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"
Related
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.
#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;
First of all, I know there are many duplicates of this question but I have tried and tried and non have been able to solve my issue.
I have the following string
string s = "asdfqasdfp";
I need to loop through the string and find which sub string appears more than once. so in this case its
asdf
I have made the following code but I do not know why it doesn't work. I start from the full string and go down one at a time. I should get occurence value of 2.
int t = s.length();
for (int i = 0; i < s.length(); i++) {
string str = s.substr(0, t);
int occurence = 0;
size_t start = 0;
while ((start = s.find(str, start)) != string::npos) {
++occurence;
start += str.length();
}
if (occurence > 1) {
cout << occurence;
}
else {
--t;
}
}
EDIT: I only want the largest substring that the string contains, in this case
"asdf"
Here's a fixed version of your code, including Daniel's suggestions (thanks! Daniel's demo)
for (size_t t = s.length(); t >= 1; --t) {
for (size_t i = 0; (i + t) <= s.length(); i++) {
std::string str = s.substr(i, t);
size_t occurence = 0;
size_t start = 0;
while ((start = s.find(str, start)) != std::string::npos) {
++occurence;
start += str.length();
}
if (occurence > 1) {
std::cout << str << " " << occurence << std::endl;
return 0;
}
}
}
You need to
loop over t, and decrement it at the end of the i loop rather than inside
limit i to s.length() - t, so that there's always a t-length string to take as str
fix your substr to start at i not 0
You can also stop at the first time you find a duplicate, since this will be a largest duplicate (e.g. if there are two pairs of duplicates of length 4 it will find one of them, but it doesn't sound like you need both). You should also use size_t throughout as your integer type since that's what's used by the string functions here.
Looks ok. Do you need an << endl; to see output in your terminal?
Having some trouble understanding parts of the code; the output I am getting is also wrong. The problem is to replace all spaces in a string with '%20'. The full code is shown below; it compiles but doesn't run exactly as it should.
#include <iostream>
#include <string>
using namespace std;
void replaceSpaces(string str){
//Getting the length of the string, counting the number of spaces
int strLen = str.length();
int i, count = 0;
for (i = 0; i <= strLen; i++) {
if(str[i]==' ')
count++;
}
//Determining the new length needed to allocate for replacement characters '%20'
int newLength = strLen + count * 2;
str[newLength] = '\0';
for (i = strLen - 1; i >= 0; i--) {
if (str[i] == ' ') {
str[newLength - 1] = '0';
str[newLength - 2] = '2';
str[newLength - 3] = '%';
newLength = newLength - 3;
}
else {
str[newLength - 1] = str[i];
newLength = newLength -1;
}
}
cout << str <<endl;
}
int main() {
string str = "hello jellybean hello";
replaceSpaces(str);
return 0;
}
I am probably missing something obvious, but when allocating for the new string length in this line:
int newLength = strLen + count * 2;
Here we are multiplying the number of spaces by 2, but if we are trying to replace all spaces with '%20', why not multiply it by 3?
str[newLength] = '\0';
Does this line indicate that the position past the last character in the string is assigned a null space?
Am also confused about the else statement.
else {
str[newLength - 1] = str[i];
newLength = newLength -1;
}
Not sure if I completely understand the circumstance when this would be executed.
When the functions are compiled and run, if
string str = "hello jellybean hello";
the expected output would be hello%20jellybean%20hello, except the output I am getting is hello%20jellybean%20h.
In terms of time complexity, since there are two independent for loops, would the time complexity be O(n)?
I know I'm asking a lot of different questions, many thanks in advance for any answers!
This is wrong:
str[newLength] = '\0';
std::string objects maintain their NUL terminator internally based on their size. You want
str.resize(newLength);
instead.
int newLength = strLen + count * 2;
says to allocate space (later), equal to the length of the string, plus the number of whitespaces found multiplied by two, which makes sense.
For example: so glad to help, should use the slots that the whitespaces live into for the % and they will need two more slots each, for the 20 part of the replacement that will come into play.
This is WRONG:
str[newLength] = '\0';
can't you see? You access memory out of the bounds of your string. You act like you actually allocated space equal to the newLength, but you haven't that anywhere in the code yet.
Out of bounds accessing result in Undefined Behavior and that's bad.
The else statement is just for copying non-whitespace characters, but you should already given up on that code (if it's not yours) and start from scratch or/and take a sneak peak at: Encode/Decode URLs in C++.
As for the wrong result, you should know by reaching that point of that answer, that this is expected.
Trying to do the modification in place is tricky. It's much easier to create a new string:
std::string new_string;
for (int i = 0; i < str.length(); ++i) {
if (str[i] == ' ')
new_string += "%20";
else
new_string += str[i];
}
return new_string;
or, if you like range-for:
std::string new_string;
for (char ch : str) {
if (ch == ' ')
new_string += "%20";
else
new_string += ch;
}
return new_string;
You can change that string argument in function to reference, then there wont be any need for new string, at other part of the code, you can use insert function to add '2' and '0', and you only need to convert space to '&'.
void replaceSpaces(string &str) {
size_t strLen = str.length();
for (int i = 0; i < strLen; i++) {
if (str[i] == ' ') {
str[i] = '%';
str.insert(str.begin() + i + 1, '2');
str.insert(str.begin() + i + 2, '0');
strLen += 2;
}
}
}
This is easy; replace examplestring with your string in the code, and use as you would:
#include <iostream> //debug output
#include <string>
using std::string;
using std::cout;
using std::endl;
//the string to convert
string examplestring = "this is the example string for spaces into %20";
int main()
{
int countspaces = 0; //its faster to fill a known size
for (auto &x : examplestring)if (x == ' ')countspaces++; //counts spaces
string newstring; //declare new string
newstring.resize(examplestring.size() + (countspaces*3)); //pre-set size to make it run faster
int newstringiterator = 0; //keep track of new string location
//if ' '(space), place %20 in newstring and add 3 to iteration
//else just place the letter and iterate
for (int i=0;i<examplestring.size();i++)
{
if (examplestring[i] == ' ')
{
newstring.insert(newstringiterator, "%20");
newstringiterator += 3;
}
else newstring[newstringiterator++] = examplestring[i];
}
//final newstring is the original with %20 instead of spaces.
cout << newstring << endl;
system("PAUSE"); //to read console output
return 0; //return to zero
}
This will output newstring, which is the old string with '%20' instead of spaces.
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;
}