st: problem with addition in var generation

[Andrew Hovel <hovel.andrew@gmail.com>]

Hello All,

I am having issues with Stata performing simple addition when I create a
variable. I am using Stata 12 for windows.

The issue is straight-forward on the face of it: Stata is adding
incorrectly.

Background:
 I am trying to construct pseudo fips codes by adding the tract code to the
5 digit state-county fips. For instance, 06037 denotes Los Angeles County,
CA, so I would add 6037000000 to the six-digit tract fips code to form a 10
digit code-- 11 digits for states later in the alphabet with fips id's
above 10.
I have two variables formatted %15.0f, fip11 and tract. tract is 6 digits
long and uniquely identifies the Census tracts in my dataset.

.gen fip11 = 0
.replace fip11 = 6001000000 + tract if T_COUNTY == "001"
.replace fip11 = 6037000000 + tract if T_COUNTY == "037"
.replace fip11 = 6073000000 + tract if T_COUNTY == "073"

When I run this, I get something like this:
-tract-        -fip11-
400200     6001400320
400500     6001400320
400400     6001400320
400300     6001400320

This happens for others in ascending order.

I noticed a trend in the way they are adding. Cases repeat values for fip11
usually about 4 or 5 times in a row. After 4 or 5 cases, the value
increases by 512 each time.

For example:

-tract6-       -fip11-
400300    6001400320
[3 cases]
400100    6001400320
400900    6001400832
[3 cases]
400600    6001400832
401300    6001401344
[3 cases]
401400    6001401344
401800    6001401856
[3 cases]
401600    6001401856
402400    6001402368
[3 cases]
402200    6001402368
402900    6001402880

Has anyone seen this problem before? I have had this issue in the past. I
am guessing I must have some sort of setting wrong to cause the addition to
occur like this.
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