.data
string1: .asciiz "Please enter a string. Make sure the string only contains RPN characters: "
string2: .asciiz "Invalid input: "
string3: .asciiz "Input is valid.\n"
string4: .asciiz "The first operator is "
string5: .asciiz "No operator found.\n"
string6: .asciiz "Please enter a single interger: \n"
string7: .asciiz "Please enter a single operator (+,-,*,/): \n"
string8: .asciiz "The result is "
input: .space 50
output: .space 3

	.text
	.globl Olu
Olu:
	la	$a0, string1
	li	$v0, 4
	syscall
	
	la	$a0, input
	li	$v0, 8
	syscall

	move $s0, $a0			 #address of input

loop:
	lb $t0, ($s0) 			     # gets a character
	beq $t0, $zero, next		# runs loop till 0 is received
	blt $t0, 48, charCheck	       # checks if char is less than 0
	bgt	$t0, 57, notValid	# checks if char is greater than 9
	add $s0, $s0, 1			 # moves along string
	j loop

notValid:
	la $a0, string2
	li $v0, 4
	syscall

	move $a0, $t0
	li $v0, 11
	syscall
	j next

charCheck:
	bne $t0, 42, notValid		#checks if char is equal to op
	bne $t0, 43, notValid
	bne $t0, 45, notValid
	bne $t0, 47, notValid
	add $s0, $s0, 1	
	j loop

next:
	li $v0, 10
	syscall