New 103 For-Loops material

-Added discussion of varlist/numlist
-Discussion of difference btw foreach in vs. of
-Eliminated overlap material from 102, made minor reference/link changes as a result

Conflicts:
	Stata 103/Pseudo-SMCL/Concepts/For-Loops and Macros.do
	Stata 103/SMCL/Concepts/For-Loops and Macros.smcl
	Stata 103/Stata 103.zip

Stata 103/Do/Concepts/For-Loops and Macros.do

@@ -7,6 +7,61 @@ if `example' == 1 {
 	if `trace' set trace on
 	noi {
 
+foreach var of varlist _all {
+    display "`var'"
+}
+
+	}
+	if `trace' set trace off
+}
+else if `example' == 2 {
+	if `trace' set trace on
+	noi {
+
+foreach var in _all {
+    display "`var'"
+}
+
+	}
+	if `trace' set trace off
+}
+else if `example' == 3 {
+	if `trace' set trace on
+	noi {
+
+foreach i of numlist 10(5)50 {
+    display "`i'"
+}
+
+	}
+	if `trace' set trace off
+}
+else if `example' == 4 {
+	if `trace' set trace on
+	noi {
+
+foreach num in 1 2 3 4 5 {
+    display "`num'"
+}
+
+	}
+	if `trace' set trace off
+}
+else if `example' == 5 {
+	if `trace' set trace on
+	noi {
+
+foreach num of numlist 1/5 {
+    display "`num'"
+}
+
+	}
+	if `trace' set trace off
+}
+else if `example' == 6 {
+	if `trace' set trace on
+	noi {
+
 foreach var in sex age educ {
     display "Checking `var' for missing values..."
     list hhid `var' if `var == .
@@ -15,7 +70,7 @@ foreach var in sex age educ {
 	}
 	if `trace' set trace off
 }
-else if `example' == 2 {
+else if `example' == 7 {
 	if `trace' set trace on
 	noi {
 
@@ -27,7 +82,7 @@ foreach var in sex age educ {
 	}
 	if `trace' set trace off
 }
-else if `example' == 3 {
+else if `example' == 8 {
 	if `trace' set trace on
 	noi {
 
@@ -39,7 +94,7 @@ foreach var in sex age educ {
 	}
 	if `trace' set trace off
 }
-else if `example' == 4 {
+else if `example' == 9 {
 	if `trace' set trace on
 	noi {
 

Stata 103/Pseudo-SMCL/Concepts/For-Loops and Macros.do

@@ -4,144 +4,112 @@
 
 {USE} 
 /* 
-{view `"{LOOPS-}##introduction"':1. Introduction}{BR}
+{view `"{LOOPS-}##introduction"':1. Variants on foreach}{BR}
 {view `"{LOOPS-}##tracing"':2. Tracing}{BR}
 {view `"{LOOPS-}##macros"':3. More on Macros}{BR}
 {view `"{LOOPS-}##loops"':4. More Loops}
 
-{hline}{marker introduction}
+{hline}{marker variants}
 
-{bf:1. Introduction}
+{bf:1. Variants on foreach}
 
 {hline}
 
-Let's do a quick review of for-loops and macros. Here is an example of a for-loop: 
+In {bf:Stata 102}, we introduced the {bf:foreach in} loop, which you
+should be familiar with. A simple example: 
 
 {TRYITCMD}
-foreach i of numlist 1/10 {{BR}
-	display "Hello world!"{BR}
+foreach x in a b c {{BR}
+	display "`x'"{BR}
 }
 {DEF}
 
-This repeats the commands enclosed by the braces ({cmd}{ }{txt})
-(here, just {cmd:display "Hello world!"})
-{cmd:10} times. It's preferable to typing the command 10 times: */
-
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-display "Hello world!"
-
-/* The for-loop was less code and more efficient,
-and it's easier to update:
-if I want to switch {cmd:"Hello world!"} to something else
-or make a larger change,
-I have to do it just once within the loop instead of 10 times.
-The loop is also less error-prone,
-because repetitively typing out the same thing can lead to mistakes.
-Indeed, there are 11 {cmd:display}s above when I meant 10.
-
-{marker style}A quick detour into style.
-Notice that after the left brace ({cmd}{{txt}), I indented once.
-The code stayed indented until the right brace ({cmd}}{txt}).
-This formatting makes the code more readable;
-it's easy to tell what's in the loop and what's not.
+In the above loop, each item following "in" is substituted into the macro `x'. 
+However, there are several very useful variants on this syntax, used
+when dealing with lists of variables or numbers. 
 
