Macro works, but error message received.

[Haikuo]

The original discussion was on SAS-L. The OP wanted to replace blanks with ',' in his macro variable strings.

eg. Have: %let a=a b c d; Want a=a,b,c,d.

So I came up with the following Macro:

%macro com (a=);

%let a1=;

%do i=1 %to %sysfunc(countw(&a));

    %let a1=%sysfunc(catx(%str(,),&a1,%scan(&a,&i)));

  %end;

  %let a=&a1;

  %put &a;

%mend;

%com (a=a b c d)

It works for the purpose, but it generate an error message as:

ERROR: %SYSEVALF function has no expression to evaluate.

Please help me to figure it out why.

Thanks in advance,

Haikuo

[Astounding · Accepted solution]

Haikuo,

OK, first I have to comment and then I'll make a guess as to the cause.

This is a very dangerous macro to use.  It is missing this key statement:

%local a1 i;

If I were to call this macro, it would change the value of existing macro variables named &A1 or &I.  Those macro variables might exist in the %GLOBAL symbol table, or they might exist in the %LOCAL symbol table for a macro that calls this macro.  But that is what makes this macro dangerous to use.

My guess as to where the error comes from is the first pass through the %DO loop, when &A1 is null.  To test this (I can't, I'm home today), try these changes.

What's there now:

%let a1=;

%do i=1 %to ...;

Try this instead:

%let a1=%scan(&a,1);

%do i=2 %to ...;

See if the error disappears.

[Haikuo]

Thanks, Astounding. Comments well received. So What it means is that %sysfunc has problem with macro variable of null value, but still process it reluctanly?

[Astounding]

My guess:

%sysfunc in combination with catx is calling %sysevalf in a way that we cannot see. 

It's a sort of a black box.  We can see what goes in and what comes out, but can only guess at the internal workings.

[FriedEgg]

15         %let a=1    2 3  4;

16         %put original -> &a;

original -> 1    2 3  4

17        

18         %let a=%sysfunc(prxchange(%quote(s/\s+/,/),-1,&a));

19         %put prxchange -> &a;

prxchange -> 1,2,3,4

20        

21         %let a=%sysfunc(translate(%quote(&a),'2c'x,'20'x));

22         %put translate -> &a;

translate -> 1,2,3,4

[Ksharp]

Actually. you don't need function catx();

%macro com (a=);

%let a1=%scan(&a,1);

%do i=2 %to %sysfunc(countw(&a));

    %let a1=&a1.,%scan(&a,&i);

  %end;

  %let a=&a1;

  %put &a;

%mend;

%com (a=a b c d)

Ksharp

[Haikuo]

Good call, Ksharp!