Examples > Strings

 

String Substring Function

The String function substring() is closely related to charAt(), startsWith() and endsWith(). It allows you to look for an instance of a particular substring within a given String.

Relevant Groundwork

None

Hardware Required

  • MSP-EXP430G2 LaunchPad

Circuit

Only LaunchPad is required for this example.

image developed using Fritzing. For more circuit examples, see the Fritzing project page.

Schematic

Code Explanation

substring() with only one parameter looks for a given substring from the position given to the end of the string. It expects that the substring extends all the way to the end of the String. For example:

  String stringOne = "Content-Type: text/html";

  // substring(index) looks for the substring from the index position to the end:
  if (stringOne.substring(19) == "html") {
   }

  //is true, while 

  String stringOne = "Content-Type: text/html";

  // substring(index) looks for the substring from the index position to the end:
  if (stringOne.substring(19) == "htm") {
   }

  //is not true, because there's 1 after the htm in the String.

 

substring() with two parameters looks for a given substring from the first parameter to the second. For example:

  String stringOne = "Content-Type: text/html";

  // you can also look for a substring in the middle of a string:
  if (stringOne.substring(14,18) == "text") {

  }

This looks for the word text from positions 14 through 18 of the String.

 

Caution: make sure your index values are within the String’s length or you’ll get unpredictable results. This kind of error can be particularly hard to find with the second instance of substring() if the starting position is less than the String’s length, but the ending position isn’t.

 

Code

/*
  String substring() 
  Examples of how to use substring in a String

  created 27 July 2010
  by Tom Igoe

  Hardware Required:
  * MSP-EXP430G2 LaunchPad

  This example code is in the public domain.
*/

void setup() {
  Serial.begin(9600);
  Serial.println("\n\nString  substring():");
}

void loop() {
  // Set up a String:
  String stringOne = "Content-Type: text/html";
  Serial.println(stringOne);

  // substring(index) looks for the substring from the index position to the end:
  if (stringOne.substring(19) == "html") {
    Serial.println("It's an html file"); 
  } 
  // you can also look for a substring in the middle of a string:
  if (stringOne.substring(14,18) == "text") {
    Serial.println("It's a text-based file"); 
  } 

  // do nothing while true:
  while(true);
}

Working Video

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Try it out:

– Use string comparison to blink an LED when strings are equal

 

See Also

 

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