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Test for Empty Strings and Missing Values

String arrays can contain both empty strings and missing values. Empty strings contain zero characters and display as double quotes with nothing between them (""). You can determine if a string is an empty string using the == operator. The empty string is a substring of every other string. Therefore, functions such as contains always find the empty string within other strings. String arrays also can contain missing values. Missing values in string arrays display as <missing>. To find missing values in a string array, use the ismissing function instead of the == operator.

Test for Empty Strings

You can test a string array for empty strings using the == operator.

You can create an empty string using double quotes with nothing between them (""). Note that the size of str is 1-by-1, not 0-by-0. However, str contains zero characters.

str = ""
str = 

Create an empty character vector using single quotes. Note that the size of chr is 0-by-0. The character array chr actually is an empty array, and not just an array with zero characters.

chr = ''
chr =

  0x0 empty char array

Create an array of empty strings using the strings function. Each element of the array is a string with no characters.

str2 = strings(1,3)
str2 = 1x3 string
    ""    ""    ""

Test if str is an empty string by comparing it to an empty string.

if (str == "")
    disp 'str has zero characters'
str has zero characters

Do not use the isempty function to test for empty strings. A string with zero characters still has a size of 1-by-1. However, you can test if a string array has at least one dimension with a size of zero using the isempty function.

Create an empty string array using the strings function. To be an empty array, at least one dimension must have a size of zero.

str = strings(0,3)
str = 

  0x3 empty string array

Test str using the isempty function.

ans = logical

Test a string array for empty strings. The == operator returns a logical array that is the same size as the string array.

str = ["Mercury","","Apollo"]
str = 1x3 string
    "Mercury"    ""    "Apollo"

str == ''
ans = 1x3 logical array

   0   1   0

Find Empty Strings Within Other Strings

Strings always contain the empty string as a substring. In fact, the empty string is always at both the start and the end of every string. Also, the empty string is always found between any two consecutive characters in a string.

Create a string. Then test if it contains the empty string.

str = "Hello, world";
TF = contains(str,"")
TF = logical

Test if str starts with the empty string.

TF = startsWith(str,"")
TF = logical

Count the number of characters in str. Then count the number of empty strings in str. The count function counts empty strings at the beginning and end of str, and between each pair of characters. Therefore if str has N characters, it also has N+1 empty strings.

str = 
"Hello, world"
ans = 12
ans = 13

Replace a substring with the empty string. When you call replace with an empty string, it removes the substring and replaces it with a string that has zero characters.

ans = 
"Hello, "

Insert a substring after empty strings using the insertAfter function. Because there are empty strings between each pair of characters, insertAfter inserts substrings between each pair.

ans = 
"-H-e-l-l-o-,- -w-o-r-l-d-"

In general, string functions that replace, erase, extract, or insert substrings allow you to specify empty strings as the starts and ends of the substrings to modify. When you do so, these functions operate on the start and end of the string, and between every pair of characters.

Test for Missing Values

You can test a string array for missing values using the ismissing function. The missing string is the string equivalent to NaN for numeric arrays. It indicates where a string array has missing values. The missing string displays as <missing>.

To create a missing string, convert a missing value using the string function.

str = string(missing)
str = 

You can create a string array with both empty and missing strings. Use the ismissing function to determine which elements are strings with missing values. Note that the empty string is not a missing string.

str(1) = "";
str(2) = "Gemini";
str(3) = string(missing)
str = 1x3 string
    ""    "Gemini"    <missing>

ans = 1x3 logical array

   0   0   1

Compare str to a missing string. The comparison is always 0 (false), even when you compare a missing string to another missing string.

str == string(missing)
ans = 1x3 logical array

   0   0   0

To find missing strings, use the ismissing function. Do not use the == operator.

See Also

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