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Rooted by: Searching substring inside another string
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  • Algorithms
    Logic
    Search
    Strings
    Data Types
    Code 
    fun findFirstSubString(string: String, subString: String): Int? =
    if (subString.isEmpty()) null
    else string.indexOf(subString).let { if (it == -1) null else it }
     Test Cases 
    import org.junit.Assert
import org.junit.Test

class SimpleSubStringSearchKtTest {

    @Test
    fun `Substring longer than string`() {
        Assert.assertNull(findFirstSubString("12345", "123456"))
        Assert.assertNull(findFirstSubString("54321", "123456"))
    }

    @Test
    fun `Empty string`() {
        Assert.assertNull(findFirstSubString("", "123456"))
    }

    @Test
    fun `Empty substring`() {
        Assert.assertNull(findFirstSubString("12323123", ""))
    }

    @Test
    fun `Empty substring and string`() {
        Assert.assertNull(findFirstSubString("", ""))
    }

    @Test
    fun `Correct finding`() {
        Assert.assertEquals(findFirstSubString("12345", "123"), 0)
        Assert.assertEquals(findFirstSubString("54321321", "321"), 2)
        Assert.assertEquals(findFirstSubString("123456789ABCDE123456789", "ABCDE"), 9)
        Assert.assertEquals(findFirstSubString("123456789ABCDE123456789", "A"), 9)
        Assert.assertEquals(findFirstSubString("123456789ABCDE123456789", "a"), null)
        Assert.assertEquals(findFirstSubString("111234", "1234"), 2)
    }
}
    View Run Output
  • Code
    • fun findFirstSubString(string: String, subString: String): Int? =
    • if (subString.isEmpty()) null
    • else subString.toRegex().find(string)?.range?.first
    • if (subString.isEmpty()) null
    • else string.indexOf(subString).let { if (it == -1) null else it }