Text.IndexOf

Summary#

Text.IndexOf(s, sub) returns the zero-based index of the first ordinal occurrence of sub in s, or -1 when sub does not occur — exactly C#'s string.IndexOf. The 3-arg overload Text.IndexOf(s, sub, startIndex) begins the search at startIndex and runs forward, matching C#'s IndexOf(value, startIndex).

Signature#

Text.IndexOf(<string> s, <string> sub) -> int
Text.IndexOf(<string> s, <string> sub, <int> startIndex) -> int

Description#

Matching is ordinal (byte-for-byte, culture-independent), the same as Contains, StartsWith, EndsWith's Contains. A miss returns -1. Text.LastIndexOf (Text.LastIndexOf) is the backward-search sibling.

The 3-arg startIndex is the C# contract: it is the first position considered, and the search runs forward from it. startIndex may run from 0 to the string's length inclusive — a start exactly at the end is legal and simply finds nothing — and anything outside that range faults rather than clamping.

⚠ Dropping the startIndex is not a simplification. The whole point of the overload is "find the next one after the one I already found", so a search that restarts at 0 answers a position the caller has already passed — and code that then compares that position against where it was looking takes the wrong branch on every input, silently.

Text.IndexOf is in-memory only — like Text.Reverse, it has no faithful SQL form, so it may be called on locals inside a function body but not pushed down into a query predicate.

Examples#

string SecondField(string line) {
  var first = Text.IndexOf(line, ",");
  if (first < 0) {
    return "";
  }
  var second = Text.IndexOf(line, ",", first + 1);   // resume AFTER the one just found
  if (second < 0) {
    return Text.Substring(line, first + 1);
  }
  return Text.Substring(line, first + 1, second - first - 1);
}
// SecondField("a,b,c")  ->  "b"

See also#

• Text.LastIndexOf — the backward-search sibling, with the same startIndex contract
• Contains, StartsWith, EndsWith — Contains / StartsWith / EndsWith, when the POSITION is not needed