Something like 
a :<= b 
is an expression, which Mata will evaluate. Perhaps you want  
an assignment on the LHS, e.g. 
c = a :<= b 
Thomas Masterson
A mata question: when I use the function affinity() [code below] I get
the results I expect, but the colon comparison operations result in the
resulting matrix being written to the Stata results window [quite a
nuisance for logging purposes]. Any thoughts why this happens?
Thanks!
Tom
real colvector affinity(real rowvector ai, real matrix aj, real
rowvector sd) {
   real matrix aij, upper, lower
   aij = J(rows(aj), 1, 0)
   upper = J(rows(aj), cols(aj), 0)
   lower = J(rows(aj), cols(aj), 0)
   upper = aj
   printf("upper has %4.0g columns and %9.0g rows\n", cols(upper),
rows(upper))
   lower = aj
   // These two operations print out the entire resulting matrix to the
screen
   upper:<= ai+sqrt(sd)
   lower:>= ai-sqrt(sd)
   upper:& lower
   aij = rowsum(upper)
   printf("aij has %4.0g columns and %9.0g rows\n", cols(aij),
rows(aij))
   return(aij)
}
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