String Parsing Examples
Here are some examples of using the >> operator to parse strings.
Example 0:
Let's see what happens when we redirect a file into a C++ program that
reads in 3 strings.
<![CDATA[
/* File: input0.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std ;
int main() {
string str1, str2, str3 ;
cin >> str1 ;
cin >> str2 ;
cin >> str3 ;
cout << "str1 = '" << str1 << "'\n" ;
cout << "str2 = '" << str2 << "'\n" ;
cout << "str3 = '" << str3 << "'\n" ;
}
]]>
Download: input0.cpp
The sample run shows that all white spaces are skipped including new
line characters. Even reading past the last word is OK. In the last run,
str3 is the empty string.
<![CDATA[
linux1% g++ input0.cpp
linux1% cat data1
hello blue world
linux1% ./a.out < data1
str1 = 'hello'
str2 = 'blue'
str3 = 'world'
linux1%
linux1% cat data2
Hello World
good
3.14159
linux1% ./a.out < data2
str1 = 'Hello'
str2 = 'World'
str3 = 'good'
linux1%
linux1% cat data3
abc def
linux1% ./a.out < data3
str1 = 'abc'
str2 = 'def'
str3 = ''
linux1%
]]>
Example 1:
In the next example, we take one of the strings and convert it to a
floating point vlaue using the
atof() function.
(See
sample run.)
The
atof() function is fairly robust and returns a 0 when
the string cannot be turned into a float. (Try changing the data
file yourself.)
<![CDATA[
/* File: input1.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h> // so you can use atof()
using namespace std ;
int main() {
string str1, str2, str3 ;
string str4 ;
cin >> str1 ;
cin >> str2 ;
cin >> str3 ;
cin >> str4 ;
cout << "str1 = '" << str1 << "'\n" ;
// can use [] with strings to retrieve characters
cout << "str1[1] = '" << str1[1] << "'\n" ;
cout << "str2 = '" << str2 << "'\n" ;
cout << "str3 = '" << str3 << "'\n" ;
cout << "str4 = '" << str4 << "'\n" ;
// convert to float
float f = atof( str4.c_str() ) ;
cout << "f = " << f << endl ;
}
]]>
Download: input1.cpp
Example 2:
The next example shows that the
getline() function can be used
to read in an entire line of text.
(See
sample run.)
The Countries file has a line of headings that we don't want to keep.
<![CDATA[
/* File: input2.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std ;
int main() {
string str1, str2, str3 ;
string str4 ;
getline(cin, str1) ; // read entire line
cin >> str2 ;
cout << "str1 = '" << str1 << "'\n" ;
cout << "str2 = '" << str2 << "'\n" ;
}
]]>
Download: input2.cpp
Example 3:
In this example, we learn how to open a file for reading.
(See
sample run.)
<![CDATA[
/* File: input3.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h> // so you can use atof()
#include <fstream> // so you can use ifstream
using namespace std ;
int main() {
string str1, str2, str3 ;
string str4 ;
ifstream ifile("data2") ; // open file for reading
getline(ifile, str1) ; // use ifile instead of cin
ifile >> str2 ; // use ifile instead of cin
cout << "str1 = '" << str1 << "'\n" ;
cout << "str2 = '" << str2 << "'\n" ;
ifile.close() ;
}
]]>
Download: input3.cpp
Example 4:
In the next example, we read in 9 fields from the second line of the
input.
(See
sample run.)
<![CDATA[
/* File: input4.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
#include <fstream>
using namespace std ;
int main() {
string firstLine ;
string str1, str2, str3, str4, str5, str6, str7, str8, str9 ;
ifstream ifile("2013WorldBankEducationCensusData.txt") ;
getline(ifile, firstLine) ; // toss first line
ifile >> str1 ; // read 9 fields
ifile >> str2 ;
ifile >> str3 ;
ifile >> str4 ;
ifile >> str5 ;
ifile >> str6 ;
ifile >> str7 ;
ifile >> str8 ;
ifile >> str9 ;
cout << "str1 = '" << str1 << "'\n" ; // print 9 fields
cout << "str2 = '" << str2 << "'\n" ;
cout << "str3 = '" << str3 << "'\n" ;
cout << "str4 = '" << str4 << "'\n" ;
cout << "str5 = '" << str5 << "'\n" ;
cout << "str6 = '" << str6 << "'\n" ;
cout << "str7 = '" << str7 << "'\n" ;
cout << "str8 = '" << str8 << "'\n" ;
cout << "str9 = '" << str9 << "'\n" ;
ifile.close() ; // put away file when done
}
]]>
Download: input4.cpp
Example 5:
Then, we put the printing in a loop.
