Does Stata have an Eval Function?

When working with string expressions in Stata, it's important to recognize that Stata does not directly evaluate mathematical operations performed on string values. Instead, you need to break down the string, convert it to a numeric value, and then perform calculations. This process can be broken down into several steps, including the use of the substring and destring commands.

Breaking Down String Expressions

A common scenario involves encountering string expressions that look like '22'. These express natural numbers, but Stata requires these to be in numeric form to perform any arithmetic operations. To achieve this, you first need to extract the relevant parts of the string and then use the appropriate Stata commands to convert them into numeric values.

Identify the String Expression:

Let's consider a string such as '22'. This is a straightforward string representing a numerical value.

Use the substring Command:

The substring command can be used to extract characters from a string. For example, to locate the portion of the string that represents the number:

. local varname "22". display `"`varname' are `  substring("`varname'",1,1)' plus `  substring("`varname'",2,2)'"'

Here, we're extracting both characters from the string '22' and assigning them to separate variables.

Concatenate the Extracted Values:

Now that we have the individual characters, we can concatenate them to form a numeric value. This can be achieved by joining the characters together:

. local numvar : display ".0fc" `var1'`var2'. display "Numeric value: `numvar'"

Note the use of the `numvar' variable, which now holds the numeric value derived from the string.

Convert to Numeric Using destring:

With the string converted to numeric, you can now proceed to perform arithmetic operations using the numeric data type:

. destring `numvar', replace

This command converts the string to numeric and updates the original variable.

Why Avoiding Eval is Safer

It's important to note that the use of an eval function in Stata (or any other programming language) is generally discouraged. This is because eval can introduce security risks, as it allows for dynamic evaluation of arbitrary expressions, which can be exploited if the input is not properly sanitized. Instead, Stata's built-in commands like destring and substring offer a safer and more straightforward approach to parsing and manipulating string values to perform mathematical operations.

For instance, consider the following example:

. local expression "22   33". di (`expression')

Running this code will result in an error, as Stata cannot evaluate the string directly. Instead, you can handle the string expression manually using the above-mentioned commands:

. local var1 "22". local var2 "33". local result : display ".0fc" `var1'   `var2'. di "`result'"

This manual approach ensures that the operations are performed safely and securely, without exposing your code to potential security risks.

Conclusion

In summary, Stata does not directly evaluate mathematical operations when provided with string values. To perform arithmetic on string expressions, it is essential to use a combination of string manipulation commands like substring and destring. While the eval function may seem convenient, its use can pose security risks. By adhering to safer practices, you can effectively parse and manipulate string expressions while ensuring the robustness and security of your code.