Error when plotting `sum(i for i in range(x))`

[Georgi Guninski]

sage: def f1(x): return sum(i for i in range(x))
sage: f1(20)
190
sage: pl1=plot(f1,2,4);pl1
verbose 0 (3965: plot.py, generate_plot_points) WARNING: When
plotting, failed to evaluate function at 200 points.
verbose 0 (3965: plot.py, generate_plot_points) Last error message:
''float' object cannot be interpreted as an integer'

[dmo...@deductivepress.ca]

The error message is correct -- you should not expect to be able to plot this function, because it is only defined for integer values of x, not all x between 2 and 4.  For example:

sage: f1(2.1)
<snip>
TypeError: 'sage.rings.real_mpfr.RealLiteral' object cannot be interpreted as an integer

[kcrisman]

If you want to plot a list of points, there are several ways to do so, including points() and list_plot().

def f1(x): return sum(i for i in range(x))

L = [(n,f1(n)) for n in [2..4]]
list_plot(L) # or points(L)