Lesson 4: Assignment Statements and Numeric Functions
Once you've read your data into a SAS data set, surely you want to do something with it. A common thing to do is to change the original data in some way in an attempt to answer a research question of interest to you. You can change the data in one of two ways: (1) You can use a basic assignment statement in which you add some information to all of the observations in the data set. Some assignment statements may take advantage of the numerous SAS functions that are available to make programming certain calculations easier (e.g., taking an average). (2) Alternatively, you can use an if-then-else statement to add some information to some but not all of the observations. In this lesson, we will learn how to use assignment statements and numeric SAS functions to change your data. In the next lesson, we will learn how to use if-then-else statements to change a subset of your data.
Modifying your data may involve not only changing the values of a particular variable, but also the type of the variable. That is, you might need to change a character variable to a numeric variable. For that reason, we'll investigate how to use the INPUT function to convert character data values to numeric values. (We'll learn how to use the PUT function to convert numeric values to character values in Stat 481 when we study character functions in depth.)
Learning objectives & outcomes
Upon completing this lesson, you should be able to do the following:
- write a basic assignment statement involving a numeric variable
- write an assignment statement that involves an arithmetic calculation
- write an assignment statement that utilizes one of the many numeric SAS functions that are available
- know how SAS handles missing values for various arithmetic calculations and functions
- write an assignment statement involving nested functions
- write a basic assignment statement involving a character variable
- use the INPUT function to convert character data values to numeric values
Our "to do" list for this lesson
In order to complete the lesson you should:
- Read the lesson pages that follow.
- Type up your answers to the homework problems in a Word file named homework04_yourPSUloginid. By now you should be used to the format. If your PSU login id is xyz123, then name your file homework04_xyz123. Upload the file to the Lesson #4 Homework Dropbox.
- Post any questions or comments you have concerning the lesson's material to the Lesson #4 General Discussion Board.
- Take the Lesson #4 Mastery Quiz.
4.1 - Assignment Statement Basics
The fundamental method of modifying the data in a data set is by way of a basic assignment statement. Such a statement always takes the form:
variable = expression;
where variable is any valid SAS name and expression is the calculation that is necessary to give the variable its values. The variable must always appear to the left of the equal sign and the expression must always appear to the right of the equal sign. As always, the statement must end with a semicolon (;).
Because assignment statements involve changing the values of variables, in the process of learning about assignment statements we'll get practice with working with both numeric and character variables. We'll also learn how using numeric SAS functions can help to simplify some of our calculations.
Example 4.1. Throughout this lesson, we'll work on modifying various aspects of the temporary data set grades that is created in the following DATA step:
The data set contains student names (name), each of their four exam grades (e1, e2, e3, e4), their project grade (p1), and their final exam grade (f1).
A couple of comments. For the sake of the examples that follow, we'll use the DATALINES statement to read in the data. We could have just as easily used the INFILE statement. Additionally, for the sake of ease, we'll create temporary data sets rather than permanent ones. Finally, after each SAS DATA step, we'll use the PRINT procedure to print all or part of the resulting SAS data set for your perusal.
Example 4.2. The following SAS program illustrates a very simple assignment statement in which SAS adds up the four exam scores of each student and stores the result in a new numeric variable called examtotal.
Note that, as previously described, the new variable name examtotal appears to the left of the equal sign, while the expression that adds up the four exam scores (e1+e2+e3+e4) appears to the right of the equal sign.
Launch and run the SAS program. Review the output from the PRINT procedure to convince yourself that the new numeric variable examtotal is indeed the sum of the four exam scores for each student appearing in the data set. Also note what SAS does when it is asked to calculate something when some of the data are missing. Rather than add up the three exam scores that do exist for John Simon, SAS instead assigns a missing value to his examtotal. If you think about it, that's a good thing! Otherwise, you'd have no way of knowing that his examtotal differed in some fundamental way from that of the other students. The important lesson here is to always be aware of how SAS is going to handle the missing values in your data set when you perform various calculations!
