Could someone explain in detail on how this palindrome code works? - c++

#include <iostream>
// Define is_palindrome() here:
bool is_palindrome(std::string text) {
std::string reversed_text = "";
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
}
if (reversed_text == text) {
return true;
}
return false;
}
int main() {
std::cout << is_palindrome("madam") << "\n";
std::cout << is_palindrome("ada") << "\n";
std::cout << is_palindrome("lovelace") << "\n";
}
Could someone please explain to me in great detail on how this for loop works? I would like to know how the code sequences each letter to figure out if the word is a palindrome.

To begin, we define the is_palindrome function to return a bool. Our input string is the function parameter.
Then, we reverse the text using the following for loop.
std::string reversed_text = "";
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
}
This code simply defines a string called reversed_string. It then reverses the input string one letter at a time (text[i] gives the i+1 letter in the input string, reversed_text += text[i] adds this letter to the string).
After that, the code is a simple if statement. It compares the original string and the reversed string. If both are the same, the function returns true. If they are not the same, the function returns false.
if (reversed_text == text) {
return true;
}
return false;
}

The ability to read code is far more important than the ability to write it, as in most of your programming life, you will read a lot more than write. More often than not, it's someone else's code. If you lack the skill, you will have a hard time working in this field.
Here's above function body commented:
// prepare a string variable to hold the reversed input
std::string reversed_text = "";
// append each character from the input to variable above, starting from the last character, effectively generating its reversed version
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
}
// it's a palindrome if the reversed input is the same as the input
if (reversed_text == text) {
return true;
}
// not a palindrome otherwise
return false;

Related

Cannot find a logical sense

i already studied c++ in school and during the last days i have been doing the beginner c++ course of codecademy. On codecademy there is an exercise in which i have to identify palindrome words and return true or false. I haven't been able to resolve it so i saw the solution and it was:
#include <iostream>
// Define is_palindrome() here:
bool is_palindrome(std::string text) {
std::string reversed_text = "";
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
}
if (reversed_text == text) {
return true;
}
return false;
}
int main() {
std::cout << is_palindrome("madam") << "\n";
std::cout << is_palindrome("ada") << "\n";
std::cout << is_palindrome("lovelace") << "\n";
}
My only doubt is with this line:
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
i know it has to do with index values but i can't understand why it has a -1.
Could somebody explain this to me?
i thanks in advance whoever read this post. i'm sorry for my english or my poor using of stacksoverflow, i'm italian and that's my first time using this site.
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
text is basically the string that you receive as input via function. size() is function that returns the size of the string i.e text.size() so in our test cases it will return
5 for madam
3 for ada
8 for lovelace
If you think about the strings as an array with exact above size then the index range will become
0-4 for madam
0-2 for ada
0-7 for lovelace
So that's why the text.size()-1 is using as the starting index of loop. text.size() will return the actual size of string and then minus 1 to get the index of last character in string.
so behind the scene, your loop iteration will look something like below
for (int i = 4; i >= 0; i--) { //for madam
}
//aca
for (int i = 2; i >= 0; i--) {
}
//lovelace
for (int i = 7; i >= 0; i--) {
}
I hope it clear out your confusion.
Thanks,
i know it has to do with index values but i can't understand why it has a -1.
If a string is n characters long, the characters in it are indexed from 0 to n−1.
Since the loop works with characters from the end of the string to the beginning, it starts with index text.size() - 1.
However, the solution you have shown is nominally inefficient. There is no reason to make a reversed copy of the string. It suffices merely to test whether each character in the first half of the string equals the character in the reflected position:
bool is_palindrome(std::string text)
{
size_t e = text.size();
for (int i = 0; i < e/2; ++i)
if (text[i] != text[e-1-i])
return false;
return true;
}
If using a for loop to reverse the string is confusing, you could also use the reverse function
std::string reversed_text = text;
reverse(reversed_text.begin(),reversed_text.end());
which just helps flip the entire string reversed_text and can achieve the same result in a simpler way.

