Motivation Vector Files
Other containers Homework
4. Vectors and STL
Jan Dugacek
October 9, 2019
Jan Dugacek        4. Vectors and STL
Motivation
Vector • Exercise
Files
• Working with files
• The easy way 9 Exercise
Other containers Homework
Jan Dugacek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
Can you write a program that gets 10 numbers and sorts them from the smallest up to the largest?
Jan Dugacek
4. Vectors and STL
Motivation Vector Files
Other Containers Homework
#i n cIu d e <vecto r >
//•••
std : : vecto r <i n t > numbers;
for   (int   i = 0;   i < 10;   i++) {
int read;
std : : ein » read;
numbers . push_back( read ) ;
}
std : : cout « "At 3 « numbers [3] « std : : endl ;
• std: :vector is a data structure that stores an indefinite number of elements
• It can store only one type of element, as defined in the < > brackets
• Elements are inserted using push_back
Jan Dugäcek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
for   (unsigned   int   i = 0;   i < n u m bers . s i z e () ;   i++) { std::cout « "At  index  " «  i « "   is   " « numbers [ i ] « std : : end I ;
}
• Elements can be accessed through the square brackets
o Number of elements can be read using the lengthO method
• The first element is at index zero
• Accessing elements at negative indexes or after the last one causes undefined behaviour, which can mean random overwriting of variables or crashes
Jan Dugacek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
for   (int& num      numbers) {
std : : cout « "We have  " « num « std : : end I ;
}
9 The for can be shortened if we don't need to know the index
numbers . erase ( numbers . begi n () + 5);
9 Erases element 5
Jan Dugacek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
Exercise
#i n cIu d e  "easy . h
a...
ii
easy :: vector <float > vec ;
Accessing elements at negative indexes or after the last one causes undefined behaviour, which can mean random overwriting of variables or crashes
Use my easy: :vector instead to receive warnings instead (at the cost of execution speed)
Jan Dugacek
4. Vectors and STL
Motivation Vector Files Other containers Homework				Exercise
			1	
O Have the user supply 5 numbers and output them afterwards
© Have the user supply 5 numbers and output them in some other ordering
O Have the user supply 5 numbers and then 1 number to set if he wants to get their arithmetic average, geometric average or harmonic average
□ S1
Jan Dugacek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
Working with files The easy way Exercise
^include <fstream>
/•••
std : : ofstream   out(" output . dat" ); out « 42 « std : : end I ;
• The changes are written to disk when the variable ceases to exist
<* Works much like std: :cout, but is faster
std :: ifstream   in("input.dat"); i n t  num ; i n » num ;
• Works much like std: :cin
Jan Dugacek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
Working with files The easy way Exercise
#i n cIu d e  "easy.h "
/•••
easy :: vector <float > n u m bers ( " n u m bers . d at " , '\n'); for   (floats num      numbers) {
std : : cout « "We have  " « num « std : : endl ;
}
• I have created this comfy library for you that does it easily
• The first argument is the file name, the second argument is the separator
Jan Dugacek        4. Vectors and STL
;y way
Motivation Vector Files
Other containers Homework
rking with files e easy way
reise
#i n cIu d e  "easy.h "
//■■■
easy:: vector <e a sy::vector<float» numbers
("numbers . dat" ,   '\n ' ,   '\t ' ); std : : cout « numbers [ 3 ] [4] « std : : end I ;
• It can also parse tables, the second argument is line separator, the third is the column separator
=
Jan Dugacek
4. Vectors and STL
Motivation Vector Files
Other containers Homework
Working with files The easy way Exercise
#i n cIu d e  "easy.h "
//•••
easy :: vector <float > numbers;
//...
easy : : write_file (numbers , "numbers, dat" , //or
numbers. write_file ("numbers, dat" ,   '\n ' );
So that you could also write files easily
Jan Dugacek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
Working with files The easy way Exercise
Have the user supply numbers in a file, output them in order from smallest to greatest
Have the number supply numbers in a file and perform a linear fit, i. e. f (line_number) = a • line_number, you may assume the slope will be between 0.01 and 1000
Jan Dugacek
4. Vectors and STL
Motivation Vector Files
Other containers Homework
std: :map allows indexing using nearly anything, but is slower (access time depends on the logarithm of size)
std: :unordered_map is not so slow, but elements are read in a strange order
Accessing an unused location will create an uninitialised variable there
^include <map>
//...
std ::map<std:: string,   int> array;
array["hi"] = 3;
a r ra y [ 1 za phod1 ] = 4 ;
array[ ] = a r r a y [ 1 za phod 1 ] — 1;
for   (std ::pair<std:: string,   int >& i t std : : cout «  it . f i rst « "="« it
array) second « std
e n d I ;
Jan Dugacek        4. Vectors and STL
Motivation Vector Files
Other containers Homework
O std: : array is faster than vector, but it has a fixed size
O Use easy: :array to get boundary checking, it suffers from undefined behaviour like vector
^include <map>
//•••
std::array<int,  4> array; a rray[3] = 3; a rray [2] = 4 ;
std : : cout « "At 1   is   " « array [1] « std : : end I ;
Jan Dugacek        4. Vectors and STL
• Read numbers from a file and print them ordered
• The numbers will be between 0 and 1000000, ordered evenly over the range
o You have two weeks to do it
• Challenge: Write it in a way that the execution time is does not depend quadratically on the size of input
Jan Dugacek        4. Vectors and STL