sum in matlab coding

what is the difference between

sum(a(1):a(3)) and sum(a(1:3))?

Answer 1

[DGM]

The first case is summing a linear increasing series between two values stored in a vector. The second is the sum of a subvector within a vector.

a = [5 -10 15 -20]

a = 1×4

5 -10 15 -20

a(1):a(3) % linear vector from 5 to 20 in steps of 1

ans = 1×11

5 6 7 8 9 10 11 12 13 14 15

a(1:3) % elements 1 through 3 of the vector a

ans = 1×3

5 -10 15

sum(a(1):a(3))

ans = 110

sum(a(1:3))

ans = 10