reset the counter in for loop

Asked by Khalid Mamdouh on 16 Nov 2019 at 20:30

Latest activity Commented on by Walter Roberson on 17 Nov 2019 at 3:27

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I want to check the values in an array but when i reset the counter it ignores the new definition and continues looping. Why that happens and if there is a better way to do it please help. Thanks in advance

v=input('Enter an array of positive numbers:');
for i=1:length(v)
    while v(i) <= 0
        v=input('Error:Enter an array of positive numbers:');
        i=1;
    end
end

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2 Answers

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Answer by JESUS DAVID ARIZA ROYETH on 16 Nov 2019 at 20:52

Edited by JESUS DAVID ARIZA ROYETH on 16 Nov 2019 at 20:53

because the for loop once defined ignores some change in its variable and tries to execute normally, but with this you can solve your problem:

v=input('Enter an array of positive numbers:');
    while sum(v<=0)>0
        v=input('Error:Enter an array of positive numbers:');
    end

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Adam Danz on 16 Nov 2019 at 21:31

Note that if the user presses enter without entering anything in response to input(), it will return an empty [ ] and since sum([]<=0) == 0, the user will not be prompted again. That may be desired or it may not be - just something to keep in mind.

Khalid Mamdouh on 16 Nov 2019 at 21:47

Is sum(v<=0) works on 2007 version because I tried it and it prompts an error 'Index exceeds matrix dimensions'

Walter Roberson on 17 Nov 2019 at 3:27

I have to wonder if you had accidentally created a variable named sum

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Answer by Adam Danz on 16 Nov 2019 at 21:01

Edited by Adam Danz on 16 Nov 2019 at 21:36

You cannot alter the for-loop counter. Here's an alternative that continually asks the user for a vector of positive numbers until those condtions are satisfied and then it moves on to your for-loop.

Instead of using input() it uses inputdlg() which is a bit cleaner IMO but that line can be replaced with input() if that is preferred. Additionally, it converts the char input to double (numeric).

v = NaN;
while any(isnan(v)) || any(v<0)
    response = inputdlg('Enter a vector of positive numbers');
    v =  str2double(strsplit((response{:})));
end
for i=1:length(v)
    % Do stuff
end

Example with input() instead.

v = [];
while isempty(v) || ~isnumeric(v) || any(v < 0)
    v = input('Enter a vector of positive numbers: ');
end
for i=1:length(v)
    % Do stuff
end

2 Comments

Walter Roberson on 16 Nov 2019 at 21:10

You can alter the for loop counter. However, the next time the for loop would execute, it will change the counter back.

Adam Danz on 16 Nov 2019 at 21:18

You can override the counter variable within the loop but I was referring to the counter controlling the number of iterations. Thanks for the clarification.

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