About th e submatrix

#1

I dont understand the logic of this and in this what us prefix matrix sum.can h help me to understand completely

#2

hello @manoj2597

first consider 1 d array instead of matrix.

and let say u want to find sum of elements between indices [i…j] .

then we can write this sum as->
sum(i,j) = sum(0,j) - sum(0,i-1) // i.e find sum from index [0 to j] and then subtract sum of [0 to i-1].
the difference will give sum(i,j).

and sum(0,j) or sum(0,i-1) is nothing but prefix sum.
so we can write
sum(i,j)=prefixsum[j]-prefixsum[i-1]

is this logic clear?

#3

can u help me to understand it on google meet

#4

sorry i cant.
i can take ur doubt here only

#5

can you help me to understand it more clearly

#6

u need to find sum of elements from index i to index j.
so first u can find the sum of first j element of array and them subtract sum of first i-1 elements . the resultant will be sum of [i…j].

image

from the yellow portion subtract red portion to get the required sum.

#7

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