Exercise 1: RPN Calculator - Hints

Hint 1: Algorithm overview

RPN uses a stack:

1. Split input by whitespace
2. For each token:
   • If it's a number, push it onto the stack
   • If it's an operator, pop two operands, apply operator, push result
3. At the end, stack should have exactly one value

#![allow(unused)]
fn main() {
let mut stack: Vec<i32> = Vec::new();
}

Hint 2: Splitting the input

Use split_whitespace() to tokenize:

#![allow(unused)]
fn main() {
for token in expr.split_whitespace() {
    // process token
}
}

Hint 3: Parsing numbers

Use parse() with error handling:

#![allow(unused)]
fn main() {
if let Ok(num) = token.parse::<i32>() {
    stack.push(num);
}
}

Hint 4: Handling operators

Match on the operator and pop two values:

#![allow(unused)]
fn main() {
match token {
    "+" => {
        let b = stack.pop().ok_or("Stack underflow")?;
        let a = stack.pop().ok_or("Stack underflow")?;
        stack.push(a + b);
    }
    // ... other operators
}
}

Note: Order matters! For a - b, you pop b first, then a.

Hint 5: Final validation

After processing all tokens, check the stack:

#![allow(unused)]
fn main() {
if stack.len() == 1 {
    Ok(stack[0])
} else if stack.is_empty() {
    Err("Empty expression".to_string())
} else {
    Err("Too many operands".to_string())
}
}