-{TECH}
-{COL}For more tips on Stata programming style, see these resources, available on{CEND}
-{COL}Box:{CEND}
-{BLANK}
-{COL}{browse "http://www.stata-journal.com/sjpdf.html?articlenum=pr0018":{it:Suggestions on Stata programming style}}{CEND}
-{BLANK}
-{COL}{browse "http://faculty.chicagobooth.edu/matthew.gentzkow/research/ra_manual_coding.pdf":{it:RA Manual: Notes on Writing Code}}{CEND}
-{BOTTOM}
+{hline}{marker varlist}
 
-For-loops and macros are distinct but related concepts.
-Every for-loop is associated with its own macro, but not every macro is associated with a for-loop.
-The loop above was associated with a local macro ({cmd:`i'}),
-though the macro wasn't used inside the loop.
-Let's look at an example of {helpb foreach:foreach}
-that incorporates its macro within the loop:
+Say we want to run a loop for 20 variables, all listed consecutively (var1, var2, ... var20).
+If you used {bf:foreach in}, you would have to list them all manually.
+However, when using {bf:foreach of varlist}, you can simply tell Stata
+to use the 1st through 20th variables, only listing those two with a dash in between.
+Thus, this code would tabulate all twenty variables: 
 
-{TRYITCMD}
-foreach var in sex age educ {
-	display "Checking `var' for missing values..."
-	list hhid `var' if `var' == .
+{CMD}			
+foreach i of varlist var1-var20 {{BR}
+	tabulate `i'{BR}
 }
 {DEF}
 
-This {cmd:foreach} command loops over the variables {cmd:sex}, {cmd:age}, and {cmd:educ}.
-In this dataset, these variables should never have missing values.
-For each variable, the loop first displays the variable name as part of a message
-({cmd:"Checking `var' for missing values..."}).
-It then lists all values of {cmd:hhid} for which the variable is missing.
-Knowing these, we can then examine the problematic observations more closely.
+If you had tried using this shorthand with {cmd:foreach in}
+Stata would get confused and would try to look for a variable that was named "var1-var20",
+and would just tell you the loop is impossible to run because that variable doesn’t exist.
+  
+Here's another very useful example. Compare the output of: */
 
-This {cmd:foreach} repeatedly sets the local macro {cmd:`var'} to each element in
-the list that follows {cmd:in}, executing the commands in the loop for each value of {cmd:`var'}.
-{cmd:var} is a name I made up, and I could have chosen something else.
+foreach var of varlist _all {
+	display "`var'"
+}
 
-{marker whats_in_a_name}{...}
-{TECH}
-{COL}{bf:What's in a name?}{CEND}
-{MLINE}
-{COL}The same rules generally apply to all Stata names, whether it's of a{CEND}
-{COL}variable, macro, or something else. From {helpb varname:help varname}:{CEND}
-{BLANK}
-{COL}"Variable names may be 1 to 32 characters long and must start with {cmd:a-z}, {cmd:A-Z},{CEND}
-{COL}or {cmd:_}, and the remaining characters may be {cmd:a-z}, {cmd:A-Z}, {cmd:_}, or {cmd:0-9}."{CEND}
-{BLANK}
-{COL}The exception to this rule is local macros, which may be 1 to 31 characters{CEND}
-{COL}long (a character shorter), but may start with {cmd:0-9}. Global macros (which{CEND}
-{COL}we will discuss later) follow the same naming conventions as variables.{CEND}
-{BOTTOM}
+*and:
+
+foreach var in _all {
+	display "`var'"
+}
 
-The first element in the list is {cmd:"sex"}.
-Thus, the local macro {cmd:`var'} is first set to {cmd:"sex"}.
-The commands in the loop are then executed:
+/* Note that there are many ways to specify a list of variables in Stata. See
+this very useful {helpb varlist:help file} for more. In particular, notice
+the use of special characters such as "*" to indicate a certain subset
+of variables. Can you think of a time when you might use them?
 
-{CMD}
-display "Checking {ul:`var'} for missing values..."{BR}
-list hhid {ul:`var'} if {ul:`var'} == .
-{DEF}
+{hline}{marker numlist}
 
-The value of {cmd:`var'} is immediately substituted for {cmd:`var'} in the commands,
-so this is the same as:
+When we want to loop over a list of numbers, {cmd:foreach of numlist} is a great
+shorthand. 
 
-{CMD}
-display "Checking {ul:sex} for missing values..."{BR}
-list hhid {ul:sex} if {ul:sex} == .
+Let's look at the following loop: 
+
+{TRYITCMD}
+foreach i of numlist 1/10 {{BR}
+	display "`i'"{BR}
+}
 {DEF}
 
-After {cmd:list},
-all the commands in the for-loop have been executed,
-so {cmd:`var'} is set to the next element in the list: {cmd:"age"} and then finally {cmd:"educ".}
+A numlist ("number list") is a list of numbers specified in shorthand.
+The shorthand used above is the one you'll see most frequently.
+The / symbol means "to" so 1/10 means 1 to 10:  1/10 is shorthand for 1 2 3 4 5 6 7 8 9 10.
+Another useful shorthand allows you to move from one number to another in steps of a third number.
+Multiples of a number follow this pattern. For example: */
 
