please explain how comparator is used for lower_bound in money change problem discussed in the video…and why do we decrease the lb variable by 1 in the code.
Money change problem
Hey @kani001
we have changed our comparator to make it work like upper bound.
thats why it will always return index of element whose value is greater than the searched key.
and to get the required value , we need to do -1.
Refer to the above code t see the value lower bound is returning .
If we dont use comparator, then the lower bound function gives the first value in the sorted array that is greater than or equal to the value to be found i.e it will ignore all values smaller than the value to be found. But, when we define a comparator same as defined in the lecture, the lower bound function ignores all the value smaller than or equal to the value to be found and hence will always return the index of key which has value greater than the value to be found. Hence we will have to subtract 1 from index in order to get the index of key which has value smaller than or equal to the required value.
Just try in your compiler for different values you will understand
using a<=b, comparator function returns true for all elements <= value to be found. When the comparator function returns false at that time we see that the current index might be the answer. If we dont use comparator function, it returns true only for elements < value to be found.
instead of using lower_bound with comparator can we use upper_bound then?
Yes you can 
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