Motivation
Parsing functions
Homework
8. Text Parsing
Ján Dugáček
November 14, 2018
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
Table of Contents
1 Motivation
2 Parsing functions
std::getline
std::stringstream
std::string
Exercise
3 Homework
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
Motivation
What if the file you need to read can’t be read by
easy::vector?
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
std::getline
std::stringstream
std::string
Exercise
std::getline
easy : : vector <std : : s t r i n g > l i n e s ( 1 ) ;
wh il e ( std : : g e t l i n e ( in , l i n e s . back ( ) , ’ \n ’ )) {
i f ( ! l i n e s . back ( ) . empty ( ) )
l i n e s . push_back ( ) ;
}
std::getline reads from the input in its first argument (it
can be std::cin, std::ifstream, ...), saves it into the
string on its second argument until it finds the separator in its
last parameter (new line by default)
This program splits the input into lines, ignoring the empty
ones
Can you spot the error in the code?
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
std::getline
std::stringstream
std::string
Exercise
Function #2
#i n c l u d e <sstream>
// . . .
easy : : vector <std : : s t r i n g > b l o c k s ;
std : : s t r i n g s t r e a m stream ( l i n e s [ i ] ) ;
std : : s t r i n g reading ;
wh il e ( std : : g e t l i n e ( stream , reading , ’ \ t ’ )) {
b l o c k s . push_back ( ) ;
std : : swap ( b l o c k s . back ( ) , reading ) ;
}
stringstream can be used to parse a string in the same way
as std::cin or std::ifstream
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
std::getline
std::stringstream
std::string
Exercise
std::string
std : : s t r i n g t e x t = "ahoy" ;
t e x t . push_back ( ’ ! ’ ) ;
char f i r s t L e t t e r = t e x t [ 0 ] ;
i f ( f i r s t L e t t e r >= ’A ’ && f i r s t L e t t e r <= ’Z ’ )
f i r s t L e t t e r += ’A ’ − ’ a ’ ;
t e x t [ 0 ] = f i r s t L e t t e r ;
i f ( t e x t == "Ahoy ! " )
t e x t . append ( " Pleased to see you ! " ) ;
std::string can do plenty of stuff, but it may be impractical
Mistakes or unhandled invalid inputs can cause crashes
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
std::getline
std::stringstream
std::string
Exercise
std::string #2
i n t number = 4;
std : : s t r i n g t e x t = " I t ’ s " + std : : to_string ( number ) ;
t e x t += " ! " ;
i n t l ength = t e x t . s i z e ( ) ;
f o r ( unsigned i n t i = 0; t e x t [ i ] != 0; i++) {
i f ( t e x t [ i ] >= ’ 0 ’ && t e x t [ i ] <= ’ 9 ’ )
length −−;
}
i n t number2 = std : : s t o i ( std : : to_string ( number ) ) ;
std : : s t r i n g theNumber = t e x t . s u b s t r (5 , 2 ) ;
std::to_string can be used to convert a number to string
std::stoi can be used to convert a string to an integer
(std::stof for float)
The final symbol in a string is always 0
The substr method (in this case) returns a string containing
2 characters starting with the fifth
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
std::getline
std::stringstream
std::string
Exercise
Exercise
1 Write a function that finds the number in string like The
number is 14 probably.
2 Write a function that splits a comma-separated list string (like
toaster,kettle,dishwasher)
3 Write a function that checks if a string is a number
4 Write a function that splits a string into capital letters, small
letters and numbers and dumps the rest
5 Write a function that multiplies all numbers placed inside a
string by a number given in the second argument
Ján Dugáček 8. Text Parsing
Motivation
Parsing functions
Homework
Homework
Write a function that parses files as shown into a vector of
vectors that are entries containing the x, y and z parametres
respectively
x=1 y=2 z=3
y=1.1 x=8.2 z=0.3
z=0.1 y=12 x=6.1
You have two weeks to do it
Ján Dugáček 8. Text Parsing