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for-Index DeclarationsYou can declare loop-index variables directly in the start
portion of the for-loop, like this:
for (int i = 0; i < n; ++i)
{
…
}
This kind of start is limited to a single declaration; it can
declare one or more variables, separated by commas, all of which are
the same basetype (int, in this example):
for (int i = 0, j = 1, *p = NULL; i < n; ++i, ++j, ++p)
{
…
}
The scope of these variables is the for statement as a whole.
See Variable Declarations for an explanation of basetype.
Variables declared in for statements should have initializers.
Omitting the initialization gives the variables unpredictable initial
values, so this code is erroneous.
for (int i; i < n; ++i)
{
…
}