Searching in std::map or in std::multimap

There are several ways to search a key in std::map or in std::multimap.

To get the iterator of the first occurrence of a key, the find() function can be used. It returns end() if the key does not exist.

std::multimap< int , int > mmp{ {1, 2}, {3, 4}, {6, 5}, {8, 9}, {3, 4}, {6, 7} };
auto it = mmp.find(6);
if (it!=mmp.end())
    std::cout << it->first << ", " << it->second << std::endl; //prints: 6, 5
else
    std::cout << "Value does not exist!" << std::endl;

it = mmp.find(66);
if (it!=mmp.end())
    std::cout << it->first << ", " << it->second << std::endl; 
else
    std::cout << "Value does not exist!" << std::endl; // This line would be executed.

Another way to find whether an entry exists in std::map or in std::multimap is using the count() function, which counts how many values are associated with a given key. Since std::map associates only one value with each key, its count() function can only return 0 (if the key is not present) or 1 (if it is). For std::multimap, count() can return values greater than 1 since there can be several values associated with the same key.

std::map< int , int > mp{ {1, 2}, {3, 4}, {6, 5}, {8, 9}, {3, 4}, {6, 7} };
if (mp.count(3) > 0) // 3 exists as a key in map
    std::cout << "The key exists!" << std::endl; // This line would be executed.
else
    std::cout << "The key does not exist!" << std::endl;

If you only care whether some element exists, find is strictly better: it documents your intent and, for multimaps, it can stop once the first matching element has been found.

In the case of std::multimap, there could be several elements having the same key. To get this range, the equal_range() function is used which returns std::pair having iterator lower bound (inclusive) and upper bound (exclusive) respectively. If the key does not exist, both iterators would point to end().

auto eqr = mmp.equal_range(6);
auto st = eqr.first, en = eqr.second;
for (auto it = st; it != en; ++it){
    std::cout << it->first << ", " << it->second << std::endl; 
}
    // prints: 6, 5
    //         6, 7

Found a mistake? Have a question or improvement idea? Let me know.

std::map:

* std::map

* Accessing elements

* Inserting elements

* Searching in std::map or in std::multimap

* Initializing a std::map or std::multimap

* Checking number of elements

* Types of maps

* Deleting elements

* Iterating over std::map or std::multimap

* Creating std map with user-defined types as key

Table Of Contents

0 Getting started

1 Literals

2 Basic type keywords

3 Operator precedence

4 Floating point arithmetic

5 Bit operators

6 Bit manipulation

7 Bit fields

8 Arrays

9 Flow control

10 const keyword

11 Loops

12 mutable keyword

13 friend keyword

14 keywords

15 Variable declaration keywords

16 auto keyword

17 Pointers

18 Type keywords (class, enum, struct, union)

19 Classes / structs

20 std::string

21 Enumeration

22 std::atomic

23 std::vector

24 std::array

25 std::pair

26 std::map

27 std::unordered_map

28 std::set and std::multiset

29 std::any

30 std::variant

31 std::optional

32 std::integer_sequence

33 std::function

34 std::forward_list

35 std::iomanip

36 Iterators

37 Basic I/O

38 File I/O

39 Streams

40 Stream manipulators

41 Metaprogramming

42 Returning multiple values from a function

43 References

44 Polymorphism

45 Value and reference semantics

46 Function call by value vs. by reference

47 Copying vs assignment

48 Pointers to class / struct members

49 The this pointer

50 Smart pointers

51 Unions

52 Templates

53 Namespaces

54 Function overloading

55 Operator overloading

56 Lambdas

57 Threading

58 Value categories

59 Preprocessor

60 SFINAE

61 Rule of three, five and zero

62 RAII

63 Exceptions

64 Implementation-defined behavior

65 Special member functions

66 Random numbers

67 Sorting

68 Regular expressions

69 Perfect forwarding

70 Virtual member functions

71 Undefined behavior

72 Overload resolution

73 Move semantics

74 Pimpl idiom

75 Copy elision

76 Fold expressions

77 Unnamed types

78 Singleton

79 ISO C++ Standard

80 User-defined literals

81 Type erasure

82 Memory management

83 Explicit type conversions

84 RTTI

85 Standard library algorithms

86 Expression templates

87 Scopes

88 Atomic types

89 static assert

90 constexpr

91 Date and time with std::chrono

92 Trailing return type

93 Function template overloading

94 Common compile linker errors

95 Design patterns

96 Optimizations

97 Compiling and building

98 Type traits

99 One definition rule

100 Unspecified behavio

101 Argument dependent name lookup

102 Attributes

103 Internationalization

104 Profiling

105 Return type covariance

106 Non-static member functions

107 Recursion

108 Callable objects

109 Constant class member functions

110 C++ vs. C++ 11 vs C++ 17

111 Inline functions

112 Client server examples

113 Header files

114 Const correctness

115 Refactoring techniques

116 Parameter packs

117 Iteration

118 type deduction

119 C++ 11 memory model

120 Build systems

121 Concurrency with OpenMP

122 Type inference

123 Resource management

124 Storage class specifiers

125 Alignment

126 Inline variables

127 Linkage specifications

128 Curiusly recurring template pattern

129 Using declaration

130 Typedef and type aliases

131 Layout of object types

132 C incompatibilities

133 Optimization

134 Semaphore

135 Thread synchronization

136 Debugging

137 Futures and promises

138 More undefined behaviors

139 Mutexes

140 Recursive mutex

141 Unit testing

142 decltype

143 Digit separators

144 C++ Containers

145 Arithmetic meta-programming

146 Contributors