Get the first occurrence of a substring within a string in Arduino

The indexOf() function within Arduino scans a string from the beginning, and returns the first index of the specified substring within the string. The syntax is −



where substr is the substring to search for. It can be of type character or string.

Optionally, you can provide a different starting point to begin the search from, in which case, the syntax is −


myString.indexOf(substr, from)

where from is the index from where the search should start. This function returns the index of the first occurrence of the substring within the string, or -1, if no match is found.


void setup() {
   // put your setup code here, to run once:
   String s1 = "Mississippi";
   String substr1 = "is";
   String substr2 = "os";
   Serial.println(s1.indexOf(substr1, 3));
void loop() {
   // put your main code here, to run repeatedly:


The Serial Monitor output is shown below −

As you can see, in the first case, the index 1 was printed (counting starts from 0). In the second case, since no match was found, -1 was printed. In the third case, we told Arduino to start searching from index 3. Therefore, the next match was found at index 4, and that was printed.

Updated on: 29-May-2021


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