[gradsusr] Proble with max function in a script

[juan pablo sierra perez]

Hello
I am writing a script and I want to extract the maximum value of a variable and save it like a single number in a single variablethe code is showed below:
'mmax=max(max('var', lat=-15, lat=20), lon=-90, lon=0)''mmin=min(min('var', lat=-15, lat=20), lon=-90, lon=0)'dif=mmax-mmin
the "dif" variable have an error, it says: Non numeric  args ti numeric operation
I have tried defining the variables mmax and mmin but it doesnt work, what can I do?
Than you very much in advance 
Juan 		 	   		  
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