I just needed to reverse a string, so I declared a new string variable and iteratively copied the elements. Now i want to print the reversed string through cout << reversed; but this is printing nothing. I can print it through a for loop through reverse[i] until the size but is there any better way?
#include <iostream>
#include <string>
using namespace std;
int main()
{
string original = "hello";
string reverse;
int i, j = 0, size = original.length();
for (i = size - 1; i >= 0; i--) // original's last as reversed's first
{
reverse[j] = original[i];
j++;
}
reverse[j] = '\0'; //last value as null
cout << "original string = " << original << endl;
cout << "reversed string = " << reverse << endl;
system("pause");
return 0;
}
A better solution is to use std::reverse:
std::string original = "whatever";
std::string rev = original;
std::reverse(rev.begin(), rev.end());
You can use reverse iterators to instantiate the reversed string from the original:
string original = "hello";
string reverse(original.rbegin(), original.rend());
Then
std::cout << "reversed string = " << reverse << std::endl;
Note: avoid using namespace std;. There is an algorithm called std::reverse, whose name you could be inadvertently pulling into the global namespace. And you do have a variable with that name.
See this working demo.
The major problem with your code is that reverse is empty, so using any index leads to you indexing out of bounds and undefined behavior.
As a side-note, you don't have to terminate std::string objects, they are automatically terminated.
you should notice that string reverse doesn't have a size , so when you do something like this
for (i = size - 1; i >= 0; i--) // original's last as reversed's first
{
reverse[j] = original[i];
j++;
}
Here, you are accessing an index that is not found in string reverse,as it's size is 0 . You should take care of something like that.
There are plenty of answers that your received about this question , I just wanted to make you notice this mistake
instead , you can make
reverse+=original[i];
You don't even need to create a new string to hold the reversed string. Just iterator over the original string.
void print_reversed(const std::string& str)
{
std::copy(str.crbegin(), str.crend(), std::ostream_iterator<char>(std::cout));
}
Related
So while working through a course on Udemy over C++ one of the challenges was to check a string to see whether it was a palindrome or not. I completed the task successfully but went about it a different way than the instructor. I understand there are a multitude of ways to complete a task but I am wondering which is more efficient and why? It may seem stupid to be wondering about this while reteaching myself coding but I feel this is something I should be keeping in mind.
//Instructors code//
# include<iostream>
using namespace std;
/*program for reverse a string and check a string is a palidrome
*/
int main()
{
string str="MADAM";
string rev="";
int len=(int)str.length();
rev.resize(len);
for(int i=0, j=len-1; i<len; i++, j--)
{
rev[i]=str[j];
}
rev[len]='\0';
if(str.compare(rev)==0)
cout<<"palindrome"<<endl;
else
cout<<"not a pallindrome"<<endl;
return 0;
}
My Approach
#include <iostream>
using namespace std;
int main(){
string str1="test";
// cout << "Enter a string to check if it is a Palindrome: ";
// getline(cin,str1);
string str2;
string::reverse_iterator it;
for(it=str1.rbegin(); it!= str1.rend(); it++)
{
str2.push_back(*it);
}
if(!str1.compare(str2))
cout << "\nPalindrome";
else
cout << "\nNot a Palindrome";
return 0;
}
Thank you in advance.
In theory the code from your instructor is more efficient, but both examples have issues.
With your instructors code the main issue is the use of
int len=(int)str.length();
In this example, it is okay because we know the size of the string will fit in a int, but if you were getting a string from an outside source, this could be a problem. A std::string using an unsigned integer type to store the size of the string and that means you can have a string who's size is larger then what can fit in an int. If that were to happen, then code is not going to work correctly.
With your code you a avoid all that, which is great, but you also leave some performance on the table. In theory your code of
for(it=str1.rbegin(); it!= str1.rend(); it++)
{
str2.push_back(*it);
}
is going to cause str2 to have multiple buffer allocations and copies from the old buffer to the new buffer as it grows. This is a lot of extra work that you don't need to do since you already know how much space you need to allocate. Having
str2.reserve(str1.size() + 1);
before the loop pre-allocates all the space you need so you don't have those potential performance hits.