Tesseract OCR (C++) Cannot Evaluate Output String

I am attempting to extract output string from an OpenCV Matrix window and evaluate it, but it seems to return something similar to "someString\n" rather "someString". This renders it difficult to compare knowing there are (x) amounts of white spaces.
I tried:
creating a char array that omits the white spaces (I am aware that I'm only evaluating 5 indexes)
std::string redef;
char charArr[100] = {NULL};
strcpy_s(charArr, str.c_str());
for (int i = 0; i < 5; i++)
{
if (charArr[i] != ' ')
{
redef += charArr[i];
}
}
std::cout << "analyseAction ran:" << redef << "white-space?";
but the string returns something like
analyseAction ran:redefString
white-space?
Relevant code running in main function:
api->Recognize(0);
outText = api->GetUTF8Text();
analyseAction(outText);
bellow, just take note that the else statement runs since redef doesn't equal to "long" when long is shown visually in window.
void analyseAction(std::string str)
{
std::string redef;
char charArr[100] = {NULL};
strcpy_s(charArr, str.c_str());
for (int i = 0; i < 5; i++)
{
if (charArr[i] != ' ')
{
redef += charArr[i];
}
}
std::cout << "analyseAction ran:" << redef << "white-space?";
//alot of missing code, trying to show what is relevant
if (redef == "long") //check if it has white space after long, seems like it new line's
{
//NOTE FOR FUTURE: Stop being lazy and make this a function of its own
//BUY
std::cout << "Long ran";
for (int i = 0; i < a; i++) //no comma with first line so 0 element
{
context += inData[i];
}
x = std::stoi(context);
for (int i = a+1; i < a1; i++)
{
context += inData[i];
}
y = std::stoi(context);
simClick(x,y);
//BUY CONFIRM
for (int i = a1+1; i < b; i++) //starting from pipeline??
{
context += inData[i];
}
x = std::stoi(context);
for (int i = b+1; i < b1; i++) //starting with comma? +1 to fix
{
context += inData[i];
}
y = std::stoi(context);
simClick(x, y);
}
else
{
std::cout << "long does not match";
}
}
I am confused, why does the string appear to new line? And how can I successfully evaluate the output? I am a noobie in C++ so any help will be greatly appreciated.
As for why it's returning a string and a line break, I cannot answer that. But I can provide you an alternative system to what you are trying to accomplish. Remove your first for loop in the analyseAction function and in your if statement for "long" pass in... if(charArr[0] == 'l') {//do stuff} This does have a limit if you are assessing many words that start with "l", in this case assess the first two or three letters of the word so long they aren't less than 2 or 3 letter words. PS. this was written on mobile.

Understanding this palindrome code in C++

I'm having trouble wrapping my head around this piece of code for finding out if something is a palindrome or not.
I was hoping someone could break it down for me and make it simple stupid.
I included the full code but I think the part that's baffling me, is this line
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
}
Thanks in advance!
#include <iostream>
// Define is_palindrome() here:
bool is_palindrome(std::string text){
std::string reversed_text = "";
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
}
if (reversed_text == text){
return true;
}
return false;
}
}
int main() {
std::cout << is_palindrome("madam") << "\n";
std::cout << is_palindrome("ada") << "\n";
std::cout << is_palindrome("lovelace") << "\n";
}
The code creates a string that is the reverse of the first one and compare if they are equal.
for (int i = text.size() - 1; i >= 0; i--) {
reversed_text += text[i];
}
Simply reverses input string. It iterates input string barwards and adds chars to reversed_text char by char. If text is warchant, then reversed_text would be tnahcraw.
More C++-ish way to do this is using std::reverse:
// don't forget to include this
#include <algorithm>
bool is_palindrome(std::string text){
std::string reversed = text;
std::reverse(reversed.begin(), reversed.end());
...
This function is not "most efficient code", but it will do the job.
The code functions by inputting the reversed version of your input word and checks if the reversed version and the original version of the word are the same. As for the for loop, it works like this:
Get the position of the last character of the input word
Input that character into your reversed_text string
Continue as i decreases by 1, until the first character of the input word is inputted into the reversed_text string
This line of code reads the word from reverse direction and concatenates with the variable reversed_text.
for (int i = text.size() - 1; i >= 0; i--)
reversed_text += text[i];
For example:
if text = "HELLO" then reversed_text="OLLEH"

C++ Palindrome program always giving 0 (false) as an output problem; Where is my code wrong?

