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api/basic_json_view/number_format.md

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+# <small>nlohmann::basic_json_view::</small>number_format
+
+```cpp
+enum class number_format {
+    shortest,
+    source
+};
+```
+
+This enumeration is used in [`dump`](dump.md) to choose how numbers are written. Two values are differentiated:
+
+shortest
+:   integers are copied from the source text -- already canonical in JSON -- except that `#!cpp -0` becomes
+    `#!cpp 0`, the way [`BasicJsonType::parse()`](../basic_json/parse.md) reads it; floats are written with the
+    library's shortest round-trip conversion, exactly as [`BasicJsonType::dump()`](../basic_json/dump.md) would (e.g.
+    `#!cpp 1.5`, `#!cpp 100.0`, `#!cpp 1e+100`)
+
+source
+:   every number is copied exactly as it appears in the source text -- `#!cpp 1.50`, `#!cpp 1E2`, `#!cpp -0`, all
+    digits of an integer literal with more digits than any number type holds -- something `BasicJsonType` cannot do,
+    since parsing already reduces every number to its parsed value
+
+## Examples
+
+??? example
+
+    The example below writes back a price list received from a supplier: with `number_format::shortest` (the
+    default), a trailing zero and scientific notation are normalized away and a long account number that overflows
+    every number type is rounded, the same way `#!cpp materialize().dump()` (or `basic_json::dump()`) would;
+    `number_format::source` keeps every number exactly as it was written in the source text instead.
+
+    ```cpp
+    #include <iostream>
+    #include <nlohmann/json_view.hpp>
+    
+    using json_document = nlohmann::json_document;
+    using json_view = nlohmann::json_view;
+    
+    int main()
+    {
+        // a price list received from a supplier feed -- prices and account
+        // numbers must be forwarded exactly, e.g. into an invoice
+        const json_document doc = json_document::parse(R"(
+          [{"sku": "A1", "price": 19.90, "account_id": 12345678901234567890123456},
+           {"sku": "A2", "price": 1E2, "account_id": 98765432109876543210987654}]
+        )");
+        const json_view list = doc.root();
+    
+        // number_format::shortest (the default) writes numbers the way
+        // basic_json::dump() would: "19.90" becomes "19.9", "1E2" becomes
+        // "100.0", and each account number -- far beyond any 64-bit integer --
+        // is rounded to the nearest double, exactly as materialize().dump()
+        // (or a plain nlohmann::json) would round it
+        std::cout << list.dump() << '\n';
+    
+        // number_format::source copies every number exactly as it was written
+        // in the source text instead -- something basic_json cannot do at all,
+        // since parsing already reduces every number to its parsed value
+        std::cout << list.dump(-1, ' ', false, json_view::number_format::source) << '\n';
+    }
+    
+    ```
+
+    Output:
+
+    ```json
+    [{"sku":"A1","price":19.9,"account_id":1.2345678901234568e+25},{"sku":"A2","price":100.0,"account_id":9.876543210987655e+25}]
+    [{"sku":"A1","price":19.90,"account_id":12345678901234567890123456},{"sku":"A2","price":1E2,"account_id":98765432109876543210987654}]
+    
+    ```
+
+## See also
+
+- [dump](dump.md) - serialize to a JSON-formatted string
+- [number_token](number_token.md) - get a single number's token text without dumping the whole value
+- [`BasicJsonType::error_handler_t`](../basic_json/error_handler_t.md) - the analogous enumeration for
+  `BasicJsonType::dump`'s decoding-error behavior
+
+## Version history
+
+- Added in version 3.13.0.