The rupiah (Rp) is the official currency of Indonesia. Issued and controlled by the Bank of Indonesia, the ISO 4217 currency code for the Indonesian rupiah is IDR. The name "Rupiah" is derived from the Indian word rupiya (रुपीया), ultimately from Sanskrit rupyakam (रूप्यकम्; silver). Informally, Indonesians also use the word "perak" ("silver" in Indonesian) in referring to rupiah. The rupiah is subdivided into 100 sen, although inflation has rendered all coins and banknotes denominated in sen obsolete. Introduced in 1946 by Indonesian nationalists fighting for independence, the currency replaced a version of the Netherlands Indies gulden which had been introduced during the Japanese occupation in World War II. In its early years the rupiah was used in conjunction with other currencies, including a new version of the gulden introduced by the Dutch. The Riau islands and the Indonesian half of New Guinea (Irian Barat) had their own variants of the rupiah in the past, but these were subsumed into the national rupiah in 1964 and 1971 respectively (see Riau rupiah and West Irian rupiah).