Then we come to the fact that both of your examples are using a second string. You don't need another string to check for a palindrome. What you can do is just check and see if the first and last characters are the same, and if they are move on to the first+1 and last-1 character and so on until you reach the middle or they don't match. You can do that using a construct like
bool is_palindrome = true;
for (auto start = str.begin(), end = str.end() - 1;
start < end && is_palindrome;
++start, --end)
{
if (*start != *end)
is_palindrom = false
}
if (is_palindrome)
std::cout << "palindrome\n";
else
std::cout << "not a pallindrome\n";
The simplest and most efficient way (no copying required) would be something like this:
inline bool is_palindrome(const std::string& u) {
return std::equal(u.begin(), std::next(u.begin(), u.length() / 2), u.rbegin());
}
I would say that both are almost the same, but as mentioned in the comments, the line:
str2.push_back(*it);
Is actually very inefficient, since std::string may copy the existing string to a new location in the memory, and then append the next char to the string, which is wasteful.
But I am wondering, why to create the copy in the first place?
It is very simple to run both from start to end, and from end to start to check it out, meaning:
bool is_polindrom(const std::string& str)
{
for (std::size_t idx = 0, len = str.length(); idx < len / 2; ++idx)
{
if (str[idx] != str[len - 1 - idx])
{
return false;
}
}
return true;
}
Running the code with:
int main()
{
const std::string right1 = "MADAM";
const std::string right2 = "MAAM";
const std::string wrong1 = "MADAAM";
const std::string wrong2 = "MEDAM";
std::cout << "MADAM result is: " << is_polindrom(right1) << std::endl;
std::cout << "MAAM result is: " << is_polindrom(right2) << std::endl;
std::cout << "MADAAM result is: " << is_polindrom(wrong1) << std::endl;
std::cout << "MEDAM result is: " << is_polindrom(wrong2) << std::endl;
}
Will yield:
MADAM result is: 1
MAAM result is: 1
MADAAM result is: 0
MEDAM result is: 0
You don't need extra memory in this case, since it is possible to iterate over a string from the end to the beginning, and you need to run on it exactly once (and notice that I stop when idx >= len / 2 since you don't really need to check each letter twice!).
I'm trying to reverse a string in my C++ code line below revStr.at(j) = str.at(size);
But it doesn't change any of the elements in revStr.
Is there another way to do it without using any libraries.
#include <iostream>
#include<sstream>
#include <iterator>
using namespace std;
int main() {
ostringstream d;
long long c = 123456789;
d << c;
//cout << c << endl;
string str = d.str();
//cout << str.at(0) << endl;
int size = str.size() - 1;
//cout << size << endl;
ostringstream e;
e << str;
string revStr = e.str();
for (int i = size; size==0; size--) {
//cout << str.at(size);
int j = 0;
revStr.at(j) = str.at(size);
j++;
} // End For
cout << "Original String is :" << str << endl;
cout << "Reversed String is :" << revStr << endl;
}
Use std::reverse:
#include <string>
#include <algorithm>
#include <iostream>
int main()
{
std::string test{"Hello"};
std::cout << "Original string: " << test << std::endl;
std::reverse(test.begin(), test.end());
std::cout << "Reversed string: " << test << std::endl;
return 0;
}
Output:
Original string: Hello
Reversed string: olleH
If you just want to reverse a string, you should use std::reverse, as described by Tyler Lewis. It is the best option.
If you want to learn C++, then writing your own version is good practice.
The line
for (int i = size; size==0; size--)
means “Create a new int called i and set it to size initially. Then, while size is zero, do the following and then decrement size”.
There are three problems with this:
Size is not zero unless you entered a one-character string
Since you never use i, there’s no point in declaring it
Inside the loop you use j which is set to zero each time.
You can fix the first by changing the middle part of the for loop to size >= 0 (but be careful—if you later change it so that size is an unsigned type, because it doesn’t make sense for it to be negative, that code won’t work; it’s generally better to increment going up instead). You can fix the second by using i everywhere in the loop statement, and not changing size. You can fix the third by using i in the loop body, and not declaring a new variable inside the loop.
I noticed you used std::string so I used std function swap and string. Depending on if you consider this as a 'library'. There are several definitions of 'reverse'. You could reverse the word order in a string, or a pure char to char reversal like I wrote. Reversal could also mean changing character case, etc... but this is simply swap first and last. Then swap the 2nd and 2nd to last, then swap the 3rd and 3rd to last, etc...
