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08-03-2006
It is possible for a if statement to be false. Do you think that if statements always must be true? Doesn't work like that.
In the worst possible case, where v[i] is always less than v[left], the routine will march along always swapping nothing. As a side effect of the (no-op) swap last is incremented. We need that side effect so the swap is performed regardless of whether it's really swapping.
But suppose that the worst possible case does not arise. If the routine should ever encounter even a single case where v[i] is not less than v[left], last and i would move out of sync. This would last for the rest of the loop, once out of sync, they could never sync up again.
If this bums you out too much, put a test in the swap routine to inhibit the swap if you are not really swapping. But now you are performing a test for every swap. The superfluous test will slow you down if it rarely inhibits a swap. But if it winds up inhibiting most of them, it might be worth it. It's your call...how pessimistic are you?
In the worst possible case, where v[i] is always less than v[left], the routine will march along always swapping nothing. As a side effect of the (no-op) swap last is incremented. We need that side effect so the swap is performed regardless of whether it's really swapping.
But suppose that the worst possible case does not arise. If the routine should ever encounter even a single case where v[i] is not less than v[left], last and i would move out of sync. This would last for the rest of the loop, once out of sync, they could never sync up again.
If this bums you out too much, put a test in the swap routine to inhibit the swap if you are not really swapping. But now you are performing a test for every swap. The superfluous test will slow you down if it rarely inhibits a swap. But if it winds up inhibiting most of them, it might be worth it. It's your call...how pessimistic are you?
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LEARN ABOUT BSD
swapon
SWAPON(8) System Manager's Manual SWAPON(8) NAME
swapon - specify additional device for paging and swapping SYNOPSIS
swapon -a swapon name ... DESCRIPTION
Swapon is used to specify additional devices on which paging and swapping are to take place. The system begins by swapping and paging on only a single device so that only one disk is required at bootstrap time. Calls to swapon normally occur in the system multi-user initial- ization file /etc/rc making all swap devices available, so that the paging and swapping activity is interleaved across several devices. Normally, the -a argument is given, causing all devices marked as ``sw'' swap devices in /etc/fstab to be made available. The second form gives individual block devices as given in the system swap configuration table. The call makes only this space available to the system for swap allocation. SEE ALSO
swapon(2), init(8) FILES
/dev/[ru][pk]?b normal paging devices BUGS
There is no way to stop paging and swapping on a device. It is therefore not possible to make use of devices which may be dismounted dur- ing system operation. swapon is not implemented in 2.11BSD. 4th Berkeley Distribution November 17, 1996 SWAPON(8)