Why this isn't a valid argument?

Question

I am having truble understanding why F1 produces an error, since both time and velocity are the same kind of object:

time = Table[i, {i, 1, 10}]

velocity = Table[0, {i, 1, Length[time]}]

F1[t_, v_, h_] := 
Do[v[[i + 1]] = v[[i]] + t[[i]], {i, 1, Length[t] - 1}]
F1[time, velocity, 1]

F2[t_, h_] := 
Do[velocity[[i + 1]] = velocity[[i]] + t[[i]], {i, 1, Length[t] - 1}]
F2[time, 1]
velocity


Out[253]= {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

Out[254]= {0, 0, 0, 0, 0, 0, 0, 0, 0, 0}

During evaluation of In[253]:= Set::setps: {0,0,0,0,0,0,0,0,0,0} in the part assignment is not a symbol. >>

During evaluation of In[253]:= Set::setps: {0,0,0,0,0,0,0,0,0,0} in the part assignment is not a symbol. >>

During evaluation of In[253]:= Set::setps: {0,0,0,0,0,0,0,0,0,0} in the part assignment is not a symbol. >>

During evaluation of In[253]:= General::stop: Further output of Set::setps will be suppressed during this calculation. >>

Out[259]= {0, 1, 3, 6, 10, 15, 21, 28, 36, 45}

and would be ofcourse thankful for any solutions which include keeping the more general F1 over F2.

Thank you!

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You are attempting to make an assignment to the argument of a function, which won't work as written. Take a look at this FAQ for a complete explanation and possible workarounds: Attempting to make an assignment to the argument of a function. — MarcoB, Jun 05 '15 at 14:57
The list[[i]]= form is not usually what you want to use. Imitating FORTRAN in Mathematica is possible, but it makes solving your problem more difficult. For this kind of problem, I generally write a function that performs a single time step, something like onestep[{t_, ...}]:={t+1, ...}, and then get the answer by applying onestep to an initial condition, {0, ...} using NestList. — John Doty, Jun 05 '15 at 17:26

Basheer Algohi · Accepted Answer · 2015-06-05T15:23:23.833

The problem is that the F1 substitutes its argument in the right hand side with out any hold.

Try this:

SetAttributes[F1, HoldAll];
F1[t_, v_, h_] := 
 Do[v[[i + 1]] = v[[i]] + t[[i]], {i, 1, Length[t] - 1}];
F1[time, velocity, 1];
velocity
(*{0, 1, 3, 6, 10, 15, 21, 28, 36, 45}*)

By the way, I am not sure what you are looking after but you can do this calculations as follows:

velocity[[2 ;;]] = Accumulate[time][[;; -2]];
velocity
(*{0, 1, 3, 6, 10, 15, 21, 28, 36, 45}*)

Why this isn't a valid argument?

1 Answers1