答

Image Analyst
Image Analyst 2011-11-6
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));

更多回答（2 个）

Sven
Sven 2011-11-6
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);
Chris
Chris 2011-11-7
Thanks for the advice guys.

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