I'm getting this error (*s = *end; line) during debugging, while trying to reverse string using pointers.
I'm using Windows 10 OS, codeblocks IDE and GDB debugger.
#include <stdio.h>
#include <string.h>
#include <limits.h>
void myreverse(char* s);
int main()
{
char* s1 = "1234";
myreverse(s1);
printf("%s", s1);
return 0;
}
void myreverse(char* s) {
char tmp;
char* end = s + strlen(s) - 1;
for(; s < end; s++, end--) {
tmp = *s;
*s = *end;
*end = tmp;
}
}
You should change s1 to char s1[] = "1234"; since you are making changes to the string.
Then in your myreverse() function, you never use the tmp variable, which makes your swap block failing.
Fixed:
#include <cstdio> // use the C++ versions of the header files
#include <cstring>
void myreverse(char* s) {
char tmp;
char* end = s + std::strlen(s) - 1;
for(; s < end; s++, end--) {
// swap
tmp = *s;
*s = *end;
*end = tmp; // use tmp
}
}
int main() {
char s1[] = "1234";
myreverse(s1);
printf("%s", s1);
}
Note that the 3 lines in the swap block can be replaced with std::swap(*s, *end); and also that myreverse() can be completely replaced with std::reverse(std::begin(s1), std::end(s1));.
Related
I've written a simple function to count occurrences of a character in a string. The compiler is fine. However, as I try to run it, it produced a segmentation fault.
#include <iostream>
using namespace std;
// To count the number of occurences of x in p
// p is a ะก-style null-terminated string
int count_x(char* p, char x)
{
if (p == nullptr)
{
return 0;
}
// start the counter
int count = 0;
while (p != nullptr)
{
if (*p == x)
{
++count;
}
}
return count;
}
int main(int argc, char const *argv[])
{
char myString[] = "Hello";
cout << count_x(myString, 'l');
return 0;
}
There's two mistakes in your code:
You only ever look at the first character in the string.
The last character of a null terminated string is a null character. You're testing the pointer itself.
You need to use std::string
#include <string>
#include <algorithm>
#include <iostream>
int main()
{
std::string str = "Hello";
std::cout << std::count(str.begin(), str.end(), 'l');
}
EXC_BAD_ACCESS occurred in *str++ = *end;. What's wrong with this?
#include <iostream>
using namespace std;
int main() {
char *hello = "abcdefgh";
char c = 'c';
char *str = hello;
//printf("%s",str);
char * end = str;
char tmp;
if (str) {
while (*end) {
++end;
}
--end;
while (str < end) {
tmp = *str;
printf("hello:%s str:%c, end:%c\n", hello, *str, *end);
*str++ = *end;
*end-- = tmp;
}
}
return 0;
}
It is undefined behavior to attempt to alter a string literal. Change your code to the equivalent below, and you will see the issue:
const char *hello = "abcdefgh";
const char *str = hello;
const char * end = str;
So do you see why the line *str++ = *end; had a problem? You're attempting to write to a const area, and you can't do that.
If you want an even simpler example:
int main()
{
char *str = "abc";
str[0] = 'x';
}
Don't be surprised if this simple program produces a crash or segmentation fault when the
str[0] = 'x';
line is executed.
Unfortunately, string-literals did not have to be declared as const char* in C, and C++ brought this syntax over. So even though it looks like you are not using const, you are.
If you want the code to actually work, declare a writeable buffer, i.e. an array of char:
char hello[] = "abcdefgh";
char str[100];
strcpy(str, hello);
char end[100];
strcpy(end, str);
It seems like you're trying to reverse the string. It also looks like you're overcomplicating things.
C-style strings declared on the stack have to be declared as const char *, which means you can't change the characters as they are constant.
