unix bc command and order of operation

Question

I'm encountering a strange issue with the basic unix "bc" command on a mac and I want to just make sure I'm understanding it correctly. From the man pages, it says it respects order of operations, which I remember from my schooldays as "My Dear Aunt Sally" (multiply, divide, add, subtract)

When evaluating the following string in bc I'm getting an unexpected value

5/2+4+6-8*4+5*7+8
23

However, if I go through and I parenthesize the order of operations I stated earlier, I get a different result:

(((5/2)+(4+6))-((8*4)+((5*7)+8)))
-63

Do I have a fundamental misunderstanding of the bc command, or math?

Sure your parenthesis is false, it should be (5/2)+4+6-(8*4)+(5*7)+8. Minus is for 8*4 product not up to the end... — Jean-Baptiste Yunès, Jan 17 '15 at 17:53

score 2 · Accepted Answer · answered Jan 17 '15 at 17:55

2

We know that x-y+z is not equal to x-(y+z). Thus you are committing a mistake by expecting 5/2+4+6-8*4+5*7+8 to be equal to (((5/2)+(4+6))-((8*4)+((5*7)+8))). Also, you need to study a bit more about operator precedence and truncation in division. For example, 5/2 though is equal to 2.5 is truncated to 2. Hence, 5/2+4+6-8*4+5*7+8 = 2+4+6-32+35+8 = 23. Hope, it helped

answered Jan 17 '15 at 17:55

Nagabhushan Baddi

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thank you- i was taking the "my dear aunt sally" rule too literally... I think now that it's coming back to me it's "(my dear) (aunt sally)". – infomofo Jan 17 '15 at 18:15
@infomofo.You always welcome. Exactly, * and / have the same precedence, and + and - have the same precedence. – Nagabhushan Baddi Jan 17 '15 at 18:20
And within that same precedence, you go from left to right. – Teepeemm Jun 07 '16 at 18:01

unix bc command and order of operation

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