So some points from your code. You only need to loop half the string length. The swap is from the ith and the ith to last. So the last is numCharacters - 1, thus the ith to last would be Last - i or numCharacters - 1 - i. I believe this is what you intended by using a farLeft(i) and a farRight(j) index.
#include <iostream>
void reverseStringInPlace(std::string &stringToReverse)
{
int numCharacters = stringToReverse.length();
for (int i=0; i<numCharacters/2; i++)
{ std::swap(stringToReverse[i], stringToReverse[numCharacters-i-1]); }
}
int main()
{
std::string stringToReverse = "reversing a string";
std::cout << stringToReverse << std::endl;
reverseStringInPlace(stringToReverse);
std::cout << stringToReverse << std::endl;
return 0;
}
Output:
reversing a string
gnirts a gnisrever
Changes made to the piece of code in question, it works.
for (unsigned int i = size; size >= 0; size--) {
revStr[j] = str[size];
j++;
}
Please tell me why my code to reverse the input string is giving me various errors.
#include<iostream>
#include<string>
#include<cstdlib>
using namespace std;
void ReverseString(string &aString);
int main(){
string info;
cout << "What's your string?" << endl;
getline(cin, info);
ReverseString(info);
cout << ReverseString(string info) << " compare with: " << info << endl;
system("pause");
return 0;
}
void ReverseString(string &aString){
for(int i = 0; i < aString.length(); i++)
{
string temp = 0; // initialize temporary string
temp = temp + aString.at(aString.length() - 1 - i); // hold temporary string
if(i => aString.length()) /*assign temp string to aString when all chars are processed*/
{
temp = &aString;
}
}
}
Hi you could simplify your code a lot by using the STL
for example:
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
std::string str = "Hello World";
cout << str << endl;
std::reverse(str.begin() , str.end());
cout << str << endl;
return 0;
}
let me know if this is not suitable to your needs as theres a few other ways to do it too.
Without STL:
There are some corrections/changes to your code required, which I have supplied below. However you may want to look at some documentation on referencing variables to get an idea of how it works, such as:
http://www.cprogramming.com/tutorial/references.html
http://www.thegeekstuff.com/2013/05/cpp-reference-variable/
http://en.wikipedia.org/wiki/Reference_(C++)
What is a reference variable in C++?
http://www.tutorialspoint.com/cplusplus/cpp_references.htm
Correct reference and pointer use is a major part of C++ and allows for some of the most powerful functionality in the language, provided it is used correctly, or major headaches and mental scarring if used incorrectly, so it is worth, even essential, to have a firm grasp of them.
And even then expect the odd misuse to crop up every-so-often. :)
#include<iostream>
#include<string>
#include<cstdlib>
using namespace std;
void ReverseString(string &aString);
int main(){
string info;
cout << "What's your string?" << endl;
getline(cin, info);
cout << info << " compare with: ";
ReverseString(info);
cout << info << endl;
system("pause");
return 0;
}
void ReverseString(string &aString)
{
int len = aString.length();
string temp = aString;// initialize temporary string
aString ="";
for(int i = 0; i < len; i++)
{
aString += temp[len - (1+ i)]; // assigns the reversed value to the referenced string
}
}
Just noticed the quote below from #zac-howland : so true, I have however left the code in as an illustrative piece. Provided some reading is done on this as well as plenty of experimentation I hope NewProgrammer will get the information and skill-set he needs to go forward.
#include<iostream>
#include<string>
#include <algorithm>
using namespace std;
string info;
string rvrs(string &str)
{
std::reverse(str.begin(),str.end());
return str;
}
int main()
{
cout<<"What is your string :: ";
getline(cin,info);
cout<<rvrs(info);
cout<<endl;
return 0;
}
You have a few syntactical errors in addition to your logical ones:
cout << ReverseString(string info) << " compare with: " << info << endl;
ReverseString(string info) will pass in an empty string to your ReverseString function (if it even compiles - which looks like it should not since you have 2 info's in the same scope). What you wanted is:
cout << ReverseString(info) << " compare with: " << info << endl;
In your reverse function, you only need to go to length() / 2.
