How to Fix Evaluate RPN Errors

In this tutorial, you'll learn about How to Fix Evaluate RPN Errors. We cover key concepts, practical examples, and best practices.

Fix evaluate rpn errors when operand order reversed for subtraction/division or eval() is dangerous.

Quick Fix

Wrong

def eval_rpn(t):
    st=[]
    for x in t:
        if x in '+-*/':
            a=st.pop(); b=st.pop()
            st.append(eval(f'{b}{x}{a}'))

eval() insecure. Operand order: b-a != a-b.

Right

def eval_rpn(t):
    st=[]
    ops={'+':lambda a,b:a+b,'-':lambda a,b:a-b,
         '*':lambda a,b:a*b,'/':lambda a,b:int(a/b)}
    for x in t:
        if x in ops:
            a=st.pop(); b=st.pop()
            st.append(ops[x](b,a))
        else: st.append(int(x))
    return st[0]

['10','6','9','3','+','-11','*','/','*','17','+','5','+'] -> 22. O(n).

Prevention

Order: b (earlier) OP a (later). Use int(a/b) for truncation toward zero.

DodaTech Tools

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