TheAlgorithms/Python · warning · ValueError

Initial orbit radius must be greater than zero.

Error message

Initial orbit radius must be greater than zero.

What it means

Raised in the __main__ interactive block of physics/orbital_transfer_work.py when the initial orbit radius entered at the prompt is <= 0. Note the surrounding CLI code is buggy: r1 is prompted inside 'if M <= 0:' and prompted a second time later (lines 56 and 66), so the validated value is overwritten by an unvalidated re-read. The exception is caught locally and printed as 'Input error: ...'.

Source

Thrown at physics/orbital_transfer_work.py:58

    work = (gravitational_constant * mass_central * mass_object / 2) * (
        1 / r_initial - 1 / r_final
    )
    return f"{work:.3e}"


if __name__ == "__main__":
    import doctest

    doctest.testmod()
    print("Orbital transfer work calculator\n")

    try:
        M = float(input("Enter mass of central body (kg): ").strip())
        if M <= 0:
            r1 = float(input("Enter initial orbit radius (m): ").strip())
        if r1 <= 0:
            raise ValueError("Initial orbit radius must be greater than zero.")

        r2 = float(input("Enter final orbit radius (m): ").strip())
        if r2 <= 0:
            raise ValueError("Final orbit radius must be greater than zero.")
        m = float(input("Enter mass of orbiting object (kg): ").strip())
        if m <= 0:
            raise ValueError("Mass of the orbiting object must be greater than zero.")
        r1 = float(input("Enter initial orbit radius (m): ").strip())
        r2 = float(input("Enter final orbit radius (m): ").strip())

        result = orbital_transfer_work(M, m, r1, r2)
        print(f"Work done in orbital transfer: {result} Joules")

    except ValueError as e:
        print(f"Input error: {e}")

View on GitHub (pinned to f5988cc097)

Solutions

  1. Enter a strictly positive initial radius such as 7000000 (meters) when prompted
  2. Prefer importing the function (from physics.orbital_transfer_work import orbital_transfer_work) and validating inputs yourself rather than using the buggy CLI
  3. If you maintain this file, remove the duplicated r1/r2 reads (lines 66-67) and the misplaced 'if M <= 0' so each value is read and checked exactly once

Example fix

# before (module __main__ block)
M = float(input(...))
if M <= 0:
    r1 = float(input(...))
if r1 <= 0:
    raise ValueError("Initial orbit radius must be greater than zero.")
...
r1 = float(input(...))  # re-read overwrites the validated value

# after
M = float(input("Enter mass of central body (kg): ").strip())
r1 = float(input("Enter initial orbit radius (m): ").strip())
if r1 <= 0:
    raise ValueError("Initial orbit radius must be greater than zero.")
Defensive patterns

Strategy: validation

Validate before calling

def read_positive(prompt):
    while True:
        v = float(input(prompt).strip())
        if v > 0:
            return v
        print("Value must be greater than zero.")

r1 = read_positive("Enter initial orbit radius (m): ")

Try / catch

try:
    ...
except ValueError as e:
    print(f"Input error: {e}")  # the script already catches and prints; keep prompts positive

Prevention

When it happens

Trigger: Running the module directly (python physics/orbital_transfer_work.py), entering a central mass M > 0 (so the first r1 prompt is skipped and r1 is undefined until line 66), then entering 0 or a negative number for the second r1 prompt at line 66 — this raises before the print, but note that r1 from line 66 is checked by the line 57 test against the OLD r1.

Common situations: Users running the script interactively and entering an altitude or negative value; entering a positive M, which skips the first r1 read and makes the flow depend on the buggy re-read; CI invoking the script with piped stdin containing invalid radii.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/49d104351fbb0b2d. Report an issue: GitHub.