how to clear memory in ubuntu 18.04

C++
# clear pagecache only.
sync; echo 1 > /proc/sys/vm/drop_caches

# clear dentries and inodes.
sync; echo 2 > /proc/sys/vm/drop_caches

# clear pagecache, dentries and inodes.
sync; echo 3 > /proc/sys/vm/drop_caches sudo sync; echo 3 > /proc/sys/vm/drop_caches# Clear PageCache, dentries and inodes.
sync; echo 3 > /proc/sys/vm/drop_caches 
sync; echo 1 > /proc/sys/vm/drop_caches
sync; echo 2 > /proc/sys/vm/drop_caches
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