In C++ we use strings, and string iterators:
#include <iostream>
#include <string>
using std::string;
using std::reverse;
using std::swap;
using std::cout;
using std::endl;
int main(int argc, const char * argv[])
{
string hello("abcdefgh");
reverse(hello.begin(), hello.end());
cout << hello << endl;
return 0;
}
Manually:
static void reverse(std::string & str)
{
string::size_type b = 0, e = str.length() - 1, c = e / 2;
while(b <= c)
{
swap(str[b++], str[e--]);
}
}
Recursively:
static void reverse_helper(std::string & str,
string::size_type b,
string::size_type e)
{
if(b >= e)
return;
swap(str[b], str[e]);
reverse_helper(str, ++b, --e);
}
static void reverse(std::string & str)
{
reverse_helper(str, 0, str.length() - 1);
}
I have to admit, i have no idea how to use pointers, but I tried non the less. the problem with my program is that it shows the string in reverse, except for what was the first letter being missing and the entire string is moved one space forward with the first element being blank.
for example it show " olle" when typing "hello".
#include <iostream>
#include <string>
using namespace std;
string reverse(string word);
int main()
{
char Cstring[50];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(string word)
{
char *front;
char *rear;
for (int i=0;i< (word.length()/2);i++)
{
front[0]=word[i];
rear[0]=word[word.length()-i];
word[i]=*rear;
word[word.length()-i]=*front;
}
return word;
}
The new code works perfectly. changed the strings to cstrings. the question technicaly asked for cstrings but i find strings easier so i work with strings then make the necesary changes to make it c string. figured out ho to initialize the rear and front as well.
#include <iostream>
#include <cstring>
using namespace std;
string reverse(char word[20]);
int main()
{
char Cstring[20];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(char word[20])
{
char a='a';
char b='b';
char *front=&a;
char *rear=&b;
for (int i=0;i< (strlen(word)/2);i++)
{
front[0]=word[i];
rear[0]=word[strlen(word)-1-i];
word[i]=*rear;
word[strlen(word)-1-i]=*front;
}
return word;
}
char *front;
char *rear;
then later
front[0]=word[i];
rear[0]=word[strlen(word)-1-i];
Not good. Dereferencing uninitialized pointers invokes undefined behavior.
Apart from that, your code is overly complicated, it calls strlen() during each iteration (and even multiple times), which is superfluous, and the swap logic is also unnecessarily complex. Try using two pointers instead and your code will be much cleaner:
void rev_string(char *str)
{
char *p = str, *s = str + strlen(str) - 1;
while (p < s) {
char tmp = *p;
*p++ = *s;
*s-- = tmp;
}
}
The thing is, however, that in C++ there's rarely a good reason for using raw pointers. How about using std::reverse() instead?
string s = "foobar";
std::reverse(s.begin(), s.end());
inline void swap(char* a, char* b)
{
char tmp = *a;
*a = *b;
*b = tmp;
}
inline void reverse_string(char* pstart, char* pend)
{
while(pstart < pend)
{
swap(pstart++, pend--);
}
}
int main()
{
char pstring[] = "asfasd Lucy Beverman";
auto pstart = std::begin(pstring);
auto pend = std::end(pstring);
pend -= 2; // end points 1 past the null character, so have to go back 2
std::cout << pstring << std::endl;
reverse_string(pstart, pend);
std::cout << pstring << std::endl;
return 0;
}
you can also do it like this:
#include <iostream>
#include <cstring>
using namespace std;
string reverse(char word[20]);
int main()
{
char Cstring[20];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(char word[20])
{
char a='a';
char b='b';
char *front=&a;
char *rear=&b;
for (int i=0;i< (strlen(word)/2);i++)
{
*front=word[i];
*rear=word[strlen(word)-1-i];
word[i]=*rear;
word[strlen(word)-1-i]=*front;
}
return word;
}
it successfully works on my system ,i.e. on emacs+gcc on windows 7
Taken from C How To Program Deitel & Deitel 8th edition:
void reverse(const char * const sPtr)
{
if (sPtr[0] == '\0')
return;
else
reverse(&sPtr[1]);
putchar(sPtr[0]);
}
I know how to program in C# and VB but not have idea about how to use C++ and have to program a little exe to a barcode scanner that use C++ :(
In this moment I try to parse a scanned barcode that have multiple data sepparated with a "/", I find that exist a strtok function, tested it "manually" and worked ok but I not implemented yet a working function to call it correctly, what I have now:
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int elemStr(char *str, char sep)
{
int cantElem;
unsigned ich;
cantElem = 0;
if (strlen(str) > 0) //at least 1 elem
cantElem++;
for (ich = 0; ich < strlen(str); ich++)
{
if (str[ich] == sep)
cantElem++;
}
return cantElem;
}
char* getElemStr(char *str, char sep[], int elem)
{
char *tempStr = NULL;
char *tok;
int currElem = 1;
// 1st data
strcpy( tempStr, str);
tok = strtok( tempStr, sep);
while( currElem != elem )
{
// Get next tokens:
tok = strtok( NULL, sep );
currElem++;
}
return tok;
}
void main( void )
{
char barcode[] = "710015733801Z/1/35";
char sep[] = "/";
char sep1 = sep[0];
char barcd[20];
char piezaChar[4];
int pieza;
char mtsChar[4];
int cantElem;
cantElem = elemStr(barcode, sep1 );
if (cantElem >= 1)
{
strcpy(barcd, getElemStr(barcode,sep,1) ); //pasa a str resultado;
printf("Cod: %s\n", barcd ); //STACK OVERFLOW HERE!
