# Computing - Syntax Diagrams

> Source: https://ollybritton.com/notes/a-level/computing/topics/syntax-diagrams/ · Updated: 2021-01-22 · Tags: computing, regular-languages

See also:
- [Computing - Backus-Naur Form](https://ollybritton.com/notes/a-level/computing/topics/backus-naur-form/)
- [Computing - Parsing](https://ollybritton.com/notes/a-level/computing/topics/parsing/)

### 2021-01-22
##### What is a syntax diagram??
A graphical equivalent of BNF.

##### ![PHOTO SYNTAX DIAGRAM TERMINAL SYMBOL](syntax-diagram-terminal-symbol.png) What does this represent in a syntax diagram??
A terminal symbol.

##### ![PHOTO SYNTAX DIAGRAM NON TERMINAL SYMBOL](syntax-diagram-non-terminal-symbol.png) What does this represent in a syntax diagram??
A non-terminal symbol.

##### ![PHOTO SYNTAX DIAGRAM REPEATED NON TERMINAL SYMBOL](syntax-diagram-repeated-non-terminal-symbol.png) What does this represent in a syntax diagram??
A non-terminal symbol that can be used more than once.

##### Can you picture what a non-zero digit would look like in a syntax diagram??
![PHOTO SYNTAX DIAGRAM NON ZERO DIGIT](syntax-diagram-non-zero-digit.png)

##### Can you picture what an integer would look like in a syntax diagram, made from a "non-zero-digit" and a "digit" category??
![PHOTO SYNTAX DIAGRAM INTEGER](syntax-diagram-integer.png)

##### ![PHOTO SYNTAX DIAGRAM DIGIT](syntax-diagram-digit.png) What does the fork on the left hand side of this syntax diagram mean??
A digit can either be a non-zero digit or a zero.

##### ![PHOTO SYNTAX DIAGRAM DIGIT](syntax-diagram-digit.png) How would you write this in BNF using a "non-zero-digit" category??
```
<digit> ::= <non-zero-digit>|0
```

##### ![PHOTO SYNTAX DIAGRAM INTEGER](syntax-diagram-integer.png) How would you write this in BNF using a "non-zero-digit" and a "digit" category??
```
<integer> ::= <non-zero-digit>|<integer><digit>
```

##### When drawing a syntax diagram, should you write recursive rules or iterative ones??
Iterative rules.

### Question
___Are you allowed to use recursive definitions in syntax diagrams??___

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