Re: Table[ ...If[...i...] ,{i,...}]

*To*: mathgroup at yoda.physics.unc.edu*Subject*: Re: Table[ ...If[...i...] ,{i,...}]*From*: John Lee <lee at math.washington.edu>*Date*: Sat, 18 Jul 92 11:34:33 -0700

Michael Trott <trott at physik.th-ilmenau.de> writes: Hi mathgroup, why is the i in Table[ v[i]'[t] == If[ x[i]>0,ex1,ex2] , {i,2,6} ] not replaced with the iterator i? thanks The reason is the unconventional way in which If evaluates its arguments. If has the HoldAll attribute, which means that its arguments are not evaluated when the statement is initially encountered, but only under control of the If itself. The result of evaluating an If statement of the form If[condition,ex1,ex1] will be of one of two things: (1) if "condition" evaluates to True or False, then the If statement will return "ex1" or "ex2" as appropriate. (2) If "condition" is neither True nor False, then the If statement remains unevaluated. Case (2) is what's happening in your example: since, at the time your Table statement is executed, Mathematica evidently doesn't know whether x[1]>0, x[2]>0, etc. are True, it simply leaves the If statement as is; this means, in particular, that the current value for i is not substituted either. When the If statement has a third argument, as in If[condition,ex1,ex2,ex3], then it always returns one of ex1,ex2,ex3, with ex3 representing the case in which "condition" is not known to be either True or False. One way to get Mathematica to do what want is this: In[4]:= Table[ v[j]'[t] == If[ x[j]>0,ex1,ex2] /. j->i, {i,2,6} ] Out[4]= {(v[2])'[t] == If[x[2] > 0, ex1, ex2], > (v[3])'[t] == If[x[3] > 0, ex1, ex2], > (v[4])'[t] == If[x[4] > 0, ex1, ex2], > (v[5])'[t] == If[x[5] > 0, ex1, ex2], > (v[6])'[t] == If[x[6] > 0, ex1, ex2]} However, depending on the application you have in mind, it might be more straightforward just to ensure that x[2],...,x[6] have actual values at the time your Table statement is executed, and then use the same Table statement you originally used. Jack Lee Dept. of Mathematics University of Washington Seattle, WA