never_giveup's blog

Strange things with C++17
By never_giveup, history, 6 years ago, In English

Hello, I was solving golf from JOI 2017 Open. I tested in polygon with author's solution. My solution got WA, then I decided to change C++17 to C++14 and magically I've got AC. Can someone tell me how is that possible. Also I submitted it to oj.uz where compiler is under Ubuntu and it also gives wrong answer code test

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6 years ago, # |
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I changed links to array, still doesn't work. code

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6 years ago, # |
  Vote: I like it +28 Vote: I do not like it

Undefined behaviour or bug in g++. It's so obvious.

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    6 years ago, # ^ |
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    So why UD should work in other compilers than C++17

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      6 years ago, # ^ |
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      It's not really that it "works" in C++14, it just happened to not fail. You know, because it's undefined behavior.

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        6 years ago, # ^ |
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        Not to fail in 50 tests? Seems quite strange to me.

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          6 years ago, # ^ |
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          You are yellow, was red, and act like a green. You are able to handle this thing by yourself.

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            6 years ago, # ^ |
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            If I was able to, I wouldn't ask to help me. This is too hard for "yellow".

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              6 years ago, # ^ |
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              Line 255-256:

              set<int> :: iterator it = st.upper_bound(its.sc.fr);
it--;

              Sometimes upper_bound returns st.begin() causing undefined behavior when you decrement the iterator. Did you even try to test for UB before you made a codeforces blog to cry about C++17?

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                6 years ago, # ^ |
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                Why are you so mad, not everyone so smart like you. I never heard that upper_bound can return begin. Also I inserted -1 and inf some lines above and I can't remove them.

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                  6 years ago, # ^ |
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                  Yes you did. You inserted -1 above.And then you proceeded to check for -inf. Inserting -1 literally does nothing. If you insert -inf then it outputs the correct answer.

                  "I never heard that upper_bound can return begin". I actually wonder how you were once red...

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                  6 years ago, # ^ |
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                  Values that I use for upper and lower bound are in range 0..2*n+2, I use -inf for removing elements and inf to inserting elements as you can see.

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                6 years ago, # ^ |
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                Oh, actually I understood what you meant with upper_bound returning begin, I thought if there is no element instead of end it returns begin. Haha :D Anyway it can't happen, you can set assert if you don't believe.

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6 years ago, # |
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Is 10 the correct answer to test?

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6 years ago, # |
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If someone was wondering what was the problem. I found bug, actually in Windows C++14 and earlier when you access element of map, it creates if wasn't firstly. I was using it to compress values on lines 216-217. So in Windows elements started from 0, and in Linux from -1.

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