Subscript range issue when doing increasing for loop

[#1]

I got error "Subscript Range in access or evaluation of 'collist[ /*###*/i]' , collist[/*###*/i]" when doing a simple new column () with increasing for loop. I able to solve it using decreasing for loop. However, I want to have created column ordered follow increasing for loop. I dont understand what causes the error when using increasing for loop here. Can someone provide suggestion how I can use increasing for loop here? if possible, can someone help explain why the error will happen for increasing for loop?

path = "my file path with filename.jmp";

dt= open(path);
collist = dt <<get column names(string);
colname = Column("test1") <<getname;
colnumber = (Loc(collist,colname))[1];

For(i=colnumber+1, i<=Ncols(dt),i++,  //give the error
//For(i=Ncols(dt), i>colnumber+1,i--,  fixed issue but created column order inverted

dt  <<new column("SG_"||collist[i], character);

);

[#2 · Accepted solution]

Hi,

I think that your problem is that N Cols (dt) increases after each iteration of your loop. Therefore, your index value "i" exceeds the number of elements in your collist list.

I suggest capturing the number of columns in dt outside the loop and using it as a fixed limit. 

path = "my file path with filename.jmp";

dt= open(path);
collist = dt <<get column names(string);
colname = Column("test1") <<getname;
colnumber = (Loc(collist,colname))[1];
num_col = N Cols (dt);

For(i=colnumber+1, i<=num_col,i++,  
   dt  <<new column("SG_"||collist[i], character);

);

Of note, I am unsure why the reverse loop works. You it should run into a similar issue with your collist variable.

Let us know if that solve your issue.

Best,

TS

[#3]

Hi TS,

Yes, the error go away. However, I always use i<= N Cols (dt) and no issue at all. I dont understand why in this situation the same method failed?

[#4]

@Thierry_S  the reverse loop captures ncols(dt) at the start of the loop, exactly once.

@dadawasozo 

• the reverse loop isn't quite the same because it uses >colnumber+1 and probably should be >=colnumber+1 to match the non-working forward loop.
• In the forward loop, ncols(dt) is re-evaluated each time it loops and keeps getting bigger, but collist is not getting bigger, so eventually the subscript fails.

I like @Thierry_S  solution.