Append to the end of a Char array in C++ - c++

Is there a command that can append one array of char onto another? Something that would theoretically work like this:
//array1 has already been set to "The dog jumps "
//array2 has already been set to "over the log"
append(array2,array1);
cout << array1;
//would output "The dog jumps over the log";
This is a pretty easy function to make I would think, I am just surprised there isn't a built in command for it.
*Edit
I should have been more clear, I didn't mean changing the size of the array. If array1 was set to 50 characters, but was only using 10 of them, you would still have 40 characters to work with. I was thinking an automatic command that would essentially do:
//assuming array1 has 10 characters but was declared with 25 and array2 has 5 characters
int i=10;
int z=0;
do{
array1[i] = array2[z];
++i;
++z;
}while(array[z] != '\0');
I am pretty sure that syntax would work, or something similar.

If you are not allowed to use C++'s string class (which is terrible teaching C++ imho), a raw, safe array version would look something like this.
#include <cstring>
#include <iostream>
int main()
{
char array1[] ="The dog jumps ";
char array2[] = "over the log";
char * newArray = new char[std::strlen(array1)+std::strlen(array2)+1];
std::strcpy(newArray,array1);
std::strcat(newArray,array2);
std::cout << newArray << std::endl;
delete [] newArray;
return 0;
}
This assures you have enough space in the array you're doing the concatenation to, without assuming some predefined MAX_SIZE. The only requirement is that your strings are null-terminated, which is usually the case unless you're doing some weird fixed-size string hacking.
Edit, a safe version with the "enough buffer space" assumption:
#include <cstring>
#include <iostream>
int main()
{
const unsigned BUFFER_SIZE = 50;
char array1[BUFFER_SIZE];
std::strncpy(array1, "The dog jumps ", BUFFER_SIZE-1); //-1 for null-termination
char array2[] = "over the log";
std::strncat(array1,array2,BUFFER_SIZE-strlen(array1)-1); //-1 for null-termination
std::cout << array1 << std::endl;
return 0;
}

If your arrays are character arrays(which seems to be the case), You need a strcat().
Your destination array should have enough space to accommodate the appended data though.
In C++, You are much better off using std::string and then you can use std::string::append()

You should have enough space for array1 array and use something like strcat to contact array1 to array2:
char array1[BIG_ENOUGH];
char array2[X];
/* ...... */
/* check array bounds */
/* ...... */
strcat(array1, array2);

There's no built-in command for that because it's illegal. You can't modify the size of an array once declared.
What you're looking for is either std::vector to simulate a dynamic array, or better yet a std::string.
std::string first ("The dog jumps ");
std::string second ("over the log");
std::cout << first + second << std::endl;

Related

Char outputting random characters at the end of the sentence

#include <iostream>
#include <string.h>
using namespace std;
void crypt(char* sMsg)
{
cout << "Original Message: '" << sMsg << "'" << endl;
int length = strlen(sMsg);
char sMsg_Crypt[3][length];
/* sMsg_Cryp[3]
[0] CRYPT LETTERS, ASCII + 3
[1] INVERT CHAR
[2] HALF+ OF SENTENCE, ASCII - 1
*/
for (int i=0; i<length; i++)
{
if (isalpha((int)sMsg[i]))
sMsg_Crypt[0][i] = sMsg[i] + 3; // DO ASCII + 3
else
sMsg_Crypt[0][i] = sMsg[i];
}
cout << "Crypt[0]: '" << sMsg_Crypt[0] << "'" << endl;
}
int main()
{
char sMsg[256];
cin.getline(sMsg,256);
crypt(sMsg);
return 0;
}
Input:
Hello World! Testing the Cryptography...
Output:
Original Message: 'Hello World! Testing the Cryptography...'
Crypt[0]: 'Khoor Zruog! Whvwlqj wkh Fu|swrjudsk|...Çi­o'
Why this Çi­o is comming out??
For starters variable length arrays like this
int length = strlen(sMsg);
char sMsg_Crypt[3][length];
is not a standard C++ feature.
You could use at least an array of objects of the type std::string like for example
std::string sMsg_Crypt[3];
Nevertheless the problem is that the array sMsg_Crypt[0] dies not contain a string. That is you forgot to append inserted characters in the array with the terminating zero character '\0'.
You could write after the for loop
sMsg_Crypt[0][length] = '\0';
provided that the array (if the compiler supports VLA) is declared like
char sMsg_Crypt[3][length+1];
Firstly, you can't define a static char array like this: char sMsg_Crypt[3][length];. That is because the length is not a const type, meaning the size of the array will be sMsg_Crypt[3][0] (this is because the size is not known at compile time). In MSVC, it'll flag an error (by IntelliSense). Since you know the size beforehand (256), you can replace the length with 256.
The second fact is that you're using C++ and you have access to std::string. So without using a char buffer, use std::string instead. It would look something like this: std::string sMsg_Crypt[3];
The last fact is that, for a string to be read correctly, it needs to be null-terminated ('\0' at the end). This means that the ending character must be '\0'. In the case of std::string, it does it for you.