Example 4.3. In the previous example, the assignment statement created a new variable in the data set by simply using a variable name that didn't already exist in the data set. You need not always use a new variable name. Instead, you could modify the values of a variable that already exists. The following SAS program illustrates how the instructor would modify the variable e2, say for example, if she wanted to modify the grades of the second exam by adding 8 points to each student's grade:
Note again that the name of the variable being modified (e2) appears to the left of the equal sign, while the arithmetic expression that tells SAS to add 8 to the second exam score (e2+8) appears to the right of the equal sign. In general, when a variable name appears on both sides of the equal sign, the original value on the right side is used to evaluate the expression. The result of the expression is then assigned to the variable on the left side of the equal sign.
Launch and run the SAS program. Review the output from the print procedure to convince yourself that the values of the numeric variable e2 are indeed eight points higher than the values in the original data set.
4.2 - Arithmetic Calculations Using Arithmetic Operators
All we've done so far is add variables together. Of course, we could also subtract, multiply, divide or exponentiate variables. We just have to make sure that we use the symbols that SAS recognizes. They are:
|Operation||Symbol||Assignment Statement||Action Taken|
|addition||+||a = b + c;||add b and c|
|subtraction||-||a = b - c;||subtract c from b|
|multiplication||*||a = b * c;||multiply b and c|
|division||/||a = b / c;||divide b by c|
|exponentiation||**||a = b ** c;||raise b to the power of c|
|negative prefix||-||a = -b;||take the negative of b|
As is the case in other programming languages, you can perform more than one operation in an assignment statement. The operations are performed as they are for any mathematical expression, namely:
- exponentiation is performed first, then multiplication and division, and finally addition and subtraction
- if multiple instances of addition, multiple instances of subtraction, or addition and subtraction appear together in the same expression, the operations are performed from left to right
- if multiple instances of multiplication, multiple instances of division, or multiplication and division appear together in the same expression, the operations are performed from left to right
- if multiple instances of exponentiation occur in the same expression, the operations are performed right to left
- operations in parentheses are performed first
It's that last bullet that I think is the most helpful to know. If you use parentheses to specifically tell SAS what you want calculated first, then you needn't worry as much about the other rules. Let's take a look at two examples.
Example 4.4. The following example contains a calculation that illustrates the standard order of operations. Suppose a statistics instructor calculates the final grade by weighting the average exam score by 0.6, the project score by 0.2, and the final exam by 0.2. The following SAS program illustrates how the instructor (incorrectly) calculates the students' final grades:
Well, okay, so the instructor should stick to statistics and not mathematics. As you can see in the assignment statement, the instructor is attempting to tell SAS to average the four exam scores by adding them up and dividing by 4, and then multiplying the result by 0.6. Let's see what SAS does instead. Launch and run the SAS program, and review the output to see if you can figure out what SAS did, say, for the first student Alexander Smith. If you're still not sure, review the rules for the order of the operations again. The rules tell us that SAS first:
- takes Alexander's first exam score 78 and multiples it by 0.6 to get 46.8
- takes Alexander's fourth exam score 69 and divides it by 4 to get 17.25
- takes Alexander's project score 97 and multiplies it by 0.2 to get 19.4
- takes Alexander's final exam score 80 and multiplies it by 0.2 to get 16.0
Then, SAS performs all of the addition:46.8 + 82 + 86 + 17.25 + 19.4 + 16.0
to get his final score of 267.45. Now, maybe that's a final score that Alexander wants, but it is still fundamentally wrong. Let's see if we can help set the statistics instructor straight by taking advantage of that last rule that says operations in parentheses are performed first.
Example 4.5. The following example contains a calculation that illustrates the standard order of operations. Suppose a statistics instructor calculates the final grade by weighting the average exam score by 0.6, the project score by 0.2, and the final exam by 0.2. The following SAS program illustrates how the instructor (correctly) calculates the students' final grades:
Let's dissect the calculation of Alexander's final score again. The assignment statement for final tells SAS:
- to first add Alexander's four exam scores (78, 82, 86, 69) to get 315
- and then divide that total 315 by 4 to get an average exam score of 78.75
- and then multiply the average exam score 78.75 by 0.6 to get 47.25
- and then take Alexander's project score 97 and multiply it by 0.2 to get 19.4
- and then take Alexander's final exam score 80 and multiply it by 0.2 to get 16.0
Then, SAS performs the addition of the last three items:47.25 + 19.4 + 16.0
to get his final score of 82.65. There, that sounds much better. Sorry, Alexander.
