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Different result when adding variables

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Jim
Jim on 19 May 2016
Edited: Jim on 19 May 2016
This is in MATLAB R2016a. The addition
1:3 + [0 0 5]
gives
1 2 3
if we assign
x = 1:3;
and
y = [0 0 5];
then x+y is
1 2 8

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Accepted Answer

Stephen Cobeldick
Stephen Cobeldick on 19 May 2016
Edited: Stephen Cobeldick on 19 May 2016
You are doing two different operation sequences, so why the surprise that you get two different results?
First Case
>> 1:3 + [0 0 5]
ans =
1 2 3
because of the operator precedence (plus is higher than colon) is equivalent to
>> 1: (3 + [0 0 5])
ans =
1 2 3
which is equivalent to
>> 1:[3,3,8]
ans =
1 2 3
and thus is just
>> 1:3
ans =
1 2 3
because the colon operator only uses the first element of the vector and ignore the rest: "If you specify nonscalar arrays, MATLAB interprets j:i:k as j(1):i(1):k(1)."
Second Case
>> x = 1:3;
>> y = [0 0 5];
>> x+y
ans =
1 2 8
performs an element-wise addition on two vectors.
Summary
Beginners often forget about operator precedence and to read the documentation...

  2 Comments

Jim
Jim on 19 May 2016
I just thought the colon operator had priority over the plus operator...
Summary: always use parentheses to be sure, especially if you are programming in different languages
Steven Lord
Steven Lord on 19 May 2016
To elaborate slightly on Stephen's answer, the plus operator is at precedence level 6 while the colon operator is at level 7.

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