Question

I just want to know the difference between CompareStr and = for comparing strings in Delphi. Both yield the same result.

if(str2[i] = str1[i]) then
  ShowMessage('Palindrome')

if(CompareStr(str2[i], str1[i]) = 0) then
  ShowMessage('Palindrome')

Both show message Palindrome.

Answer 1

Use CompareStr not when you just want to see whether two strings are equal, but when you want to know how one string compares relative to another. It will return a value less than 0 if the first argument appears first, asciibetically, and it will return a value greater than zero if the first argument belongs after the second.

Without CompareStr, you might have code like this:

if str1[i] = str2[i] then begin
  // They're equal
end else if str1[i] < str2[i] then begin
  // str1 comes first
end else begin
  // str2 comes first
end;

That compares str1 and str2 twice. With CompareStr, you can cut out one of the string comparisons and replace it with a cheaper integer comparison:

x := CompareStr(str1[i], str2[i]);
if x = 0 then begin
  // They're equal
end else if x < 0 then begin
  // str1 comes first
end else begin
  // str2 comes first
end;

As Gerry’s answer explains, the function is particularly useful in sorting functions, especially since it has the same interface as other comparison functions like CompareText and AnsiCompareStr. The sorting function is a template method, and each of the functions serves as a comparison strategy.

If all you want to do is test for equality, use the = operator — it’s easier to read. Use CompareStr when you need the extra functionality it provides.