C++ - How to use std::string to get length of the string?

[flaymond]

Hello, I still can't figure this out of how to get the value string length and save in a variable, in C++ like in Python.

C++ code:

#include <iostream>
#include <cstdlib>
#include <cstring>
#include <string>
using namespace std;

// Program that print strings backward


void revencrypt() {

string translate;
string message;
int i;

cout << "Please enter message to encrypt: ";
cin >> message;

output = "";
i = std::string str(message) -1; // This not working :(
// I want to save string length value in i, but not working. It produce error 'expected ";" in std::string<here> str(message)'
while ( i >= 0)
output = output + message[i];
i = i - 1;
cout << output << endl;
}

int main() {
revencrypt();
return 0;
}



In Python, this code working:



def encrypt():
message = raw_input ("Please enter your message to encrypt: ")
if message != 0:
output = ''
i = len(message) -1 # This code not working in C++ program above
while i >= 0:
output = output + message[i]
i = i - 1
print (output)


def main()
encrypt()

main() # code starts here



I expected the both program produce same output, Unfortunately, the data type and STL functions is confusing me in C++. I don't know which function or method will fix this out. Anyone can help? Thanks.

[matt_symes]

Hi

There are a number of things wrong with the program but to get it to compile and run for you i have highlighted the changes required.

Two of the problems are syntactic and one is semantic.

Work through my changes to understand what i have done. After that try to write it more succinctly and remove any cruft on the code.



#include <iostream>
#include <cstdlib>
#include <cstring>
#include <string>
using namespace std;

// Program that print strings backward
void revencrypt() {

string translate;
string message;
int i;

cout << "Please enter message to encrypt: ";
cin >> message;

std:string output("");
i = message.length() -1;
while ( i >= 0) {
output = output + message[i];
i = i - 1;
}

cout << output << endl;
}

int main() {
revencrypt();
return 0;
}

And it running.



matthew-laptop:/home/matthew:1 % g++ t.cpp
matthew-laptop:/home/matthew:1 % ./a.out
Please enter message to encrypt: test
tset
matthew-laptop:/home/matthew:1 %

Kind regards

[flaymond]

Thanks! It's worked. I think I should explore the C/C++ libraries more often!

[spjackson]

Thanks! It's worked. I think I should explore the C/C++ libraries more often!
Perhaps so. A more idiomatic C++ way of reversing a string might be:


#include <string>
#include <algorithm>
#include <iostream>

int main(int argc, char * argv[])
{
if (argc > 1) {
std::string output(argv[1]);
std::reverse(output.begin(), output.end());
std::cout << output << std::endl;
}
}

[matt_symes]

Hi


Perhaps so. A more idiomatic C++ way of reversing a string might be:


#include <string>
#include <algorithm>
#include <iostream>

int main(int argc, char * argv[])
{
if (argc > 1) {
std::string output(argv[1]);
std::reverse(output.begin(), output.end());
std::cout << output << std::endl;
}
}


Yep :)

Kind regards

[dwhitney67]

Another alternative is to use the std::string range constructor:


#include <string>
#include <iostream>

int main(int argc, char** argv)
{
if (argc > 1)
{
std::string original(argv[1]);
std::string reverse(original.rbegin(), original.rbegin() + original.length());

std::cout << "original: " << original << "\n"
<< "reverse : " << reverse << std::endl;
}
}