How to resolve dcast error with reshape2 package in R?

Question

I\'m trying to convert some data from long to wide using the reshape2 package, but I\'m getting an error that I haven\'t been able to resolve. In the example below, I\'ve cr

Answer 1

Just put credits in quotation marks:

df.wide = dcast(df, ID ~ subj, value.var="credits", fun.aggregate=sum)
df.wide
   ID CHEM ENGL HIST MATH
1 100    6    3    3    3
2 101    3    6    3    3
3 102    3    3    6    3
4 103    3    3    3    6