What is i=0 then i-1 would be -1

#1

as shown in this lecture that the formula for sum is cs[j]-cs[i-1] so what of i is 0 then i-1 becomes -1 shouldn’t it give errot?

#2

yes it may give run time error
you have to check it explicitly
like this

if(i==0) 
  sum = cs[j];
else 
  sum = cs[j] - cs[i-1];
#3

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