Launch and run the SAS program to see how we did. Review the output from the print procedure to convince yourself that the final grades have been calculated as the instructor wishes. By the way, note again that SAS assigns a missing value to the final grade for John Simon.
In this last example, we calculated the students' average exam scores by adding up their four exam grades and dividing by 4. We could have instead taken advantage of one of the many numeric functions that are available in SAS, namely that of the MEANfunction.
4.3 - Numeric Functions
Just as is the case for other programming languages, such as C++ or S-Plus, a SAS function is a pre-programmed routine that returns a value computed from one or more arguments. The standard form of any SAS function is:functionname(argument1, argument2,…);
For example, if we want to add three variables, a, b and c, using the SAS function SUM and assign the resulting value to a variable named d, the correct form of our assignment statement is:
d = sum(a, b, c) ;
In this case, sum is the name of the function, d is the target variable to which the result of the SUM function is assigned, and a, b, and c are the function's arguments. Some functions require a specific number of arguments, whereas other functions, such as SUM, can contain any number of arguments. Some functions require no arguments. As you'll see in the examples that follow, the arguments can be variable names, constants, or even expressions.
SAS offers arithmetic, financial, statistical and probability functions. There are far too many of these functions to explore them all in detail, but let's take a look at some examples.
Example 4.6. In the previous example, we calculated students' average exam scores by adding up their four exam grades and dividing by 4. Alternatively, we could use the MEANfunction. The following SAS program illustrates the calculation of the average exam scores in two ways — by definition and by using the MEAN function:
Launch and run the SAS program. Review the output from the PRINT procedure to convince yourself that the two methods of calculating the average exam scores do indeed yield the same results:
Oooops! What happened? SAS reports that the average exam score for John Simon is 82 when the average is calculated using the MEAN function, but reports a missing value when the average is calculated using the definition. If you study the results, you'll soon figure out that when calculating an average using the MEAN function, SAS ignores the missing values and goes ahead and calculates the average based on the available values.
We can't really make some all-conclusive statement about which method is more appropriate, as it really depends on the situation and the intent of the programmer. Instead, the (very) important lesson here is to know how missing values are handled for the various methods that are available in SAS! We can't possibly address all of the possible calculations and functions in this course. So ... you would be wise to always check your calculations out on a few representative observations to make sure that your SAS programming is doing exactly as you intended. This is another one of those good programming practices to jot down.
Although you can refer to SAS Help and Documentation (under "functions, by category") for a full accounting of the built-in numeric functions that are available in SAS, here is a list of just some of the numeric functions that can be helpful when performing statistical analyses:
|INT: the integer portion of a numeric value||a = int(x);|
|ABS: the absolute value of the argument||a = abs(x);|
|SQRT: the square root of the argument||a = sqrt(x);|
|MIN: the minimum value of the arguments||a = min(x, y, z);|
|MAX: the maximum value of the arguments||a = max(x, y, z);|
|SUM: the sum of the arguments||a = sum(x, y, z);|
|MEAN: the mean of the arguments||a = mean(x, y, z);|
|ROUND: round the argument to the specified unit||a = round(x, 1);|
|LOG: the log (base e) of the argument||a = log(x);|
|LAG: the value of the argument in the previous observation||a = lag(x);|
|DIF: the difference between the values of the argument in the current and previous observations||a = dif(x);|
|N: the number of non-missing values of the argument||a = n(x);|
|NMISS: the number of missing values of the argument||a = nmiss(x);|
I have used the INT function a number of times when dealing with numbers whose first few digits contain some additional information that I need. For example, the area code in this part of Pennsylvania is 814. If I have phone numbers that are stored as numbers, say, as 8142341230, then I can use the INT function to extract the area code from the number. Let's take a look at an example of this use of the INT function.
