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POHow to read ints from a text file with MPI and C++
 

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  1. POHow to read ints from a text file with MPI and C++
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    <p>I'm having a bit of a problem with parallel reading</p> <p>I have a text file which looks like this:</p> <pre><code>1 4 30 46 0 2 3 29 1 11 12 -1 1 4 5 -1 0 3 13 14 3 6 7 8 5 10 -1 -1 13 10 -1 -1 5 9 27 -1 </code></pre> <p>and I'm trying to read these ints 4 at a time in each process, the number of lines of the files equals the number of processes and every line contains 4 ints</p> <pre><code>int bufsize, count; int *buf; MPI::Status status; MPI::File top = MPI::File::Open(MPI::COMM_WORLD, "top.txt", MPI::MODE_RDONLY, MPI::INFO_NULL); MPI::Offset filesize = top.Get_size(); filesize = filesize / sizeof(int); bufsize = filesize / wasteland_size + 1; buf = new int[bufsize * sizeof(int)]; top.Set_view(my_rank * bufsize * sizeof(int), MPI_INT, MPI_INT, "native", MPI::INFO_NULL); top.Read(buf, bufsize, MPI_INT, status); count = status.Get_count(MPI_INT); top.Close(); </code></pre> <p>this is the code i'm using.</p> <p>It compiles without errors or warnings, but it outputs something line :</p> <pre><code>540287025 874524723 805969974 857748000 </code></pre> <p>for every process.</p>
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    1. UScpp_ninja
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    1. COYou're going to have problems here because your lines aren't of all the same length; there are 8,9,10,11, and 12-character lines. So simply dividing the file size by the number of processors (or something) and reading it in is unlikely to work. You could use the same approach [as in this answer](http://stackoverflow.com/a/12942718/463827) to divide up the file, and as a post-processing step do any loadbalancing as necessary. But in general text files aren't great for parallel I/O.
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      1. USJonathan Dursi
    2. COi'm creating that file from another one because i need it to be formatted like this (I'm reading the neighbors of a node in a graph with it) so i can modify it, but i don't know what kind of file to use so i can read it properly, should i make it binary ? what is the best file type to use in this situation ?
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    3. COI would either: (a) create it as a binary file; (b) preprocess it to split it up into the right number of sub files before running (eg, `split --lines=N top.txt` where N is the number of lines per processor) & have each processor read its own file; or, if the file isn't huge, (c) read it in with one processor and then distribute the data using `MPI_Scatter()` or `MPI_Scatterv()`. We could probably cobble something together that would use MPI-IO using the approach in the linked answer but unless there's some other compelling reason I'd tend to think it would be more trouble than it's worth.
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      1. USJonathan Dursi
 

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