At first this seemed easy to do but I was mistaken. I want to 'replace' a char (a variable) in a string with a int (value). But how?
I tried replace() because I'm working with a string and it worked but it will not work if I wish to change the value of the variable again to another
value because then the original variable will not be found. I have been struggling with this for the passed 2 days. Any help will be much appreciated on how to do this.
changeVar(string startingExpr, char var, int val)
{
for(int i = 0; i < startingExpr.length(); i++)
{
if(startingExpr[i] == var)
{
cout << "I found x! Now to replace it!";
startingExpr[i] = val; //'Replace' x with 5, but x but how?
}
}
}
Your help on this one will be much appreciated.
J
If you want to be able to replace the "variable" by the "value" as many times as you want, you should keep a copy of the original string and do the replacement from there only.
Another option is to undo the replacement (replace the value by the variable) and redo with another value, provided the undo can be done unambiguously.
I am sorry to hear this problem wasted your more than 2 days. You shouldcarefully read a basic C++ textbook. As your problem is a very basic one. If you understand the arguments passing for function, you will sort this out!
More specific, you function is not wrong, but just did not deliver the results you want. Because your function using argument, which will make a copy of argument startingExpr inside the function body, when you make replacement using "startingExpr[i] = val", the replacement is happened on the copy of startingExpr (which is a local variable just visible inside the function), your original startingExpr does change at all.
The solution is very simple, change argument passing with its reference, just add &, now the declaration your function should be: "changeVar(string startingExpr, char var, int val)"
Try the following code, which will demonstrate my explanation:
#include <iostream>
#include <string>
using namespace std;
//the original function which was reported had problem
// This because the function using parameter not reference
void changeVar(string startingExpr, char var, int val)
{
for(int i = 0; i < startingExpr.length(); i++)
{
if(startingExpr[i] == var)
{
cout << "I found x! Now to replace it!(But the replace is happend inside the function, the original string is not changed!)"<<endl;
startingExpr[i] = val; //'Replace' x with 5, but x but how?
}
}
}
// updating using the reference
//void changeVar(string & startingExpr, char var, int val)
void changeVar_refe(string & startingExpr, char var, int val)
{
for(int i = 0; i < startingExpr.length(); i++)
{
if(startingExpr[i] == var)
{
cout << "I found x! Now to replace it!(Now using reference, the replace is happend inside the original string, which will be changed!)"<<endl;
startingExpr[i] = val; //'Replace' x with 5, but x but how?
}
}
}
int main()
{
//lets test the original function
string my_name="Hi there, I am C++ taoism .";
cout<<"The original string is: "<<my_name<<endl;
//lets change a to A
changeVar(my_name,'a',65);
cout<<"The changed string is: "<<my_name<<endl;
cout<<endl;
cout<<"Using the reference to test ... "<<endl;
cout<<endl;
cout<<"The original string is: "<<my_name<<endl;
//lets change a to A
changeVar_refe(my_name,'a',65);
cout<<"The changed string is: "<<my_name<<endl;
//cout <<"Char A is int 65:"<<int('A')<<endl;
}
Your mistake is to assign the number that is different from the ASCII character represntation. In ASCII table characters '0', '1' .. '9' goes one after another. So you can rewrite your code as:
startingExpr[i] = '0' + val;
But please aware that this is good for one character case only. If you need to replace multiple chars then your solution without need for a function is that:
#include <iostream>
#include <string>
int
main()
{
std::string a("SomeVar1"), b("SomeVar356");
std::string::size_type index = std::string::npos;
std::cout << "Before: " << a << std::endl;
if ((index = a.rfind("1")) != std::string::npos)
a.replace(index, 1, std::to_string(2));
std::cout << "After: " << a << std::endl;
std::cout << "Before: " << b << std::endl;
if ((index = b.rfind("356")) != std::string::npos)
b.replace(index, 3, std::to_string(673));
std::cout << "After: " << b << std::endl;
return 0;
}
This is slightly optimized since it is using rfind (search from the end of the string).
P.S. As the comments suggested - you can use std::replace with reverse iterators and lambda for condition. Since it is available in C++11 I wrote a small example in universal style.
