'for loop' for beginners?

Question

Andrew Ardolino 2014-10-8

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评论： Andrew Ardolino 2014-10-8

I'm having trouble figuring out a for loop command. I'm trying to make a matlab script that allows the user to input a row vector, and then it outputs a new vector of 0's and 1's. 0's for negative numbers and 1's for non negative numbers.

so far what i have is:

v=input('Enter elements of vector: \n');
 % after entering v,run below code
 for i = 1:length(v)
     if v(i) > 0
        sprintf('v(%i)>0',i)
     elseif v(i) <0
        sprintf('v(%i)<0',i)
     end
 end

but when I finish, i get "Error: Unexpected MATLAB expression." So I guess I just need to know where I'm going wrong and what I need to do differently. I'm not sure how to make the loop output it when it's done either. Thanks in advance for your help!

edit: I also tried:

v=input('Enter elements of vector: \n');
 % after entering v,run below code
 for i = 1:length(v)
     if v(i) > 0
        v(i)=1
     elseif v(i) <0
        v(i)=0
     else
        v(i)=0
     end
 end

which also doesn't work. I'm not sure how to get the program to replace the values from the old vector with 0's and 1's based on whether or not they are positive. Thanks again.

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Answer 1

Image Analyst 2014-10-8

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编辑：Image Analyst 2014-10-8

在 MATLAB Online 中打开

Here's one way that works.

str = input('Enter elements of vector: \n', 's');
v = cell2mat(textscan(str, '%f,'))
% After entering v,run below code
for i = 1:length(v)
     if v(i) > 0
        v(i)=1
     elseif v(i) <= 0
        v(i)=0
     end
end

Of course the for loop could be replaced by a vectorized solution

v = v>0;

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Image Analyst 2014-10-8

在 MATLAB Online 中打开

At the prompt when it's waiting for your input, type

1, 0, -3, 6, 2, -7

instead of

[1 0 -3 6 2 -7]

Andrew Ardolino 2014-10-8

oh wow, jeez thank you so much.

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Answer 2

Iain 2014-10-8

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在 MATLAB Online 中打开

The error is because you're trying to get the user to enter a list of numbers with a function that only allows ONE number to be input.

If you replace:

v=input('Enter elements of vector: \n');

with

v = [4 5 3 6 1 -34 -56];

How you get the list of numbers from the user is up to you - you can either put it in a loop (while loop), or you can change your code to accept an input, then have the user call it, like sin([0 pi/2 pi])

You'll see that it works.