How do you replace vector values?

2012 10 9
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Consider the following vector:
A=[6 8 12 -9 0 5 4 -3 7 -1]
Write a program using a for loop to produce a new vector, B which is related to A in the following way: All values of A which are not less than 1 should be replaced with the natural logarithm of that number, all numbers that are less than 1 should be replaced with the original number plus 20. Output the new vector.

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Thomas
Thomas 2012-10-9
Sounds like a homework question.. What have you done so far?
Madeline
Madeline 2012-10-9
This is what I have so far:
A = [6 8 12 -9 0 5 4 -3 7 -1];
k=1;
p=20;
for A
true(A>1)
Bvector=k.*log(A);
for A
false(A>1)
Bvector=p+A;
end
end
I know it's incorrect but one of the people in my group has the correct code in a while loop.
A=[6,8,12,-9,0,5,4,-3,7,-1];
n=1;
while n<=length(A)
if A(n)<1
A(n)=A(n)+20;
else
A(n)=log(A(n));
end
n=n+1;
end
disp(A)
So now I can't figure out how to write the code using a for loop.

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Matt Tearle
Matt Tearle 2012-10-9

1 个投票

What you have inside the while loop is basically what you need:
if A(n)<1
A(n)=A(n)+20;
else
A(n)=log(A(n));
end
You just need to convert the while structure to for. The strange thing is that for is much simpler. You want to loop over n from 1 to length(A). I'm not sure I can say much more than that without just doing it for you.
However, one other thing: the assignment says to produce a new vector B.
And while I'm here... I hate these kinds of assignments. I really hope that your instructor makes it absolutely clear that you should not use a loop for this kind of operation. Here it is done properly:
A = [6,8,12,-9,0,5,4,-3,7,-1];
B = A;
idx = (B<1);
B(idx) = B(idx) + 20;
B(~idx) = log(B(~idx));

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