CODEWITHSUNDEEP
pythonmath
niicepants
niicepants

Reputation: 11

Arithmetic in python, can't figure out simple basic operations

I am doing some very basic excersizes in python. I'm using the hard way to learn python book, which in excersize 3 have a expression I should understand.

3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6, is coming out as 7.

For me, the answer is 6.

6 - 5 + 0 - 1 / 4 + 6

1 - 1 / 4 + 6

6.

This is clearly wrong, but can anyone help me with priorty in mathematics etcetera? I seem to have forgotten everything if it's not inside a parenthesis!

EDIT: Thank you very much for the response. I've clearly learned something about the very basic stuff, which I think is important before moving on! My order of operations was definately way off!

Upvotes: 1

Views: 2217

Answers (10)

vikki
vikki

Reputation: 2814

1/4 rounds down (or floors) to 0 since 1 and 4 are integers. Therefore 1-0+6 = 7

Upvotes: 2

Luca Davanzo
Luca Davanzo

Reputation: 21548

Priority is for division and multiplication, if you don't use brackets!

In this way: 1-1/4+6=7 because 1/4, working with integer, is = 0

Upvotes: 1

chrtan
chrtan

Reputation: 1724

3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6

The order of precedence for this is multiplication and division evaluated left to right and then addition and subtraction. So..

3 + 2 + 1 - 5 + (4 % 2) - (1 / 4) + 6

The ones inside the parentheses are evaluated first because of order of operations. Because you only have integers present (and not a float which would allow the value 0.25), 1/4 will floor to 0 because of python.

3 + 2 + 1 - 5 + 0 - 0 + 6

Afterwards, when you evaluate from left to right for addition/subtraction, you are left with:

6 - 5 + 6  => 1 + 6 => 7

Remember that division and multiplication are evaluated before addition and subtraction. For more information, Order of Operations - Wikipedia

Upvotes: 0

David Webb
David Webb

Reputation: 193814

If we look at the rules for Operator precedence in Python we can see that:

3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6

is being treated as:

3 + 2 + 1 - 5 + (4 % 2) - (1 / 4) + 6

(For the arithmetic operators this is the same order as the standard order of operations in mathematics.)

Now 4 % 2 is 0 since the remainder when dividing 4 by 2 is 0. 1 / 4 is also 0 as Python will return a value of the same type as the operands and 0.25 when "floored" is 0.

So I think your mistake is applying the / to the whole expression to the left. In fact as / has a higher precedence than - the division evaluated is 1 / 4.

Upvotes: 3

MK.
MK.

Reputation: 34597

4 % 2 = 0 because the remainder of 4 / 2 is 0

1 / 4 is also 0 because it is doing integer division and .25 is floored to 0.

Upvotes: 5

Samvel
Samvel

Reputation: 182

In Python3.x it'd be 6.75 b/c 1/4 = 0.25 (true division). However, in Python 2.x 1/4 = 0 (convert to the most generic type of the arguments used, that is int in given case). Therefore, if it'd be 1. / 4 or 1 / 4. then in Python2.x you'd get 0.25 and the result would be 6.75

Upvotes: 1

ezdazuzena
ezdazuzena

Reputation: 6790

Rather than mathematics you should have a look at the operators:

a%b is a modular b, which in case of a=4 and b=2 yields 0 (ok, thanks mathematics ;) )

1/4: here it's important to take into account that 1 and 4 are integers. So the result will also be an integer, i.e. 0.25 becomes 0.

That's why you end up with 

  3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6
= 3 + 2 + 1 - 5 +   0   -   0   + 6
=                                 7

For operators in python have a look at the python docs.

Upvotes: 0

zch
zch

Reputation: 15278

See the table of operator precedences in Python.

http://docs.python.org/reference/expressions.html#summary

Upvotes: 2

Marcin
Marcin

Reputation: 49886

You seem to think that python should evaluate everything to the left of a / operator before dividing by the first token to the right. That would be an odd evaluation order by any measure.

As always with programming: if you have a complex expression with infix operators, use brackets to force the correct order.

Upvotes: 1

Odobenus Rosmarus
Odobenus Rosmarus

Reputation: 5998

operands are integer, so 1/4 integer dividing = 0

Upvotes: 0

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