How to use string in if statement?

When using comparisons with strings, you need to use the string comparison operator $==. For example

whichmodel = "4f_2c";

if (whichmodel $== "2f_2c");
    print "We found a match!";
else;
    print "No match this time";
endif;

You can use the or statement with the string comparisons like this

whichmodel = "4f_2c";

if (whichmodel $== "2f_2c") or (whichmodel $== "4f_2c");
    print "We found a match!";
else;
    print "No match this time";
endif;

However, if your list of items gets longer, I would recommend using the GAUSS contains function. contains takes two inputs: haystack and needle. It answers the question "Can 'needle' be found in haystack. Let's modify your example to use contains

// Use the vertical string concatenation operator
// $| to make a vector of strings
models = "2f_2c" $| "4f_2c" $| "2f_4c";
whichmodel = "4f_2c";

// Does 'models' contain the value of 'whichmodel'
if contains(models, whichmodel);
    print "We found a match!";
else;
    print "No match this time";
endif;