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CCPR Computing ServicesMore Efficient Programming
July 13, 2006
Outline
Thinking through a programming task Ways of efficiently documenting and organizing your
project Naming variables, programs, files Commenting code Including file header Implementing directory structure
Programming constructs Raw data -> finished product: are your results
replicable?
Before you start coding…
Think Clearly define the problem in writing Write down the solution/algorithm in English
Modularity Create test (if reasonable)
Translate one section to code Test the section thoroughly Translate/Test next section, etc.
Documentation - File Header
Each do-file/program/file you create should include: Your name Project name Project location Date Software Version Purpose of program Inputs, Outputs Special Notes
Naming Files, Variables, and Functions Use language standard (if it exists) Be aware of language-specific rules
Max length, underscore, case, reserved words Differentiating log files:
Programs MergeHH.sas, MergeHH.do Log files MergeHHsas.log, MergeHHsta.log
Meaningful variable names: LogWt vs. var1 AgeLt30 vs. x
Procedure that cleans missing values of Age: fixMissingAge
Matrix multiplication X transpose times X matXX
Commenting Code
Good code is self-commenting Naming conventions, structure/formatting, header should
explain 95% Comments should explain
Purpose of code, not every detail Tricks used Reasons for unusual coding
Comments do not fix sloppy code translate syntax
If it takes longer to read the comment than to read the code, don’t add a comment!
Commenting Code - Stata example
SAMPLE 2*Convert names in dataset to
lowercase.program def lowerVarNames foreach v of varlist _all { local LowName = lower("`v'")
if `"`v'"' != `"`LowName'"' { rename `v' `=lower("`v'")' }
}end
SAMPLE 1program def function1foreach v of varlist _all {local x = lower("`v'")if `"`v'"' != `"`x'"' {rename `v' `=lower("`v'")'}}end
Compare formatting, comments, variable name and function names
Directory Structure
A project consists of many different types of files
Use folders to separate files in a logical way
Be consistent across projects if possible
ATTIC folder for older versions
HOME
PROJECT NAME
DATA
RESULTS
LOG
PROGRAMS
ATTIC
Stata example: using directory structure** Paths:
global parentpath "C:\Documents and Settings\piersol\Summer06\prog\progtips"global pgmsloc "$parentpath\pgms"global logsloc "$parentpath\logs"global cleandataloc "$parentpath\data\clean"global rawdataloc "$parentpath\data\raw"
capture log closelog using "$logsloc\test200607", text replace**********************************************************************INSERT FILE HEADER HERE...then it’s included in log file.*********************************************************************macro list
webuse union, clearsave "$rawdataloc\union.dta", replace
*keep idcode year age gradesave "$cleandataloc\unionLJP.dta", replace
log close
Programming Constructs
Tools to simplify and clarify your coding Available in virtually all languages Constructs
Loops - for, foreach, do, while If/elseif/else– if, then, else, case continue exit
Loop Example 1 Problem: Given 4 indicator variables (south, union, black,
not_smsa) and 2 discrete variables (age, grade), generate 8 new indicator variables:
south_age21 = south and age > 21, south_gr12 = south and grade > 12 Similarly for union, black, not_smsa
Solution without loop 8 lines of code similar to:
generate newvar = (south==1 & age>21 & age<.) generate newvar = (south==1 & grade>12 & grade<.)
Solution with loopforeach j in south union black not_smsa {
gen `j'_age21 = (age>21 & age<. & `j'==1)
gen `j'_gr12 = (grade>12 & grade<. & `j'==1)
}
Loop Example 1, cont.*CHECK GENERATED VARIABLES AGAINST ORIGINAL VARIABLESforeach j in south union black not_smsa { qui count if `j'==1 & age>21 & age<. local origCount = r(N) qui count if `j'_age21==1 if `origCount' ~= `r(N)' { display "Counts do not match for `j'_age21!" } else display "Counts match for `j'_age21."
