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POStrange behavior with std::nth_element
 

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  1. POStrange behavior with std::nth_element
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    <p>I'm trying to find the median of vectors of (x,y) points using nth_element</p> <pre><code> cv::Point2f medOffset; vector&lt;float&gt; tempOffsetsX = offsetsX; int medLoc = tempOffsetsX.size()/2; nth_element(tempOffsetsX.begin(), tempOffsetsX.begin()+medLoc, tempOffsetsX.end()); // sort(tempOffsetsX.begin(), tempOffsetsX.end()); medOffset.x = tempOffsetsX[medLoc]; vector&lt;float&gt; tempOffsetsY = offsetsY; ****** debug out line 1 ********* nth_element(tempOffsetsY.begin(), tempOffsetsY.begin()+medLoc, tempOffsetsY.end()); // sort(tempOffsetsY.begin(), tempOffsetsY.end()); medOffset.y = tempOffsetsY[medLoc]; ****** debug out line 2 ********* </code></pre> <p>tempOffsetsX is working just fine but, occasionally, tempOffsetsY is giving very strange results after nth_element. Here is sample output at the marked debug lines</p> <pre><code>tempOffsetsY1: 5.184135 -1.564125 3.751759 0.221855 -0.742348 1.737648 tempOffsetsY2: -0.742348 -1.564125 -8885092352.000000 -8850636800.000000 0.000000 0.000000 </code></pre> <p>The results are pretty repeatable until I recompile, at which point the specifics change the the general problem remains. Clearly the vector is getting corrupted somehow but I can't think of how. </p> <p>Also, if I use sort instead of nth_element it works without problem. For debugging, I tried doing a sort and then nth_element which worked just fine. So somehow the reordering that happens inside nth_element is getting messed up but I can't think of how.</p> <p>Any ideas how this is happening?</p> <p><em>edit</em> - More info about my environment. I'm running Arch Linux. I just did a system update. I should note that this same code did work without problem before the update, and this is the first time I've ran it after the update. But that's a gap of several days and I'm always hesitant to point at system libraries for what is usually my own problem.</p> <pre><code>[]$ uname -r 3.11.6-1-ARCH []$ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/4.8.2/lto-wrapper Target: x86_64-unknown-linux-gnu Configured with: /build/gcc/src/gcc-4.8.2/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared --enable-threads=posix --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch --enable-gnu-unique-object --enable-linker-build-id --enable-cloog-backend=isl --disable-cloog-version-check --enable-lto --enable-gold --enable-ld=default --enable-plugin --with-plugin-ld=ld.gold --with-linker-hash-style=gnu --disable-install-libiberty --disable-multilib --disable-libssp --disable-werror --enable-checking=release Thread model: posix gcc version 4.8.2 (GCC) []$ pacman -Qi glibc Name : glibc Version : 2.18-8 .... </code></pre>
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    1. USryan0270
    1. USryan0270
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    1. COAre you positive offsetsX and offsetsY are of the same size?
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      1. POStrange behavior with std::nth_element
      1. USice-phoenix
    2. COYeah, I checked that to be sure. For sanity I also tried using separate medLocX and medLocY variables with no change
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      1. POStrange behavior with std::nth_element
      1. USryan0270
    3. COOk, what's your execution environment? Compiler versions, etc.
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      1. USice-phoenix
 

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