# Form Logic Overview {#form-logic-overview}


MQUAD surveys support four kinds of form logic. They look similar — all four are
small expressions typed into the question properties panel — but each one does a
different job:

| Feature | What it does | Designer location |
|---|---|---|
| **Validation** | Decides whether an *entered* answer is acceptable | **Validation Rules** (Advanced section) |
| **Skip Logic** | Decides whether a question is *shown* | **Skip Logic** (Advanced section) |
| **Default Value** | Supplies an *initial* answer when the form starts | **Default Value** (Advanced section) |
| **Auto-Populate / Calculation** | *Derives* an answer from other answers | **Calculated Value** or **Auto-Populate** (Advanced section) |

Two of these run once (default values, at form start). Two of them re-run every
time a dependency changes (skip logic and calculations). Validation runs every
time a value changes.

> **Tip:** Not sure which feature you need? Jump to
> [Choosing the right tool](#choosing-the-right-tool) at the end of this page.

---


## Referencing another question {#referencing-another-question}


Expressions reference questions by their **variable name** (the `name` field),
never by their displayed label.

```text
${age}                 — the answer to the question named "age"
${visit_date}          — the answer to the question named "visit_date"
```

Examples using realistic survey questions:

| Question label | Variable name | Reference |
|---|---|---|
| "What is your date of birth?" | `dob` | `${dob}` |
| "How old are you?" | `age` | `${age}` |
| "Are you currently pregnant?" | `pregnant` | `${pregnant}` |
| "Weight (kg)" | `weight_kg` | `${weight_kg}` |
| "Select your district" | `district` | `${district}` |
| "Visit date" | `visit_date` | `${visit_date}` |

**Correct:**

```text
${dob}
```

**Incorrect:**

```text
What is your date of birth?
```


### Why the variable name matters {#why-the-variable-name-matters}


The variable name is the stable identifier used everywhere behind the scenes:

- **Translations** — the label changes per language, the variable name does not.
- **Label edits** — you can reword a label without breaking any logic.
- **Data exports** — exported columns use the variable name.
- **Stable calculations** — a calculation keeps working even if someone renames
  the label.

The variable name must start with a letter or underscore and contain only
letters, numbers, and underscores (`[a-zA-Z_][a-zA-Z0-9_]*`).

---


## Two different reference grammars {#two-different-reference-grammars}


MQUAD uses **two different reference styles**, depending on which feature you
are configuring:

| Feature | Reference style | Example |
|---|---|---|
| Validation, Skip Logic, Calculation | `${question_name}` | `${age} >= 18` |
| Default Value | `#question_name` | `#country#district` |

A default value like `#country#res_num` concatenates the answers of `country`
and `res_num`. The `#` grammar is **only** for default values — you cannot use
it in a calculation, and you cannot use `${...}` inside a default value.

This is a common source of confusion, so it is called out explicitly in each
chapter below.

---


## The current-field value: `.` {#the-current-field-value-}


Inside a **constraint** (validation) expression, the dot `.` refers to the
current question's own value:

```text
. >= 18
```

means "the answer to *this* question must be at least 18". The validation
builder generates `.`-based expressions automatically (e.g. `. >= 18`,
`string-length(.) >= 3`). You can also combine `.` with other questions:

```text
. >= ${min_age}
```

"this answer must be at least the value of question `min_age`".

---


## Choosing the right tool {#choosing-the-right-tool}


| Scenario | Use |
|---|---|
| A question must be answered | Mark it **Required** in the question properties |
| An answer must be within a range, match a pattern, or satisfy a condition | **Validation Rules** |
| A question should only appear under certain conditions | **Skip Logic** |
| A field should start pre-filled (and stay editable) | **Default Value** |
| A value should be computed from other answers and update when they change | **Auto-Populate** (on a normal question) |
| A read-only computed result should never be edited | **Calculated Value** (type = `calculate`) |

---


## What you'll find in this section {#what-youll-find-in-this-section}


| Page | Covers |
|---|---|
| [Validation](/user-manual/validation.md) | Constraints, operators, validation messages |
| [Skip Logic](/user-manual/skip-logic.md) | Relevance expressions, conditions, nested logic |
| [Default Values](/user-manual/default-values.md) | `#ref` syntax, `today`, `now`, system variables |
| [Auto-Populate and Calculations](/user-manual/calculation-expressions.md) | `${ref}` syntax and the full function set |
| [Expression Reference](/user-manual/expression-reference.md) | Every operator and every built-in function |
| [Date and Time Reference](/user-manual/date-time-reference.md) | Dates, datetimes, plain times, and the frozen rules |
| [Common Recipes](/user-manual/common-recipes.md) | Copy-and-adapt examples for real surveys |
| [XLSForm Mapping](/user-manual/xlsform-mapping.md) | How these features map to XLSForm columns |
| [Blank and Invalid Values](/user-manual/blank-and-invalid-values.md) | What happens with null, blank, and bad input |
| [Troubleshooting Expressions](/user-manual/troubleshooting-expressions.md) | Common mistakes and how to fix them |

If you are new to survey logic, start with [Validation](/user-manual/validation.md) and
[Skip Logic](/user-manual/skip-logic.md), then read [Common Recipes](/user-manual/common-recipes.md) for
real examples.