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The order of precedence MDAS

Discussion in 'C' started by Player, Jan 21, 2009.

  1. Player

    Player New Member

    Hi all.

    I have been here before but not for a while. I hope my question is in the right section :). I am new to learning C and seem to be picking it up and enjoying it. I am a little confused about how the order of precedence works. I have wrote the following simple program but can't workout how the compiler gets it answer. I have shown my workings using the MDAS rule but i'm stuck. Sorry to all the professionals out there at what may seem a dumb question but i really want to understand everything i'm learning.

    Thanks for reading :)
    int main()
        /*               18/9-2+6*2
                          my answer = -12
                          compiler = 12*/
        /*               12-32/2*8+10
                          my answer = 0
                          compiler = -106*/
    Last edited by a moderator: Jan 22, 2009
  2. xpi0t0s

    xpi0t0s Mentor

    The order of multiple equal precedence operators is left to right, just as in maths. So 2-2+12 is not 2-(2+12) but (2-2)+12.

    So the first is 18/9-2+6*2= 2-2+12= 0+12= 12.

    Similarly 12-2+10=(12-2)+10, not 12-(2+10), and 32/2*8=(32/2)*8 not 32/(2*8), so 12-32/2*8+10= 12-16*8+10= 12-128+10= -116+10= -106.

    So you seem to be working right-to-left for some reason. Some things in C work that way, but expression evaluation isn't one of them.
    shabbir likes this.
  3. Player

    Player New Member

    Thanks you very much for your answer. This has made it a lot clearer. I was looking at the following and i guess i miss understood :). I'm sure i will be back with more questions in the not too near future.

    Thanks again.
  4. xpi0t0s

    xpi0t0s Mentor

    You didn't get right-to-left from that page, because is presents left to right evaluation: 100+10-20 -> 110-20, not 100-10. Also it shows 5*2 evaluated before 60/3, again in left to right order.
  5. Player

    Player New Member

    I have miss understood that then too. The important bit is that i now understand it from your explination. I have tested myself with long random sums and i'm getting it right everytime now. Again, many thanks for taking the time to explain.:)
  6. shabbir

    shabbir Administrator Staff Member

    I removed the attachment from this post as I did not see anything related to it in the post
  7. xpi0t0s

    xpi0t0s Mentor

    "I was looking at the following" referred to the attachment.
  8. Player

    Player New Member

    I was just trying to show how i had gone wrong. I was doing the following:

    I was going to the multiflication first as in.


    Then the division


    Then the addition


    Then the subtraction


    But now i know its.



    Thanks again, i hope i have made myself clear as to where i was going wrong.
  9. asadullah.ansari

    asadullah.ansari TechCake

    See inline message .....
    Remeber one thing only " Capture the higher precendency operator it may be more than one If it is then go for operation on that operator which if in the left means operation should be from left to right on same precedence Operators"
    Last edited: Jan 23, 2009
  10. Player

    Player New Member

    I was working multiplication first before division. I was working from left to right but skipping division if i came across it first. I didn't realise that M and D was the same precedence and A and S was also the same. I was putting them in order 1=M 2=D 3=A 4=S instead of 1=M&D 2=A&S.
    I hope this has cleared this up, i certainly understand it now thanks to you guys :)

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