(See
sample run.)
<![CDATA[
/* File: input5.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
#include <fstream>
using namespace std ;
int main() {
string firstLine ;
string str1, str2, str3, str4, str5, str6, str7, str8, str9 ;
ifstream ifile("2013WorldBankEducationCensusData.txt") ; // open file
getline(ifile, firstLine) ; // toss first line
while(true) { // keep reading until done
ifile >> str1 ; // read 9 fields
ifile >> str2 ;
ifile >> str3 ;
ifile >> str4 ;
ifile >> str5 ;
ifile >> str6 ;
ifile >> str7 ;
ifile >> str8 ;
ifile >> str9 ;
if ( ifile.eof() ) break ; // done?
cout << "str1 = '" << str1 << "'\n" ; // print country name
/* cout << "str2 = '" << str2 << "'\n" ; // skip printing rest of data
cout << "str3 = '" << str3 << "'\n" ;
cout << "str4 = '" << str4 << "'\n" ;
cout << "str5 = '" << str5 << "'\n" ;
cout << "str6 = '" << str6 << "'\n" ;
cout << "str7 = '" << str7 << "'\n" ;
cout << "str8 = '" << str8 << "'\n" ;
cout << "str9 = '" << str9 << "'\n" ;
*/
}
ifile.close() ; // put away file when done
}
]]>
Download: input5.cpp
Example 6:
We look for "N/A" in the input.
(See
sample run.)
<![CDATA[
/* File: input6.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
#include <fstream>
using namespace std ;
int main() {
string firstLine ;
string str1, str2, str3, str4, str5, str6, str7, str8, str9 ;
long int population ;
ifstream ifile("2013WorldBankEducationCensusData.txt") ;
getline(ifile, firstLine) ;
while(true) {
ifile >> str1 ;
ifile >> str2 ;
ifile >> str3 ;
ifile >> str4 ;
ifile >> str5 ;
ifile >> str6 ;
ifile >> str7 ;
ifile >> str8 ;
ifile >> str9 ;
if ( ifile.eof() ) break ;
cout << "country = '" << str1 << "' " ;
// use atol() to convert to long
population = atol(str2.c_str()) ;
cout << "population = " << population << endl ;
// use == for string comparison
if ( str3 == "N/A" ) cout << " str3 has no value\n" ;
/* cout << "str2 = '" << str2 << "'\n" ;
cout << "str3 = '" << str3 << "'\n" ;
cout << "str4 = '" << str4 << "'\n" ;
cout << "str5 = '" << str5 << "'\n" ;
cout << "str6 = '" << str6 << "'\n" ;
cout << "str7 = '" << str7 << "'\n" ;
cout << "str8 = '" << str8 << "'\n" ;
cout << "str9 = '" << str9 << "'\n" ;
*/
}
ifile.close() ;
}
]]>
Download: input6.cpp
Example 7:
In this version, we store a -1 where there was an "N/A".
(See
sample run.)
<![CDATA[
/* File: input7.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
#include <fstream>
using namespace std ;
int main() {
string firstLine ;
string str1, str2, str3, str4, str5, str6, str7, str8, str9 ;
long int population ;
float litRate ;
ifstream ifile("2013WorldBankEducationCensusData.txt") ;
getline(ifile, firstLine) ;
while(true) {
ifile >> str1 ;
ifile >> str2 ;
ifile >> str3 ;
ifile >> str4 ;
ifile >> str5 ;
ifile >> str6 ;
ifile >> str7 ;
ifile >> str8 ;
ifile >> str9 ;
if ( ifile.eof() ) break ;
cout << "country = '" << str1 << "' " ;
population = atol(str2.c_str()) ;
cout << "population = " << population << " " ;
// check if field is N/A. Store -1 if so.
//
if ( str3 == "N/A" ) {
litRate = -1.0 ;
} else {
litRate = atof(str3.c_str()) ;
}
cout << "literacy rate = " << litRate << endl ;
/* cout << "str2 = '" << str2 << "'\n" ;
cout << "str3 = '" << str3 << "'\n" ;
cout << "str4 = '" << str4 << "'\n" ;
cout << "str5 = '" << str5 << "'\n" ;
cout << "str6 = '" << str6 << "'\n" ;
cout << "str7 = '" << str7 << "'\n" ;
cout << "str8 = '" << str8 << "'\n" ;
cout << "str9 = '" << str9 << "'\n" ;
*/
}
ifile.close() ;
}
]]>
Download: input7.cpp
Example 8:
This next version is a little bit more robust.