Related
#include <iostream>
#include <string.h>
using namespace std;
void reverse_Text(char* str);
int main()
{
int i, j, Text, revText;
const int [arrSize] = 101;
char Text[arrSize], revText[arrSize];
cout << "Please enter a sentence: ";
cin >> Text;
reverse_Text(Text);
cout << "The reversed text is " << Text << endl;
return 0;
}
int reverse_Text(char ori_str, char rev_str)
{
int i = 0;
for(j = strlen(ori_str)-i, j >= 0, j--){
rev_str[i] = ori_str[j];
rev_str[i+1] = '0';
i += 1;
}
}
I cant seem to find the error, im still a newbie in coding, it says that text and revtext are not declared, i cant find the next error when there is already an error at the top
int reverse_Text(char ori_str, char rev_str)
This function takes two characters as parameters. You actually need to pass the addresses of the characters.
int reverse_Text(const char* ori_str, char* rev_str)
Notice that I've declared the first parameter as const. This tells the compiler that the original string is not supposed to be changed. The compiler will then generate an error if any code inside the function tries to modify any character in the original string.
You will need to pass both parameters, not just one, when you call the function.
reverse_Text(Text, revText);
This is actually passing the address of the first character in each string.
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++;
}
this script is supposed to output array values that were inputted by the user into array "store." I am trying to store all the char array values into string temp. I get the error on line 12: "[Error] invalid conversion from 'char*' to 'char' [-fpermissive]." Would appreciate any help!
Edit: so I fixed the declaration and now at least it compiles, but the answer I get on my cmd is all jumbled up. Why is this so? The cmd only correctly couts the first string but after the space, it messes up.
#include <iostream>
#include <cstdlib>
using namespace std;
void coutArray(char[], int);
int main()
{
char store[50];
cout << "enter text: " << endl;
cin >> store;
coutArray(store, 50);
system("pause");
return 0;
}
void coutArray(char store[], int max)
{
string temp = "";
int i = 0;
while (i < max)
{
temp += store[i];
i++;
}
cout << temp << endl;
}
Using input from all answerers I finally got the fixed code:
#include <iostream>
#include <cstdlib>
#include <string>
using namespace std;
void coutArray(char[], int);
int main()
{
char store[50] = {0};
cout << "enter text: " << endl;
cin.getline(store, 50);
coutArray(store, 50);
system("pause");
return 0;
}
void coutArray(char store[], int max)
{
string temp = "";
int i = 0;
while (i < max && store[i]!=0)
{
temp += store[i];
i++;
}
cout << temp << endl;
}
Thanks everyone. i learned a lot!!!
When you get an input using "cin" your input automatically ends with 0 (NULL).
You just need to add one little piece of code to your while statement.
instead of this :
while (i < max)
use this :
while (i < max && store[i]!=0)
Now it will stop when the input string is finished and won't print any garbage existed in the array beforehand.
To show that cin does add terminating zero, i initialized the array to 46, and put a breakpoint after the cin
so I fixed the declaration and now at least it compiles, but the answer I get on my cmd is all jumbled up. Why is this so?
Not sure what you mean by jumbled up. But since you did not tell us what you typed its hard to know it looks like it worked to me:
> ./a.out
enter text:
Plop
Plop�ȏU�
Notice that since my input is only 4 characters long. This means that a lot of the characters in the array still have undefined (ie random values). This is why I am seeing junk. To get past this initialize the array to have all 0 values.
char store[50] = {0};
Even bettern use a C++ object than handles longer strings.
std::string store;
std::getline(std::cin, store);
Note: passing arrays to functions by value is not a good idea. On the other end they have decayed to pointers and thus do not act like arrays anymore (they act like pointers whose semantics are similar but not identical).
If you must pass an array pass it by reference. But I would use a C++ container and pass that by reference (it is much safer than using C constructs). Have a look at std::string
The declaration of the function is wrong. Should be void coutArray(char *, int);
Look at the Implicit Conversion rules to understand what the compiler can do and what it cannot to do for you.
The issue with your program was that you were probably entering in less characters than the maximum size of the buffer. Then when you passed the maximum size as the parameter to coutArray, you assigned unfilled slots in the char array to temp. These unfilled slots could contain anything, as you have not filled them up to that point.
Your program is still correct, but what would be better would be to use read so that the number of bytes you specify is the minimum number of bytes that can be entered:
std::cin.read(store, 50);
Even better solution would be to use std::string:
std::string store;
std::cin >> store;
// or for the entire line
std::getline(std::cin, store);
It also follows that your coutArray should be changed to:
void coutArray(std::string);
// ...
void coutArray(std::string str)
{
std::cout << str << std::endl;
}
Look at this way
template<typename T, size_t N>
void MyMethod(T (&myArray)[N])
{
//N is number of elements, myArray is the array
std::cout<<"array elements number = "<<N<<endl;
//put your code
string temp;
temp.resize(N+1);//this is for performance not to copy it each time you use += operator
int i = 0;
while (i < max)
{
temp += store[i];
i++;
}
cout << temp << endl;
}
//call it like this
char arr[] = "hello world";
MyMethod(arr);
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