The Excel Hlookup function 'looks up' a given value in the top row of a data array (or table), and returns the corresponding value from another row of the array.
The syntax of the function is:
Where the function arguments are as follows:
lookup_value    The value that you want to search for, in the first row of the supplied data array.  
table_array    The data array or table, containing the data to be searched in the top row, and the return values in any other row.  
row_index_num    The row number, within the supplied table_array, that you want the corresponding value to be returned from.  
[range_lookup]    An optional logical argument, which can be set to TRUE or FALSE, meaning:  

Wildcards
In textrelated Hlookups, when the [match_type] argument is set to 0, the lookup_value can contain the following wildcard characters:
?  matches any single character
*  matches any sequence of characters
Cells A2F6 of the spreadsheet below, show the exam scores for 5 students in 4 different subjects. If you want to look up a specific score (e.g. Biology) for one of the students (e.g. Ed), this can be done using the Hlookup function, as shown in cell B10 of the spreadsheet.
Formulas:  Results: 
In the above example, the Hlookup function searches through the top row of the table_array (the range A2F2), to find a match for the lookup_value (the name "Ed"). When the the name "Ed" is found, the function returns the corresponding value from the 5th row of the table_array.
This is illustrated in the above spreadsheet on the right. The function finds the name 'Ed' in the top row of the table_array and then returns the value '61%' from the 5th row of the table_array.
If we change the name in cell A10 of the spreadsheet from 'Ed' to 'Cara', the Hlookup function would automatically recalculate the function to display the exam result for Cara.
Note that although the top row of the table_array is in ascending order in this example, this is not essential, because the [range_lookup] argument is set to FALSE (requiring an exact match).
Cells A1F3 of the following spreadsheet show body types relating to body mass index (BMI), for the ranges 018.4, 18.524.9, 25.029.9 and over 30.
Cell C6 shows the user's current BMI, which is 23.5, and cell C7 shows the Hlookup function that is used to look up the body type that relates to this BMI.
The Hlookup function in the above spreadsheet returns the result "Normal Weight", which is the correct body type for a BMI of 23.5.
Note that, in this example, the [range_lookup] argument is set to TRUE, to tell that function that, if it cannot find an exact match to the supplied lookup_value, it should use the closest match below this value. Therefore, for all BMIs up to and including 18.4 the function would return "Underweight", for all BMIs between 18.5 and 24.9, the function would return "Normal Weight", etc.
For a practical example of the HLOOKUP function being used to create a variable dropdown list, see the Variable DropDown List page.
Also, further examples are provided on the Microsoft Office website.
If you get an error from the Excel Hlookup function this is likely to be one of the following:
#N/A    Occurs if the Hlookup function fails to find a match to the supplied lookup_value The cause of this will generally depend on the supplied [range_lookup]:  
 
#REF!    Occurs if the supplied row_index_num argument is greater than the number of rows in the supplied table_array.  
#VALUE!    Occurs if either:
