four-ops-interpret

Now handle all four operators: +, -, *, /. Try "3+4*5" (left-to-right, no precedence yet -- so 35, not 23). Try "5-3", "4*7", "8/2".

For division, use @divTrunc(a, b) -- Zig wants you to be explicit about how to round.

In the interpreter loop, read the operator first, then dispatch.

while (input_pos < input.len) {
    const op: u8 = input[input_pos];
    input_pos += 1;
    const next_digit: i64 = @as(i64, input[input_pos]) - '0';
    input_pos += 1;

    // YOU: dispatch on op for '+', '-', '*', '/'.
    //      for anything else, printString("error").
}

if (op == '+') {
    result = result + next_digit;
} else if (op == '-') {
    result = result - next_digit;
} else if (op == '*') {
    result = result * next_digit;
} else if (op == '/') {
    result = @divTrunc(result, next_digit);
} else {
    printString("error: unknown operator\n");
    return;
}

9-3-1 is 5 (left-to-right: (9-3)-1). 8/2/2 is 2. Precedence -- where * and / outrank + and - -- comes much later.