下推自动机：
下推自动机 (PDA) 是具有附加堆栈存储的有限状态机。除了输入符号和当前状态之外，额外的堆栈用于做出转换的决定。它包含以下 7 个元组：
有限自动机：
有限自动机是任何机器的数学模型，我们可以通过它计算每个输入符号的状态转换。有限自动机中的每个转换取决于输入符号和当前转换状态。它包含以下 5 个元组：
让我们看看下推自动机和有限自动机之间的区别：
| S.NO | Pushdown automata | finite automata |
|---|---|---|
| 1. | For Type-2 grammar we can design pushdown automata. | For Type-3 grammar we can design finite automata. |
| 2. | Non – Deterministic pushdown automata has more powerful than Deterministic pushdown automata. | Non-Deterministic Finite Automata has same powers as in Deterministic Finite Automata. |
| 3. | Not every Non-Deterministic pushdown automata is transformed into its equivalent Deterministic pushdown Automata . | Every Non-Deterministic Finite Automata is transformed into an equivalent Deterministic Finite Automata |
| 4. | Context free languages can be recognized by pushdown automata. | Regular languages can be recognized by finite automata. |
| 5. | Pushdown automata has the additional stack for storing long sequence of alphabets. | Finite Automata doesn’t has any space to store input alphabets. |
| 6. | It gives acceptance of input alphabhets by going up to empty stack and final states. | It accepts the input alphabets by going up to final states. |