qui count if `j'==1 & grade>12 & grade<. local origCount = r(N) qui count if `j'_gr12==1 if `origCount' ~= `r(N)' { display "Counts do not match for `j'_gr21!" } else display "Counts match for `j'_gr21."}
Loop Example 2
Given indicator variables white, black, other, and continuous variable educyrs, create interaction variables
Solution using loop:local allraces "white black other"
foreach race of varlist `allraces' {
generate `race'_educ=`race'*educyrs
}
Loop Example 3
Problem: Dataset contains variables over multiple years (1970-1990) Need to perform a number of commands separately for 1970, 1975,
1980, 1985. Solution without loop
bysort year: command1 if year==70 | year==75 | year==80 | year==85bysort year: command2 if year==70 | year==75 | year==80 | year==85
Solution with loopforeach year in 70 75 80 85 { di as result "***Regression for year = `year':" regress ln_wage grade tenure ttl_exp if year==`year' di as result "***Summarize for year = `year':" summarize ln_wage if year==`year'}
Loop Example 4 – pulling from 2 lists From Stata FAQ websiteCode:local agrp "cat dog cow pig"local bgrp "meow woof moo oinkoink"local n : word count `agrp'
forvalues i = 1/`n' { local a : word `i' of `agrp' local b : word `i' of `bgrp' di "`a' says `b'" }Resulting output:cat says meowdog says woofcow says moopig says oinkoink
Constructs - If/then/else Execute section of code if condition is true:
if condition then
{execute this code if condition true}
end
Execute one of two sections of code: if condition then
{execute this code if condition true}
else
{execute this code if condition false}
end
If/Else Example
Problem: need to execute commands on an operating system, but only if the os is Unix…the commands will fail if os is anything else
Solution:if "`c(os)'"~="Unix" { di as err "Sorry; this section requires Unix OS."}else { ** continue with unix commands…}
Constructs - Elseif/case Elseif - Execute one of many sections of code:
if condition1 then{execute this code if condition1 true}
elseif condition2 then{execute this code if condition2 true}
else{execute this code if condition1, condition2 are all false}
end
Case- same idea, different name
case condition1 then{execute this code if condition1 true}
case condition2 then{execute this code if condition2 true}
etc.
Elseif Example
Problem: Continue example from if…else, but execute different section of code for Unix, Windows, and Mac
Solution:if "`c(os)'"=="Unix" {
di "This is a Unix environment"
}
else if "`c(os)'" == "Windows" {
di "This is a Windows environment"
}
else if "`c(os)'" =="MacOSX" {
di "This is a MacOS” environment."
}
else {
di as err "`c(os)' not recognized."
}
Stata- If command vs. if qualifier ifcmd was designed to be used with a single expression Example:
Given variable x with 5 observations: 1, 1, 2, 1, 3, Compare the following three pieces of Stata code:if x==2 { replace x=99}
if x==1 { replace x=99}
replace x=99 if x==2
Stata- If command vs. if qualifier
Constucts -- Continue Example from Stata online help Continue is used to exit current iteration of loop and
continue with next iteration The following two loops produce the same result:
forvalues x = 1/10 { if mod(`x',2)==1 { display "`x' is odd" continue } display "`x' is even"}
forvalues x = 1/10 { if mod(`x',2)==1 { display "`x' is odd" } else { display "`x' is even" }}
Constructs – Exit
Stop execution of program Examples:
Do-file contains a number of data checks followed by analysis commands. If data checks reveal something unacceptable, you can exit out of do-file before running analysis.
Program requires user input. If user enters “bad” information, need to quit program.
Debugging. If particular error occurs then break. Check denominator prior to dividing. If equals zero, exit.
Raw data to finished product
Raw data
Analysis data
Runs/results
Finished product
Raw Data -> Analysis Data
Always have two distinct data files- the raw data and analysis data
A program should completely re-create analysis data from raw data
NO interactive changes!! Final changes must go in a program!!
Raw Data -> Analysis Data
Document all of the following: Outliers? Errors? Missing data? Changes to the data?
Remember to check- Consistency across variables Duplicates Individual records, not just summary stats “Smell tests”
Analysis Data -> Results
All results should be produced by a program Program should use analysis data (not raw) Have a “translation” of raw variable names ->
analysis variable names -> publication variable names
Analysis Data -> Results
Document- How were variances estimated? Why? What algorithms were used and why? Were
results robust? What starting values were used? Was
convergence sensitive? Did you perform diagnostics? Include in
programs/documentation.
Log files
Your log file should tell a story to the reader. As you print results to the log file, include
words explaining the results Include not only what your code is doing, but
your reasoning and thought process Don’t output everything to the log-file- use quietly and noisily in a meaningful way.
Project Clean-up
Create a zip file that contains everything necessary for complete replication
Use a readme.txt file to describe zip contents Delete/archive unused or old files Include any referenced files in zip When you have a final zip archive containing
everything- Open it in it’s own directory and run the script Check that all the results match
Questions/Feedback