It can deal with the situation where some lines may have more words than
others. It uses getline() to read an entire line of input from
the file. Then it converts the string into a stringstream. Finally, the
stringstream can be used like cin to get separate words.
We did not encounter any issues trying to read 11 words from each line,
even though each line only has 9 words.
(See sample run.)
<![CDATA[
/* File: input8.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
#include <fstream>
#include <sstream> // so you can use istringstream
using namespace std ;
int main() {
string oneLine ;
string str1, str2, str3, str4, str5, str6, str7, str8, str9 ;
string str10, str11 ;
long int population ;
float litRate ;
ifstream ifile("2013WorldBankEducationCensusData.txt") ;
getline(ifile, oneLine) ;
while(true) {
getline(ifile, oneLine) ; // read entire line
// cout << oneLine << endl ;
if ( ifile.eof() ) break ;
// Convert string to stringstream
// new one each loop!
//
istringstream istrm(oneLine) ;
istrm >> str1 ; // read from istrm
istrm >> str2 ;
istrm >> str3 ;
istrm >> str4 ;
istrm >> str5 ;
istrm >> str6 ;
istrm >> str7 ;
istrm >> str8 ;
istrm >> str9 ;
istrm >> str10 ; // extra read attempts OK
istrm >> str11 ;
cout << "country = '" << str1 << "' " ;
population = atol(str2.c_str()) ;
cout << "population = " << population << " " ;
if ( str3 == "N/A" ) {
litRate = -1.0 ;
} else {
litRate = atof(str3.c_str()) ;
}
cout << "literacy rate = " << litRate << endl ;
/* cout << "str2 = '" << str2 << "'\n" ;
cout << "str3 = '" << str3 << "'\n" ;
cout << "str4 = '" << str4 << "'\n" ;
cout << "str5 = '" << str5 << "'\n" ;
cout << "str6 = '" << str6 << "'\n" ;
cout << "str7 = '" << str7 << "'\n" ;
cout << "str8 = '" << str8 << "'\n" ;
cout << "str9 = '" << str9 << "'\n" ;
*/
}
ifile.close() ;
}
]]>
Download: input8.cpp
Example 9:
In this final version, we store each country's name in a
vector of strings.
At the end of the program, we loop through the vector and
print out each country's name.
(See sample run).
<![CDATA[
/* File: input9.cpp
Testing file I/O using >> with strings.
*/
#include <iostream>
#include <string>
#include <stdlib.h>
#include <fstream>
#include <sstream>
#include <vector> // so you can use vector
using namespace std ;
int main() {
string oneLine ;
string str1, str2, str3, str4, str5, str6, str7, str8, str9 ;
string str10, str11 ;
long int population ;
float litRate ;
vector<string> names ;
ifstream ifile("2013WorldBankEducationCensusData.txt") ;
getline(ifile, oneLine) ;
while(true) {
getline(ifile, oneLine) ;
// cout << oneLine << endl ;
if ( ifile.eof() ) break ;
istringstream istrm(oneLine) ; // new one each loop!
istrm >> str1 ;
istrm >> str2 ;
istrm >> str3 ;
istrm >> str4 ;
istrm >> str5 ;
istrm >> str6 ;
istrm >> str7 ;
istrm >> str8 ;
istrm >> str9 ;
istrm >> str10 ;
istrm >> str11 ;
cout << "country = '" << str1 << "' " ;
population = atol(str2.c_str()) ;
cout << "population = " << population << " " ;
if ( str3 == "N/A" ) {
litRate = -1.0 ;
} else {
litRate = atof(str3.c_str()) ;
}
cout << "literacy rate = " << litRate << endl ;
names.push_back(str1) ; // add country name to vector
/* cout << "str2 = '" << str2 << "'\n" ;
cout << "str3 = '" << str3 << "'\n" ;
cout << "str4 = '" << str4 << "'\n" ;
cout << "str5 = '" << str5 << "'\n" ;
cout << "str6 = '" << str6 << "'\n" ;
cout << "str7 = '" << str7 << "'\n" ;
cout << "str8 = '" << str8 << "'\n" ;
cout << "str9 = '" << str9 << "'\n" ;
*/
}
ifile.close() ;
// print out country names from vector
//
cout << "\n\n\n**** Vector test *****\n" ;
int n = names.size() ;
for (int i = 0 ; i < n ; i++) {
cout << names[i] << endl ;
}
}
]]>
Download: input9.cpp