for loop
显示 更早的评论
Hi,I have created the following loop:
for i = 1:5 x = i*2 end
I wish to be able to record each value of the iteration i.e. 2,4,6,8,10 aswell as arrive at the final value of 10 which the above code dies.Could you tell me how to do this? Thanks
采纳的回答
Grzegorz Knor
2011-9-9
x = zeros(5,1);
for i = 1:5
x(i) = i*2;
end
disp(x)
更多回答（3 个）
Andrei Bobrov
2011-9-9
for i1 = 5:-1:1, x(i1) = i1*2;end
for your case
x = (1:5)*2
or
x = 2:2:10
3 个评论
Grzegorz Knor
2011-9-9
If I do it this way:
for i1 = 5:-1:1, x(i1) = i1*2;end
it doesn't have to define the variable x before the loop to preallocate memory?
Andrei Bobrov
2011-9-9
Hi Grzegorz! I begin with last number 'x' and this way preallocate memory. Please read this http://www.mathworks.com/matlabcentral/answers/7039-how-to-multiply-a-vector-with-each-column-of-a-matrix-most-efficiently
Grzegorz Knor
2011-9-9
nice trick :)
mohammad
2011-9-9
x=[];
for i = 1:5
x =[x;i*2]; %for column matrix
end
OR:
x=[];
for i = 1:5
x =[x,i*2]; %for row matrix
end
2 个评论
Andrei Bobrov
2011-9-9
Hi mohammad! Please read this http://www.mathworks.com/help/techdoc/matlab_prog/f8-784135.html#f8-793781
Jan
2011-9-9
This is working and fast for tiny vectors. But it is really worth to pre-allocate arrays to get familiar with good programming methods. Without pre-allocation, this happens:
1. x is created as empty DOUBLE vector. This needs about 100 bytes for the internal header and 0 bytes for the data.
2. Inside the loop another vector x is created, again 100 bytes header, 8 bytes for the data. The former x is released.
3. In the 2nd iteration the next vector is created, again 100 bytes header, 16 bytes of data, 8 bytes of the formar array are copied. The former x is released.
4. etc.
If the loop runs until N, a total of N*100+prod(1:N)*8 bytes are allocated, filled with zeros, partially released later and prod(1:N-1)*8 bytes are copied.
In consequence you apporach is horribly slow for e.g. 1000 elements and might even crash with an out-of-memory error for larger vectors. Therefore *always* pre-allocate.
类别
在 帮助中心 和 File Exchange 中查找有关 Loops and Conditional Statements 的更多信息
2011-9-9
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!
Translated by 
