contains and indexOf

Learn search with contains and indexOf.

contains and indexOf

To ask whether a substring exists inside a String you use the .contains() method: "welcome".contains("come") evaluates to true. The return type is bool; either true or false.

If you want the position of the substring you use .indexOf(): "welcome".indexOf("come") returns 3. Indexing is zero-based. If the substring is not found .indexOf() returns -1.

-1 when not found

When the substring is not found .indexOf() returns -1. This value is often used to check whether a substring exists: if (email.indexOf("@") != -1) roughly tests whether a text might be an email.

A more readable way is to use .contains(): if (email.contains("@")). Both forms give the same result; .contains() is clearer. Practical rule: use .contains() for existence, .indexOf() for position.

Exercise Find the @ index

Declare a String variable email with the value "alex@dart.com". Declare a new variable at and assign the result of email.indexOf("@") to it. Then print at with print(at);. The output on a single line must be 4.

Show example solution

String email = "alex@dart.com";
var at = email.indexOf("@");
print(at);

Test yourself

What does "welcome".contains("come") return?
```
"welcome".contains("come")
```
- true
- false
- 3
- -1
Show answer

true — "come" exists inside "welcome"; the result is true.
Match each method to its job

contains(x)

Exists (bool)
indexOf(x)

First index (-1 if absent)
substring(0, 3)

Takes a part
.length

Character count

Write the indexOf call

String email = "a@b.com";
print(email____);

Show answer

String email = "a@b.com";
print(email.indexOf("@"));

Next lesson: Summary - Strings

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