Navigate to the relevant doc page, in this case:

http://shoup.net/ntl/doc/ZZ_pX.cpp.htmlScroll down a few lines, and voila:

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Accessing coefficients

The degree of a polynomial f is obtained as deg(f),

where the zero polynomial, by definition, has degree -1.

A polynomial f is represented as a coefficient vector.

Coefficients may be accesses in one of two ways.

The safe, high-level method is to call the function

coeff(f, i) to get the coefficient of X^i in the polynomial f,

and to call the function SetCoeff(f, i, a) to set the coefficient

of X^i in f to the scalar a.

One can also access the coefficients more directly via a lower level

interface. The coefficient of X^i in f may be accessed using

subscript notation f[i]. In addition, one may write f.SetLength(n)

to set the length of the underlying coefficient vector to n,

and f.SetMaxLength(n) to allocate space for n coefficients,

without changing the coefficient vector itself.

After setting coefficients using this low-level interface,

one must ensure that leading zeros in the coefficient vector

are stripped afterwards by calling the function f.normalize().

NOTE: the coefficient vector of f may also be accessed directly

as f.rep; however, this is not recommended. Also, for a properly

normalized polynomial f, we have f.rep.length() == deg(f)+1,

and deg(f) >= 0 => f.rep[deg(f)] != 0.

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