Example 4.7. The following SAS program uses the INT function to extract the area codes from a set of ten-digit telephone numbers:
In short, the INT function returns the integer part of the expression contained within parentheses. So, if the phone number is 8145562314, then int(phone/10000000) becomes int(814.5562314) which becomes, as claimed, the area code 814. Now, launch and run the SAS program, and review the output from the PRINT procedure to convince yourself that the area codes are calculated as claimed.
Example 4.8. One really cool thing is that you can nest functions in SAS (as you can in most programming languages). That is, you can compute a function within another function. When you nest functions, SAS works from the inside out. That is, SAS performs the action in the innermost function first. It uses the result of that function as the argument of the next function, and so on. You can nest any function as long as the function that is used as the argument meets the requirements for the argument.The following SAS program illustrates nested functions when it rounds the students' exam average to the nearest unit:
For example, the average of Alexander's four exams is 78.75 (the sum of 78, 82, 86, and 69 all divided by 4). Thus, in calculating avg for Alexander, 78.75 becomes the argument for the ROUND function. That is, 78.75 is rounded to the nearest one unit to get 79. Launch and run the SAS program, and review the output from the PRINT procedure to convince yourself that the exam averages avg are rounded as claimed.
4.4 - Assigning Character Variables
So far, all of our examples have pertained to numeric variables. Now, let's take a look at adding a new character variable to your data set or modifying an existing characteric variable in your data set. In the previous lessons, we learned how to read the values of a character variable by putting a dollar sign ($) after the variable's name in the INPUT statement. Now, you can update a character variable (or create a new character variable!) by specifying the variable's values in an assignment statement.
Example 4.9. When creating a new character variable in a data set, most often you will want to assign the values based on certain conditions. For example, suppose an instructor wants to create a character variable called status which indicates whether a student "passed" or "failed" based on their overall final grade. A grade below 65, say, might be considered a failing grade, while a grade of 65 or higher might be considered a passing grade. In this case, we would need to make use of an if-then-else statement. We'll learn more about this kind of statement in the next lesson, but you'll get the basic idea here. The following SAS program illustrates the creation of a new character variable called status using an assignment statement in conjunction with an if-then-else statement:
Launch and run the SAS program. Review the output from the PRINT procedure to convince yourself that the values of the character variable status have been assigned correctly. As you can see, to specify a character variable's value using an assignment statement, you enclose the value in quotes. Some comments:
- You can use either single quotes or double quotes. Change the single quotes in the above program to double quotes, and re-run the SAS program to convince yourself that the character values are similarly assigned.
- If you forget to specify the closing quote, it is typically a show-stopper as SAS continues to scan the program looking for the closing quote. Delete the closing quote in the above program, and re-run the SAS program to convince yourself that the program fails to accomplish what is intended. Check your log window to see what kind of a warning statement is generated.
4.5 - Converting Data
Suppose you are asked to calculate sales income using the price and the number of units sold. Pretty straightforward, eh? As long as price and units are stored in your data set as numeric variables, then you could just use the assignment statement:sales = price * units;
It may be the case, however, that price and units are instead stored as character variables. Then, you can imagine it being a little odd trying to multiply price by units. In that case, the character variables price and units first need to be converted to numeric variables price and units. How SAS helps us do that is the subject of this section. To be specific, we'll learn how the INPUT function converts character values to numeric values.
The reality though is that SAS is a pretty smart application. If you try to do something to a character variable that should only be done to a numeric variable, SAS automatically tries first to convert the character variable to a numeric variable for you. The problem with taking this lazy person's approach is that it doesn't always work the way you'd hoped. That's why, by the end of our discussion, you'll appreciate that the moral of the story is that it is always best for you to perform the conversions yourself using the INPUT function.
