Vectors

1
Vectors

• CSE 1341Wael Kdouh

2
Agenda

• Go Through The Exercise Solution
• Introduction to Vectors

3
Exercise

• Write a function to return the sum of the two
  largest elements in an array.

4
Overview

• Definition of Vectors
• Vectors VS Arrays
• Some Vector Operations

5
Note

• For consistency, through the presentation I am
  always going to assume that I have a variable
• myVector of type vector.

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What Is A Vector?

• The vector container resembles a C array in
  that it holds zero or more objects of the same
  type, and that each of these objects can be
  accessed individually.

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When To Use Vectors?

• Vectors are good when we have an unknown sequence
  of similar items to store and we want to access
  them by their sequence numbers.

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How Can We Use Vectors?

• Vectors are part of the STL.
• You can use it by including the following into
  your program include ltvectorgt
• include ltvector.hgt gt should not be used as
  it is deprecated.

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Vector Declaration(1)

• In order to declare a vector variable you use the
  following syntaxvectorlttypegt myVector
  (initial_size)
• -lttypegt could be integer, double, float,char,
  or any other supported type.
• -initial_size is the initial size( could be
  extended throughout the lifetime of
  the program).
• -myVector is the name of the variable.
• Example
• vectorltdoublegt myVector()
• Compared to
• double myArray10

Vector
Array
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Vector Declaration(2)

• Here are some different ways to declare vectors
• - vectorltintgt myVector(100, 1) This will
  declare a vector of size 100 with all
  of them initialized to 1.
• - vectorltintgt myVector( const vector c )This
  will declare a vector that is a
  copy of the given vector c.

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Vectors vs. Arrays(1)

• Both serve as a way to collect elements of the
  same type .
• Vectors are a recent addition to C, so many
  older programs use arrays instead.

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Vectors vs. Arrays(2)

• The array size must be set when the program is
  compiled. (You can't ask the user how many
  elements and then allocate a sufficient number).
  But with vectors the size can change at run time.
• Example
• const int size 100
• int myArraysize
• for (int i0 iltsize i) myArrayii
• Same code using a vector
• const int size 100
• vectorltintgt myVector(size) //Only line
  that changed
• for (int i0 iltsize i)
  myVectorsizei
• myVector.resize(200)

Array
Vector
What is dangerous about this?
13
Question?

• What if the new size is smaller than the number
  of elements in the vector?

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Answer

• All the additional elements are truncated.

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Vector Operations(1)

• myVector. back ()Returns a reference to the last
  element in the vector.
• myVector.front() Returns a reference to first
  element of vector.
• myVector.push_back() Appends (inserts) an
  element to the end of a vector,
  allocating memory for it if
  necessary.
• myVector.pop_back() Erases the last element of
  the vector, possibly reducing
  capacity
• Example
• vectorltintgt myVector
• for( int i 0 i lt 5 i )
• myVector.push_back(i)
• cout ltlt "The first element is " ltlt
  myVector.front()
• ltlt " and the last element is " ltlt
  myVector.back() ltlt endl

How does it differ from vi?
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Vector Operations(2)

• myVector. size () Returns the number of
  elements in the current vector.
• myVector.empty() Returns true if the vector's
  size is 0.
• myVector. capacity () Returns the number of
  elements that the vector can hold before it will
  need to allocate more space.
• Example
• vectorltintgt v(10)
• cout ltlt "The capacity of v is " ltlt
  v.capacity() ltlt endl
• cout ltlt "The size of v is " ltlt v.size()
  ltlt endl
• v.resize(4)
• cout ltlt "The capacity of v is " ltlt
  v.capacity() ltlt endl
• cout ltlt "The size of v is " ltlt v.size()
  ltlt endl

What is the output?
What about now?
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Vector Operations(3)

• What is wrong with this code?
• vectorltintgt v( 5, 1 )
• for( int i 0 i lt 10 i )
• cout ltlt "Element " ltlt i ltlt " is " ltlt vi ltlt
  endl

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Vector Operations(3.1)

• Answer It overruns the end of the vector.
• Solution Use the at operation.
• vectorltintgt v( 5, 1 )
• for( int i 0 i lt 10 i )
• cout ltlt "Element " ltlt i ltlt " is " ltlt vi ltlt
  endl
• The following code would be much safer
• vectorltintgt v( 5, 1 )
• for( int i 0 i lt 10 i )
• cout ltlt "Element " ltlt i ltlt " is " ltlt v.at(i)
  ltlt endl

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Vectors vs. Arrays(3)

• When defining an array, you must guess on the
  maximum number of elements you need to store.
  With Vectors you DONT.
• You must keep a constant to hold the capacity of
  the array. With Vectors you DONT, but rather you
  can use myVector.size()

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Passing Vectors As Function Parameters(1)

• Unlike arrays when you pass a vector by value it
  will not reflect the changes after you return
  from the function. You have to pass it by
  reference.
• Example 1
• vectorltdoublegt myVector(10,50)
• ProcessInput(myVector)
• coutltltmyVector0ltlt\tltltmyVector1
• void ProcessInput( vectorltdoublegt invector)
• invector0100

What will the output be after it returns from the
function
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Passing Vectors As Function Parameters(2)

• Example 2
• vectorltdoublegt myVector(10,50)
• ProcessInput(myVector)
• coutltltmyVector0ltlt\tltltmyVector1
• void ProcessInput( vectorltdoublegt invector)
• invector0100

What about now?
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Vectors vs. Arrays(4)

• In spite the fact that vectors have a lot of
  advantages and flexibility over arrays there are
  some cases where arrays are still superior.
• 1) When efficiency is vitally important.
• 2) One can't afford the overhead of a huge
  library like vector. Example Embedded
  Systems.

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Conclusion

• Main advantages of vectors over arrays
• 1)They can increase/decrease in size at
  run-time.
• 2)They allocate memory from the free space
  when increasing in size.
• 3)They can do range checking (using at()).

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• Thank You