-Don't think of {cmd:display} as searching for and then accessing the contents of the local macro {cmd:`var'}.
-Instead, the value of {cmd:`var'} is essentially "copied and pasted" into the command before it is executed.
-Stata completes the substitution, replacing {cmd:`var'} with its value,
-and {it:then} {cmd:display} is executed.
-{cmd:display} has no idea that a local macro was used,
-because the substitution was done before the command was executed.
+foreach i of numlist 10(5)50 {
+	display "`i'"
+}
+
+/*#1(#d)#2 means "#1 to #2 in steps of #d". Thus, 10(5)50 means 10 to 50 in steps of 5,
+in other words, all the multiples of 5 between 10 and 50, inclusive.
+These aren't all the numlist shortcuts: see the {help numlist:help file} for more.
+
+{hline}
 
-In Stata, "macros" are objects that can be substituted into commands in this way.
-They are not variables;
-that term is used in reference to the variables in the dataset,
-which have as many values as there are observations.
+{bf:Note:} Almost all Stata users will at some point get mixed up between {bf:in} and {bf:of}
+when using the {cmd:foreach} command. Remember that when using {bf:in}, you must spell
+out all of the items that will be pasted into the resulting macro. Using {bf:of}, 
+when combined with {bf:varlist} or {bf:numlist}, allows you to use some shorthand
+specifically when the items that you want to loop through are either all variables
+or all numbers. 
 
-Some additional terminology: {help quotes:"double quotes"} ({cmd:""}) are used to enclose strings.
-For example, {cmd:"abc"}, {cmd:"123"}, and {cmd:"Hello world"} are each a single string enclosed by double quotes.
-"Single quotes" ({cmd:`} and {cmd:'}) is the term for the characters used to enclose locals,
-such as {cmd:`var'}.
+Note how the output of the following two loops are the same: */
+
+foreach num in 1 2 3 4 5 {
+	display "`num'"
+}
+
+foreach num of numlist 1/5 {
+	display "`num'"
+}
 
-{hline}{marker tracing}
+/*{hline}{marker tracing}
 
 {bf:2. Tracing}
 
@@ -150,8 +118,7 @@ such as {cmd:`var'}.
 Loops can be confusing, and they're also difficult to debug,
 because for a loop that results in an error,
 Stata doesn't clearly indicate which command in the loop is causing the problem.
-For example, here's the loop from above,
-but with a bug introduced: */
+For example, here's a loop with a bug introduced: */
 
 foreach var in sex age educ {
 	display "Checking `var' for missing values..."
@@ -352,7 +319,7 @@ foreach var of varlist _all {{BR}
 
 display "The dataset has `i' variables."
 
-/* {...}
+/*{...}
 {TECH}
 {COL}{bf:More on macros}{CEND}
 {MLINE}
@@ -372,11 +339,9 @@ display "The dataset has `i' variables."
 {hline}
 
 {cmd:foreach} isn't the only loop or even the only for-loop in
-Stata. If you're not already familiar with the very useful variants on {cmd:foreach in},
-{cmd:foreach of varlist} and {cmd:foreach of numlist}, check out {bf:Stata 102} for examples. 
-{helpb forvalues} loops over certain {it:numlists}, and especially abbreviated as
-{cmd:forv} or {cmd:forval}, is easier to type than {cmd:foreach of numlist}. The {helpb while} loop, 
-is a more advanced, but useful loop that we'll look at below: 
+Stata. For instance, {helpb forvalues} loops over certain {it:numlists}, and especially abbreviated as
+{cmd:forv} or {cmd:forval}, is easier to type than {view `"{LOOPS-}##numlist"':foreach of numlist}.
+The {helpb while} loop is a more advanced, but useful loop that we'll look at below: 
 
 Instead of designating text/variables/numbers to substitute within the loop, 
 {cmd:while} simply sets a condition that must be met for the loop to run. 
@@ -389,8 +354,8 @@ while x = true {
 }
 {DEF}
 
-Try this loop (with a local thrown in for good measure) if you feel comfortable with loops so far.
-If you are already confused, it might be best to come back to it later – it’s a bit more advanced. */  
+Try this loop (with a local thrown in for good measure) if you feel comfortable
+with loops so far – it’s a bit more advanced. */  
 
 local i = 1
 while  `i' < 15 {

Stata 103/Pseudo-SMCL/Concepts/if.do

@@ -129,8 +129,7 @@ foreach var of varlist _all {
 
 These are the observation numbers of the two observations with {cmd:hhid "1802011"}.
 
-Touching on {view `"{LOOPS-}##style"':style} again,
-notice above that for every left brace, I indented once.
+Touching on style, notice above that for every left brace, I indented once.
 My code stays indented until the right brace.
 Thus, it's easy to tell what's in the loop and what's not, and what's in the {cmd:if} block and what's not. */