Mathcad prime3.0中的教程中编写for循环

There are two types of subscript in Mathcad: literal subscripts and vector subscripts. You get a literal subscript with Ctrl -, and it's just part of the variable name. It is cosmetic only. That's what you have in the loop, so vec_i is a different variable to the function argument vec. What you need is a vector subscript, which you get with [. Then i is an index into the variable vec, and the program will replace the first three values with 0.