}
}
if I use strtok witout a function "getElemStr" it work ok but I try to use it on other places too.
Can I use strtok like this? You have a working example?
pd: I not have idea about pointers (sorry), good doc to learn about that?
Since you specifically asked about C++, I'm going to ignore your very c-style code and show you how to do this in C++:
#include <boost/algorithm/string.hpp>
#include <iostream>
#include <string>
#include <vector>
int main()
{
std::string barcode = "710015733801Z/1/35";
std::string sep = "/";
std::vector<std::string> v;
boost::split(v, barcode, boost::is_any_of(sep));
for(size_t i=0; i<v.size(); ++i)
std::cout << v[i] << '\n';
}
strtok destroys your original string. So i don't think it can be used with a char* that points to a static string. Static strings get copied to a read only portion of the executable.
Here is a C++ solution that doesn't use boost.
#include <string>
#include <sstream>
#include <iostream>
int main()
{
std::string barcode = "710015733801Z/1/35";
std::stringstream ss(barcode);
std::string elem;
while(std::getline(ss, elem, '/'))
{
//do something else meaningful with elem
std::cout << elem << std::endl;
}
return 0;
}
Output:
710015733801Z
1
35
// ExampleCodes.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include<stdio.h>
#include<iostream>
using namespace std;
char* stringReverse(char* s)
{
char temp, *p,*q;
q = s;
while( *(++q));
for( p = s; p < --q; p++)
{
temp = *p;
*p = *q;
*q = temp;
}
return s;
}
int _tmain(int argc, _TCHAR* argv[])
{
stringReverse("StringReverse");
return 0;
}
You can't modify constant string literals.
stringReverse("StringReverse");
You could use a character array instead:
char str[] = "StringReverse";
String literals, like "StringReverse", are not allowed to be modified in C. Use an array:
char str[] = "StringReverse";
stringReverse(str);
Note that your stringReverse() function has undefined behaviour if you feed it a zero-length string.
You never allocated any memory for p. You can try p = new char[sizeof(s)];
Perhaps you should write something like this:
// in - string to reverse
// out - buffer for reversed string
// l - size of in
StringReverse(char* in, char* out, int l) {
for (int i=0;i<l;i++) {
out[l-(i+1)] = in[i];
}
}
*There is very very little to no speed difference between [] and *(p++) / *(p--)
Well I was going to clean up the awful source in the OP but instead I'll just post a cleaner version:
#include <stdio.h>
#include <string.h>
void reverse_string(char *s)
{
char *q = s + strlen(s);
for (char *p = s; p < --q; p++)
{
char temp = *p;
*p = *q;
*q = temp;
}
}
int main()
{
char s[] = "StringReverse";
reverse_string(s);
puts(s);
return 0;
}
I hope for your sake Amit that you're still a student.
Update0
This is actually pure C, so learn from and awe at its performance but don't start writing C++ like this.