Example 4.11. The following SAS program illustrates how SAS tries to perform an automatic character-to-numeric conversion of standtest and e1, e2, e3, and e4 so that arithmetic operations can be performed on them:
Okay, first note that for some crazy reason all of the data in the data set have been read in as character data. That is, even the exam scores (e1, e2, e3, e4) and the standardized test scores (standtest) are stored as character variables. Then, when SAS goes to calculate the average exam score (avg), SAS first attempts to convert e1, e2, e3, and e4 to numeric variables. Likewise, when SAS goes to calculate a new standardized test score (std), SAS first attempts to convert standtest to a numeric variable. Let's see how it does. Launch and run the SAS program, and before looking at the output window, take a look at the log window. You should see something that looks like this:
The first NOTE that you see is a standard message that SAS prints in the log to warn you that it performed an automatic character-to-numeric conversion on your behalf. Then, you see three NOTES about invalid numeric data concerning the standtest values 1,210, 1,010, and 1,180. In case you haven't figured it out yourself, it's the commas in those numbers that is throwing SAS for a loop. In general, the automatic conversion produces a numeric missing value from any character value that does not conform to standard numeric values (containing only digits 0, 1, ..., 9, a decimal point, and plus or minus signs). That's why that fifth NOTE is there about missing values being generated. The output itself:
shows the end result of the attempted automatic conversion. The calculation of avg went off without a hitch because e1, e2, e3, and e4 contain standard numeric values, whereas the calculation of std did not because standtest contains nonstandard numeric values. Let's take this character-to-numeric conversion into our own hands.
Example 4.12. The following SAS program illustrates the use of the INPUT function to convert the character variable standtest to a numeric variable explicitly so that an arithmetic operation can be performed on it:
The only difference between the calculation of std here and of that in the previous example is that the standtest variable has been inserted here into the INPUT function. The general form of the INPUT function is:INPUT(source, informat)
- source is the character variable, constant or expression to be converted to a numeric variable
- informat is a numeric informat that allows you to read the values stored in source
In our case, standtest is the character variable we are trying to convert to a numeric variable. The values in standtest conform to the comma5. informat, and hence its specification in the INPUT function.
Let's see how we did. Launch and run the SAS program, and again before looking at the output window, take a look at the log window. You should see something that now looks like this:
Ahhaa! No warnings about SAS taking over our program and performing automatic conversions. That's because we are in control this time! Now, looking at the output:
we see that we successfully calculated std this time around. That's much better!
A couple of closing comments. First, I might use our discussion here to add another item to your growing list of good programming practices. Whenever possible, make sure that you are the one who is in control of your program. That is, know what your program is doing at all times, and if it's not doing what you'd expect it to do for all situations, then rewrite it in such a way to make sure that it does.
Second, you might be wondering "geez, we just spent all this time talking about character-to-numeric conversions, but what happens if I have to do a numeric-to-character conversion instead?" Don't worry ... SAS doesn't let you down. If you try to do something to a character variable that should only be done to a numeric variable, SAS automatically tries first to convert the character variable to a numeric variable. If that doesn't work, then you'll want to use the PUT function to convert your numeric values to character values explicitly. We'll address the PUT function in Stat 481 when we learn about character functions in depth.
4.6 - Summary
In this lesson, we learned how to write basic assignment statements, as well as use numeric SAS functions, in order to change the contents of our SAS data set. One thing you might have noticed is that almost all of our examples involved assignment statements that changed every observation in our data set. There may be situations, however, when you don't want to change every observation, but rather want to change just a subset of observations, those that meet a certain condition. To do so, you have to use if-then-else statements, which we'll learn about in the next lesson. In doing so, we'll also learn a few more good programming practices.
The homework for this lesson will give you practice with assignment statements and numeric functions so that you become even more familiar with how they work and can use them in your own SAS programming.
An assignment statement gives a value to a variable. For example,
gives the value 5.
The value of a variable may be changed. For example, if has the value 5, then the assignment statement
will give the value 6.
The general syntax of an assignment statement is
- the must be declared;
- the may be a simple name, or an indexed location in an array, or a field (instance variable) of an object, or a static field of a class; and
- the expression must result in a value that is compatible with the type of the . In other words, it must be possible to cast the expression to the type of the variable.
An assignment "statement" is not really a statement (although it is typically used that way), but is an expression. The value of the expression is the value that is assigned to the variable. For example, the expression
sets all of , , and to zero.