Why do I get a strange error in a For loop?

Questions
1

Can some one please explain why the first version of this program gets the error:
Set::write: Tag Times in mst Null is Protected. >>
Twice, but the second version works fine?

f[x_] = x^2;
s = x /. Solve[x^2 == x, x]
For[i = 1, i <= Length[s], i++,
 lst = NestList[f, s[[i]] - 0.05, 10];
 Print[lst] 
 mst = NestList[f, s[[i]] + 0.05, 10];
 Print[mst]
 ]

f[x_] = x^2;
s = x /. Solve[x^2 == x, x]
For[i = 1, i <= Length[s], i++,
 lst = NestList[f, s[[i]] - 0.05, 10];
 mst = NestList[f, s[[i]] + 0.05, 10];
 Print[lst]
  Print[mst]
 ]
2

Because you are trying to evaluate the expression

Print[lst] mst = NestList[f, s[[i]] + 0.05, 10];

The left hand side is not a parameter, but rather it is the product of Print[lst] and mst.

3

Thank you; that explains a number of strange errors I’ve gotten. Now my question is how do I tell the processor that they are two statements?

4

Easy! put a semicolon after each complete statement. So instead of

Print[lst] mst = NestList[f, s[[i]] + 0.05, 10];

you will have

Print[lst]; mst = NestList[f, s[[i]] + 0.05, 10];
5

Thank you

6

Dear David
do you know what the problem is when i run the code below :

For[k = 1, k < 11, k++,
If[fitnessfnew[[i, k]] > fitnessfnew[[i - 1, k]], pbest1[[k]] = xx1[[k]]; pbest2[[k]] = xx2[[k]];pbest3[[k]] = xx3[[k]]; pbest4[[k]] = xx4[[k]];]

vv1[[k]] = vv1[[k]] + c1 RandomReal[{0, 1}] (pbest1[[k]] - xx1[[k]]) + c2 RandomReal[{0, 1}] (gbest1 - xx1[[k]]);

vv2[[k]] = vv2[[k]] + c1 RandomReal[{0, 1}] (pbest2[[k]] - xx2[[k]]) + c2 RandomReal[{0, 1}] (gbest2 - xx2[[k]]);

vv3[[k]] = vv3[[k]] + c1 RandomReal[{0, 1}] (pbest3[[k]] - xx3[[k]]) + c2 RandomReal[{0, 1}] (gbest3 - xx3[[k]]);

vv4[[k]] = vv4[[k]] + c1 RandomReal[{0, 1}] (pbest4[[k]] - xx4[[k]]) + c2 RandomReal[{0, 1}] (gbest4 - xx4[[k]]);

xx1[[k]] = xx1[[k]] + vv1[[k]];

xx2[[k]] = xx2[[k]] + vv2[[k]];

xx3[[k]] = xx3[[k]] + vv3[[k]];

xx4[[k]] = xx4[[k]] + vv4[[k]];];

it says :

Set::write: Tag Times in -2.79834 Null is Protected. >>

when i run it part by part with k=1,…,10 it works correctly !!! i really get confused…

7

I would have to see your code in a notebook to be sure, but try putting a; after your For statement and see if that fixes things.