如何计算 Chapel forall 循环中的迭代次数

Question

我想知道在使用 Chapel 的 forall 时循环执行了多少次。使用 CDO Library 的这段代码失败了，我确信这样做是正确的。谁能给我一个很好的例子吗？

var j:int = 0;
writeln("\n=== FORALL LOOP ===");
forall row in cursor {
  writeln("from: ", row['from_nm'], "\tto: ", row['to_nm']);
  j += 1;
}
writeln("Expected 10 rows, got ", j);

错误是

faerr.chpl:59: error: illegal lvalue in assignment
faerr.chpl:57: note: The shadow variable 'j' is constant due to forall intents in this loop

Answer 1

默认情况下，大多数变量类型（数组除外）都被复制到 forall 循环中，因此您无法修改它们。这样做是为了避免常见情况下的竞争条件。如果你想在 forall 循环中修改 j，你需要使用 forall intent。

所有意图都在 forall 循环中 Chapel language spec. Since this code wants to reduce the j values of the tasks to a single value, you can use a reduce intent 的 "Data Parallelism" 部分定义以计算迭代次数：

var j:int = 0;
writeln("\n=== FORALL LOOP ===");

forall row in cursor with (+ reduce j) {
  writeln("from: ", row['from_nm'], "\tto: ", row['to_nm']);
  j += 1;
}

writeln("Expected 10 rows, got ", j);

如何计算 Chapel forall 循环中的迭代次数

How do I count iterations in a Chapel forall loop

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