Since you are passing by reference, changes you make to the string within the function will be reflected in the object you passed into it. That is, the original info will be reversed. If you want it to operate on a copy, you need to pass it by copy, not by reference.
Finally, cout << ReverseString(info) is not useful (if it even compiles) as ReverseString returns a void. You should have it return a string (the reversed string).
You have a number of problems.
string temp = 0; // initialize temporary string
It doesn't really make sense to initialize a string to 0. Just string temp; would be fine here.
temp = temp + aString.at(aString.length() - 1 - i); // hold temporary string
That's not quite how I'd do things, but I guess it should work.
if(i => aString.length())
This condition doesn't seem to make sense. Your loop is defined to iterate with i going from 0 to the length of the string -1, so it can never be greater than or equal to the string length.
/*assign temp string to aString when all chars are processed*/
{
temp = &aString;
}
Here the code doesn't match the comment. The comment says you're going to assign to aString, but the code assigns something to temp. The comment is probably closer to what you really want. But you still need to fix the condition, and probably want to do this after the loop has finished executing. So in pseudo-code, you'd end up with something like:
for (all characters in the string)
add the next character in the string to the end of temp
assign temp back to the original string
What i'm trying to do is create a template array class that will store values of a data type into an array. I have it working fine with int values, however working with string objects things start to break down.
I've taken out the block of code and tried it on it's own and I do get the same error. I'm sure I've learnt this, and I'm almost positive that the answer is something simple, trying to wrap my head around the pace in which we're learning c++ is a little crazy at times!
My best guess right now, is that I would need to tokenize the string and look for spaces. I tend to over think things though which lead to more confusion - thus me seeking out a answer here!
The code:
// Test String: Hello World this is a String Object
int stringSize = 7;
int count = 0;
string s[stringSize];
cout << "\nEnter " << stringSize << " one-word string values:\n";
while (count < stringSize) {
string tmpVal;
cin >> tmpVal;
s[count] = tmpVal;
count ++;
}
string s[stringSize]; is illegal because stringSize is not a constant. You must either use dynamic memory (i.e. string* s = new string [stringSize];), include stringsize as a template argument (don't do this, it doesn't actually solve the problem), use a fixed size value, or use an existing structure (I'd suggest vector, as in Bill's answer). The code below works fine on my compiler:
int main(int argc, char *argv[]) {
int stringSize = 7;
int count = 0;
string* s = new string [stringSize];
cout << "\nEnter " << stringSize << " one-word string values:\n";
while (count < stringSize) {
string tmpVal;
cin >> tmpVal;
s[count] = tmpVal;
count ++;
}
delete[] s;
}
I am a little confused as to exactly what you're looking for, but I suggest looking into the standard library.
Perhaps something like:
list<string> s;
and then, in the loop use push_back.
I am also confused what is your actual question, because your code works. However, FWIW, I would suggest the following. The changes are: (1) use of const (already suggested by others), (2) use of size_t, (3) change of variable name stringSize to numStrings (because of this I was confused at first glance), and (4) avoiding string copy.
#include <iostream>
#include <string>
using namespace std;
int main()
{
const size_t numStrings = 7;
size_t count = 0;
string s[ numStrings ];
cout << "\nEnter " << numStrings << " one-word string values:\n";
while (count < numStrings) {
cin >> s[ count ];
count++;
}
return 0;
}
Why not read in the entire line, then find all spaces and using the substr method, split the string?
You will need the following methods:
getline()
find_first_of()
substr()
Also, searching around this site for splitting strings in c++ will give you a lot of tips.
First of all, the size of your array should be constant:
const int stringSize = 7;
Secondly, as dbrien said, you should use std::vector unless you're doing this for the learning experience:
std::string tmpVal;
std::vector<std::string> s;
cout << "\nEnter " << stringSize << " one-word string values:\n";
while (cin >> tmpVal)
{
s.push_back(tmpVal);
}
First, the array dimension must be constant, so it should be const int stringsize = 7; Also, I would suggest using std::vector rather than std::list, additionally What was the error?
