Desmos traps: “why is it not working ?”

It’s not uncommon to have either unexpected results or puzzling error messages in Desmos. Here are some classical examples and solutions.

Misleading functions

  • Desmos log means log10. Not to be confused with ln , that also be noted log_e.
  • Desmos stdev means the statistical estimator, not to be confused with the real standard deviation of a set, noted stdevp. ( stdev = sqrt(N/(N-1))*stdevp ).

 Misleading span of variables

  • There is no local variable. Sum_n prevents you to use n anywhere else again (and the error message will be puzzling)… but in other sums.
  • Lists are kind of vectors, but with common alignment for all expressions in your session. Don’t expect to enlarge the combinatory by passing an extra vector where a constant was expected.

Ambiguous syntax

  • The syntax for tracing the locus where a variable or function equals something looks a lot like what you would do for defining it, and Desmos easily think it’s a second definition of the same thing, which it allows… as long as its not used in another expression (then you’ll get a puzzling error message).→To avoid f(x) = 3 ambiguity, either trace 3 = f(x) or f(x)+0 = 3
  • Conditions look like a separate statement, but indeed it’s part of the expression.That’s why ( x, y ) { cond } can’t work, for a point is not a value. Use (x, y { cond } ) .

Copy-pasting

  • From Desmos Graph to exterior, you will always get LaTeX form of equations. So Desmos can be used as a LaTeX editor, but you can’t get simple text version.
  • From outside text to Desmos Graph: it works… but for operators like sqrt, |.|, {.}, sum …Indeed Desmos except some LaTeX source, and just extend syntax to some text names like sin so that simple text expressions accidentally work. Conversely x_a = \sum{\sqrt{x}} works, or any complex LaTeX formula.But \{, that block the pasting.

Out of conceptual space

Many limitations can be twisted, like drawing in 3D, making random numbers, or emulating vector algebra. But some concepts remain definitively out of reach:

  • In fact all expressions are precomputed by Desmos. So don’t expect to reproduce really evolving states, like dynamics simulation.Even recursivity won’t work, for conditions are just functions like the others, rather than algorithmic structures.
  • There is no way you can get the inverse of a function. But you can trace it.
  • or between conditions is near impossible, appart duplicating the function and applying the alternate condition (for plotting)  or putting the whole content in piecewise form (for definitions): f(x) = { cond1: val, cond2: same-val }
    (and can be obtained with the m < x < M syntax for simple cases or by nesting { cond } statements.
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