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Selecting an Element from a Map

Selecting from an Map: Solution 1

btcdeb "[ 
	

	0x02			# On the stack is the index we want to access
	TOALTSTACK

	IF
		IF
				0x0d 0x04		# Element at index 4
		ELSE
				0x11 0x03		# Element at index 3
		ENDIF
	ELSE
		IF
				0x13 0x02		# Element at index 2
		ELSE
			IF
				0x17 0x01		# Element at index 1
			ELSE
				0x1d 0x00		# Element at index 0
			ENDIF
		ENDIF
	ENDIF


	# Verify that the hint was correct
	FROMALTSTACK
	EQUALVERIFY


	# We're done. The corresponding element is on the top stack

#]" 0x01 0x 		# The hint provides the binary representation of the index

Selecting from an Map: Solution 2

The following script selects an element from a map. The input can select an element from an array with five elements. In this example, the fourth element (the element at index 0x03), is selected. So we will have 23 on the stack.

btcdeb "[ 

	OP_DUP
	OP_0NOTEQUAL OP_NOTIF
		13 0
	OP_ENDIF

	OP_1SUB
	OP_DUP
	OP_0NOTEQUAL OP_NOTIF
		17 0
	OP_ENDIF

	OP_1SUB
	OP_DUP
	OP_0NOTEQUAL OP_NOTIF
		19 0
	OP_ENDIF

	OP_1SUB
	OP_DUP
	OP_NOTIF
		23 0
	OP_ENDIF

	OP_1SUB
	OP_NOTIF
		29
	OP_ENDIF

	OP_NIP

# ]" 0x03

This can be easily generalized for maps of tuples.