Not sure what error you're getting, but this is wrong because you need to use a constant integral value to allocate arrays on the stack.. Change:
int stringSize = 7;
int count = 0;
string s[stringSize];
... to:
const int stringSize = 7;
int count = 0;
string s[stringSize];
You can and probably should also use a vector instead of using C-style arrays, or trying to hand roll your own templated array class:
vector<string> s;
const int stringSize = 7;
cout << "\nEnter " << stringSize << " one-word string values:\n";
while (s.size() < stringSize) {
string tmpVal;
cin >> tmpVal;
s.push_back(tmpVal);
}
So it turns out it was the compiler. I was using xCode and getting:
cin_cout(7307) malloc: *** error for object 0x1000072c0: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
Running the same block in Visual c++ seemed to be ok... Sorry for my stupidity and thanks kindly for all the quick feedback!
I'm working on a small little thing here for school. After hours of researching, and a ton of errors and logic reworking I've almost completed my little program here.
I'm trying to take user input, store it into the string, get a character array from the string ( dont ask why, I just have to put this into a character array ), then get the reversed order of the phrase that the user entered. Here is my code:
#include "stdafx.h"
#include <iostream>
#include <String>
#include <cstring>
using namespace std;
using namespace System;
#pragma hdrstop
char* getCharArray(string);
string reversePhrase( int, char* );
void main(void)
{
string sPhrase = "";
int sSize = 0;
string sReversed = "";
char* cPhrase = NULL;
cout << "Welcome to the You Type It & We'll Reverse it! [Version 1.0] " << endl;
cout << "This program will reverse your phrase, and count how many characters are in it!" << endl;
cout << "To begin just enter a phrase." << endl;
cout << "Enter a phrase: ";
getline( cin, sPhrase);
sSize = sPhrase.length();
cout << endl;
cPhrase = getCharArray(sPhrase);
sReversed = reversePhrase( sSize, cPhrase );
cout << sReversed;
system("pause");
}
string reversePhrase(int size , char* cPhrase)
{
string sReversed = "";
int place = size;
for ( int i = 0; i < size ; i ++ )
{
sReversed.append(1, cPhrase[place]);
cout << "Current string: " << sReversed << endl;
cout << "Current character: " << cPhrase[place] << endl;
place--;
}
return sReversed;
}
char* getCharArray(string sPhrase)
{
int size = 1;
size = sPhrase.length();
char* cArray = NULL;
cArray = new char[size];
for (int i = 0 ; i < size ; i++)
{
cArray[size] = sPhrase.at(i);
}
return cArray;
}
When I type in "ownage" into the program, this is what I get returned:
It is almost like my Character Array is getting garbage collected before it can use all of the characters. This is probably an easy fix but, I just don't see how I can get around this one.
Try rewriting getCharArray like this
char* getCharArray(string sPhrase)
{
int size = 1;
size = sPhrase.length();
char* cArray = NULL;
cArray = new char[size+1]; // NOTE
for (int i = 0 ; i < size ; i++)
{
cArray[i] = sPhrase.at(i); // NOTE
}
}
cArray[size]=0; // NOTE
return cArray;
}
Note that the assignment in the loop now uses the index variable. Also, you need to allocate one extra char in the array to set the null terminator for the string and then you need to set it at the end.
You'll also need to think about deallocating the array at some point
The bug is in this line:
cArray[size] = sPhrase.at(i);
That size should be your loop index.
You should probably look at using std::string more, and not poke around with character arrays when there's no need to.
Why use a char array at all? It's not only useless – it complicates the code substantially (the usage of your function is more difficult, and you've forgotten to free the memory allocated by new!). Why not just have the following function:
string reverse(string const& input);
(Passing the argument by const reference instead of by value saves you a copy!)
In fact, implementing the function only takes a single line using the features of the string class (one of its constructors takes two iterators):
string reverse(string const& input) {
return string(input.rbegin(), input.rend());
}
reversePhrase is also not correct. Try something like this:
string reversePhrase(int size , char* cPhrase)
{
string sReversed = "";
sReversed.resize(size);
int place = size - 1;
for ( int i = 0; i < size ; i ++ )
{
sReversed [i] = cPhrase[place];
cout << "Current string: " << sReversed << endl;
cout << "Current character: " << cPhrase[place] << endl;
place--;
}
return sReversed;
}
First, start the array with -1. After that, use a for loop with -1 and increment inside the loop. Then, you can get the first element of the array.