How can I resolve 1 macro variable but not another?

[benjamin_2018]

I am using a program to download data from the Census API. I can successfully download the data, however, I am receiving a warning message: 

WARNING: Apparent symbolic reference FOR not resolved.

The reason is that the syntax required for the GET request requires a &for clause to define geographic parameters. In the example below, the request is for the total population of Alabama according to 5-year ACS estimates from the 2016-2020 survey. However, I do want to resolve a macro parameter ACS_YEAR in that same URL string. 

%let ACS_YEAR=2020;
filename hdrs TEMP;
filename out TEMP;

proc http
url="https://api.census.gov/data/&ACS_YEAR/acs/acs5?get=NAME,B01001_001E&for=state:01"
method="GET"
out=out
headerout=hdrs;
run;

DATA TEST;
INFILE OUT DLM = ',' FIRSTOBS = 2;
INPUT B01001_001E $ state $;
RUN;

I tried to add %LET FOR=&FOR thinking that FOR would resolve to &FOR but that resulted in an error. 

[ballardw · Accepted solution]

Try

url="https://api.census.gov/data/&ACS_YEAR/acs/acs5?get=NAME,B01001_001E%nrstr(&for)=state:01"

The %nrstr macro function masks special characters  like the & from being processed by the macro processor. Likely could just use the %nrstr(&) but I think including the "for" as well may make it easier to follow later why the mask is needed.

[Tom]

Add macro quoting.

58   %let ACS_YEAR=2020;
59   %put "https://api.census.gov/data/&ACS_YEAR/acs/acs5?get=NAME,B01001_001E%str(&)for=state:01" ;
"https://api.census.gov/data/2020/acs/acs5?get=NAME,B01001_001E&for=state:01"