The problem is that it always outputs 0 (false) as a result. Probably the problem is in the isPalindrome function, but I cannot figure where exactly. Would be grateful if someone helped.
#include <iostream>
#include <cmath>
#include <string>
using namespace std;
bool isPalindrome(string word)
{
bool result;
for (int i = 0; i <= word.length() - 1; i++)
{
if (word.at(i) == word.length() - 1)
{
result = true;
}
else
{
result = false;
}
return result;
}
}
int main()
{
string word1;
int count;
cout << "How many words do you want to check whether they are palindromes: " << flush;
cin >> count;
for (int i = 0; i < count; i++)
{
cout << "Please enter a word: " << flush;
cin >> word1;
cout << "The word you entered: " << isPalindrome(word1);
}
}
Try this one:
bool isPalindrome(string word)
{
bool result = true;
for (int i = 0; i < word.length() / 2; i++) //it is enough to iterate only the half of the word (since we take both from the front and from the back each time)
{
if (word[i] != word[word.length() - 1 - i]) //we compare left-most with right-most character (each time shifting index by 1 towards the center)
{
result = false;
break;
}
}
return result;
}
In this statement
if (word.at(i) == word.length() - 1)
the right side expression of the comparison operator is never changed and have the type std::string::size_type instead of the type char. You mean
if (word.at(i) == word.at( word.length() - 1 - i ))
However there is no sense to use the member function at. You could us the subscript operator. For example
if ( word[i] == word[word.length() - 1 - i ] )
And the loop should have word.length() / 2 iterations.
Also within the loop you are overwriting the variable result. So you are always returning the last value of the variable. It can be equal to true though a string is not a palindrome.
Also the parameter should be a referenced type. Otherwise a redundant copy of the passed argument is created.
The function can be defined the following way
bool isPalindrome( const std::string &word )
{
std::string::size_type i = 0;
std::string::size_type n = word.length();
while ( i < n / 2 && word[i] == word[n - i - 1] ) i++;
return i == n / 2;
}
Another approach is the following
bool isPalindrome( const std::string &word )
{
return word == std::string( word.rbegin(), word.rend() );
}
Though this approach requires to create a reverse copy of the original string.
The simplest way is to use the standard algorithm std::equal. Here is a demonstrative program
#include <iostream>
#include <string>
#include <iterator>
#include <algorithm>
bool isPalindrome( const std::string &word )
{
return std::equal( std::begin( word ),
std::next( std::begin( word ), word.size() / 2 ),
std::rbegin( word ) );
}
int main()
{
std::cout << isPalindrome( "123454321" ) << '\n';
return 0;
}
I hope this one helps you also (corrected also warnings):
bool isPalindrome(string word)
{
bool result = false;
int lengthWord = (int)word.length();
for (int i = 0; i <= (lengthWord / 2); ++i)
{
if (word.at(i) == word.at(lengthWord - i -1))
{
result = true;
continue;
}
result = false;
}
return result;
}
Two possible problems.
You appear to be comparing a character to a number
if (word.at(i) == word.length() - 1)
shouldn't this be
if (word.at(i) == word.at(word.length() - i)) ?
There are 3 returns within the if statement, so no matter what the outcome it's only going to compare one character before returning to the calling function.
As a point of technique, repeated calls to .length inside the loop, which always returns the same value, wastes time and makes the code more difficult to understand.
You need to return as soon as you find a mismatch. If you are looking for a palindrome you only need to compare the first half of the word with the second half in reverse order. Something like
bool isPalindrome(string word)
{
for (int i = 0, j= word.length() - 1; i<j; i++, j--)
// i starts at the beginning of the string, j at the end.
// Once the i >= j you have reached the middle and are done.
// They step in opposite directions
{
if (word[i] != word[j])
{
return false;
}
}
return true;
}
The loop in the function isPalindrome will only execute once, because the return statement is unconditionally executed in the first iteration of the loop. I am sure that this is not intended.
To determine whether a string is a palindrome, the loop must be executed several times. Only after the last character has been evaluated (in the last iteration of the loop) will it be time to use the return statement, unless you determine beforehand that the string is not a palindrome.
Also, in the function isPalindrome, the following expression is nonsense, as you are comparing the ASCII Code of a letter with the length of the string:
word.at(i) == word.length() - 1
Therefore, I suggest the following code for the function:
bool isPalindrome(string word)
{
for (int i = 0; i < word.length() / 2; i++)
{
if (word.at(i) != word.at( word.length() - i - 1) ) return false;
}
return true;
}
As discussed in the comments under your question. You made some mistakes in the code.
Your function should more or less look like this:
bool isPalindrome(string word) {
bool result = true;
for (int i = 0; i <= word.length() - 1; i++)
{
if (word.at(i) != word.at(word.length() - 1 -i))
{
return false;
}
}
return result;
}

