Determine the number of y-values less than the number 2.0

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Benjamin Trivers
Benjamin Trivers 2020-1-16
回答： Dominik Mattioli 2020-1-16
y = 2.5 + 1 * randn(100,1)
sum=0;
N=length(y);
for i = 1 : N
sum= sum + y(i);
end
average=sum/N;
fprintf('the average of this list is %f\n', average)
%%
count=0;
N=length(y);
for y=1:N
if y<2
count=[y<2]
sum(count)
end
disp('The number of values less than two is')
disp(sum(count))
end
above is my code. I can't seem to get the count of values less than 2. need help
  1 个评论
Star Strider
Star Strider 2020-1-16
The problem could be that using a variable named ‘sum’ overshadows the sum function.
It is not obvious what your code is supposed to do.

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回答（2 个）

David Hill
David Hill 2020-1-16
y = 2.5 + randn(100,1);
a=mean(y);
fprintf('the average of this list is %f\n', a);
t=nnz(y<2);
fprintf('The number of values less than two is %d\n', t);
Dominik Mattioli
Dominik Mattioli 2020-1-16
First, your variable 'sum' in your first for-loop is problematic because it is overwriting a built-in MATLAB function called 'sum', rendering it unusable.
A few problems in the second for-loop - here's psuedo code of what you're doing.
y = array_of_random_numbers
For y equal to 1 through the length of y (this reassigns your variable array y to an integer, losing all information):
if y < 2, which is only true for your first iteration:
create a 1x1 logical array detailing whether my iterator is less than 2 (only true for your first iteration)
sum this 1x1 logical array
display the sum of the 1x1 array (in this case, you're only referring to the value of 'count' from your first iteration for all iterations).
Try this instead:
y = 2.5 + 1 * randn(100,1)
y_sum = sum( y, 1 ); % Rename this variable.
N = length( y );
for index = 1:N
y_sum = y_sum + y( index );
end
average = y_sum / N;
fprintf( 'The average of this list is: %f\n', average )
%%
count = 0;
N = length( y );
for iter = 1:N
if y( iter ) < 2
count = count + 1;
end
disp( ['The number of values less than two is: ', num2str( count )] )
end
Alternatively, you could vectorize your code to make it more efficient and cleaner-looking:
y = 2.5 + 1 * randn(100,1)
% y_sum= sum( y, 1 ); % Your first for-loop is unneccessary if you use the built-in function.
average = mean( y )
fprintf('the average of this list is %f\n', average)
%%
count = sum( y < 2, 1 );
disp( ['The number of values less than two is: ', num2str( count )] )

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