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features/modifying_values/index.md

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+# Modifying values
+
+Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page gives an overview of the available operations. For read access, see [element access](https://json.nlohmann.me/features/element_access/index.md).
+
+## Adding to arrays
+
+New elements are appended to an array with [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md) or constructed in place with [`emplace_back`](https://json.nlohmann.me/api/basic_json/emplace_back/index.md). If the value is `null`, it is converted to an array first, so these functions can also be used to build an array from scratch.
+
+```
+json j;                 // null
+j.push_back(1);         // [1]
+j.push_back(2);         // [1,2]
+j.emplace_back(3);      // [1,2,3]
+
+// operator+= is a shorthand for push_back
+j += 4;                 // [1,2,3,4]
+```
+
+## Adding to objects
+
+The most common way to add or replace a member is [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md), which inserts the key if it does not exist yet:
+
+```
+json j;
+j["name"] = "Mary";     // {"name":"Mary"}
+j["name"] = "John";     // {"name":"John"}  (replaced)
+```
+
+[`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md) inserts a member only if the key is not already present, and reports whether the insertion happened — useful for "add if absent" semantics.
+
+## Merging objects
+
+To merge one object into another, [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) copies all members from another object, overwriting existing keys (similar to Python's `dict.update`). This is the idiomatic way to combine two objects.
+
+Example
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+int main()
+{
+    // create two JSON objects
+    json o1 = R"( {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} )"_json;
+    json o2 = R"( {"color": "blue", "speed": 100, "names": {"en": "plane"}} )"_json;
+    json o3 = o1;
+
+    // add all keys from o2 to o1 (updating "color", replacing "names")
+    o1.update(o2);
+
+    // add all keys from o2 to o1 (updating "color", merging "names")
+    o3.update(o2, true);
+
+    // output updated object o1 and o3
+    std::cout << std::setw(2) << o1 << '\n';
+    std::cout << std::setw(2) << o3 << '\n';
+}
+```
+
+Output:
+
+```
+{
+  "color": "blue",
+  "names": {
+    "en": "plane"
+  },
+  "price": 17.99,
+  "speed": 100
+}
+{
+  "color": "blue",
+  "names": {
+    "de": "Flugzeug",
+    "en": "plane"
+  },
+  "price": 17.99,
+  "speed": 100
+}
+```
+
+For a recursive merge that follows [RFC 7386](https://tools.ietf.org/html/rfc7386), see [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md). To apply a sequence of well-defined edit operations, see [JSON Patch](https://json.nlohmann.me/features/json_patch/index.md).
+
+## Removing elements
+
+Elements are removed with [`erase`](https://json.nlohmann.me/api/basic_json/erase/index.md), which accepts an object key, an array index, or an iterator. [`clear`](https://json.nlohmann.me/api/basic_json/clear/index.md) empties a value while keeping its type, and [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) combined with assignment can overwrite a value entirely.
+
+```
+json j = {{"a", 1}, {"b", 2}, {"c", 3}};
+j.erase("b");           // {"a":1,"c":3}
+
+json a = {1, 2, 3, 4};
+a.erase(1);             // [1,3,4]  (erase by index)
+```
+
+## See also
+
+- [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md) / [`emplace_back`](https://json.nlohmann.me/api/basic_json/emplace_back/index.md) - append to an array
+- [`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md) - insert into an object if the key is absent
+- [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) - merge objects
+- [`erase`](https://json.nlohmann.me/api/basic_json/erase/index.md) / [`clear`](https://json.nlohmann.me/api/basic_json/clear/index.md) - remove elements
+- [JSON Patch and Diff](https://json.nlohmann.me/features/json_patch/index.md) and [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md) - structured modifications