C++ ::toupper not allowing equality comparison?

I am trying to convert a string to uppercase so I can manipulate it, but while I can successfully manipulate natural uppercase strings, as well as convert lowercase to uppercase, using this method of conversion fails to allow the manipulation.
For example, if I pass "hello" through the encryption, my encrypted string becomes "HELLO", but when I pass "HELLO" through (naturally capitalized), it correctly shifts.
Is there a different way of forcing uppercase that I need to be using or am I doing something wrong?
int Caesar::encrypt (const std::string &message, std::string &emessage) {
int count = 0;
emessage = message;
std::transform(emessage.begin(), emessage.end(), emessage.begin(), ::toupper);
for (std::string::size_type i = 0; i < message.size(); i++) {
for (int j = 0; j < 26; j++) {
if (emessage[i] == std_alphabet[j]) {
std::replace(emessage.begin(), emessage.end(), message[i], c_alphabet[j]);
}
}
count++;
}
return count;
}
constructor:
Caesar::Caesar (int shift) {
// loop to populate vector with 26 letters of English alphabet
// using ASCII uppcase letter codes
for (int i = 0; i < 26; i++) {
std_alphabet.push_back(i + 65);
}
// fills Caesar alphabet with standard generated alphabet
c_alphabet = std_alphabet;
// shifts Caesar alphabet based off the constructor parameter
std::rotate(c_alphabet.begin(), c_alphabet.begin() + shift, c_alphabet.end());
}
test file:
void testCaesar() {
Caesar test(4);
std::string original = "HELLO";
std::string encrypted = "";
test.encrypt(original,encrypted);
std::cout << encrypted << std::endl;
std::cout << original << std::endl;
}
int main() {
testCaesar();
return 0;
}
Obviously there is a header and includes and stuff but that is the basic code
the header file includes the two private vectors
The specific issue you are seeing is that you're replacing the wrong thing here:
std::replace(emessage.begin(), emessage.end(), message[i], c_alphabet[j]);
If message was lowercase, then emessage will be all upper-case letters - none of which will be message[i]. so that replacement won't do anything. You meant:
std::replace(emessage.begin(), emessage.end(), emessage[i], c_alphabet[j]);
^^^^^^^^^^^
That said, your algorithm is totally wrong as HELLO encrypts as BCBBA with a shift of 4. There is a 1-1 mapping on letters, so H and L cannot both go to B. What you want to do is shift each letter as you go by just replacing it with what its next letter should be. That is:
for (std::string::size_type i = 0; i < emessage.size(); ++i) {
emessage[i] = c_alphabet[emessage[i] - 'A'];
}
With which you don't actually need the initial transformation step:
emessage = message;
for (std::string::size_type i = 0; i < emessage.size(); ++i) {
emessage[i] = c_alphabet[::toupper(emessage[i]) - 'A'];
}
The whole thing can be abridged quite a bit by just dropping your count (which is just the size anyway, so is redundant) and taking the message by-value:
std::string encrypt(std::string from) { // intentionally copying
for (char& c : from) {
c = c_alphabet[::toupper(c) - 'A'];
}
return from;
}