When creating a char array, its length is different from required

I need to create a newStr array with length of str array. But after its created the strlen(newStr) is totally different. For example if a strlen(str) is 5, then strlen(newStr) would be 22. What am I doing wrong?
#include <iostream>
using namespace std;
int main()
{
char *str = "Hello";
int strLength = strlen(str);
std::cout << "str = " << str << "\t" << "strLength = " << strLength << std::endl;
char *newStr = new char[strLength];
std::cout << "newStrLength = " << strlen(newStr) << std::endl;
system("pause");
return 0;
}
In the console will be
str = Hello strLength = 5
newStrLength = 22
You are mixing up two different concepts:
new[] allocates uninitialized memory block to your program,
strlen(...) counts characters in a C string before null terminator '\0' is reached.
The size of the allocated block cannot be measured with strlen. In fact, it cannot be measured at all - your program must know how much memory it has requested, and make sure that it does not go past the limit.
Once you allocated new char[n], you can safely copy a C string of length up to n-1 into that block. C++ guarantees that enough memory would be there for you to complete the operation successfully:
char *newStr = new char[strLength+1]; // Note +1 for null terminator
strcpy(newStr, str);
std::cout << "newStrLength = " << strlen(newStr) << std::endl;
delete[] newStr;
The way strlen works is that it examines the contents of the string passed to it, and counts how many characters there are until the first terminating character. The terminating character for a string is '\0' (or 0).
What you've done is asked for the length of a string that you've not assigned any value to; leading to strlen examining random memory; looking for the first 0. In this case, it found it 22 bytes further down; but it could be anything. It could even crash because you start looking into memory you don't have read access to.
The best way to resolve this is to use std::string and then you can call length and other helper functions without having to worry about the underlying pointers too much; which will also resolve your memory leak.

How do I combine two strings?

The question requires combining two strings(the longer string in the front and the shorter one after the longer one) without using <string> header file.Each string inputted can't exceed 20 characters.
My logic behind this is:
first use strlen to get the length of the str1 and str2,
use str3 to store the longer string, and str4 to store the shorter.
add str3 and str4 to str5
Here is my code:
#include<iostream>
using namespace std;
int main()
{
// combine two strings , longer in the front, and shorter
// after it. do not use strcat
char str1[20],str2[20],str3[20],str4[20],str5[40];
// str1 and str2 stores original data, str3 stores longer
// and str4 stores shorter, str5 stores total
int j=0;
cin.getline(str1,20);
cin.getline(str2,20);
if(strlen(str1)<=strlen(str2))
// give longer string value to str3,shorter to str2
{
for (int i=0;i<20;i++)
{
str3[i]=str2[i];
str4[i]=str1[i];
}
}
else
{
for (int i=0;i<20;i++)
{
str3[i]=str1[i];
str4[i]=str2[i];
}
}
for(j=0;str3[j]!='\0';j++)
{
str5[j]=str3[j];
}
for(int i=j;i<40;i++)
for(int m=0;m<20;m++)
{
str5[i]=str4[m];
}
cout<<str5<<endl;
return 0;
}
Here is the ouput:
What's my problem here? What are those characters in between the two strings? Thank you!!
Especially since you explicitly mentioned being a beginner, the solution is to use std::string:
#include <iostream>
#include <string>
int main() {
std::string a;
getline(std::cin, a);
std::string b;
getline(std::cin, b);
// Ensure that the longer string goes to the front.
if (a.size() < b.size());
swap(a, b);
std::string result = a + b;
std::cout << result << '\n';
// Or, simply:
std::cout << a << b << '\n';
}
The message here is that C++, despite its quirks, is a very high level language if you rely on its library instead of implementing every low level operation from scratch.
Everything is fine (!) up to this point
for(int i=j;i<40;i++)
for(int m=0;m<20;m++) // This loop runs m=0 to 20 for each position of i
{
str5[i]=str4[m];
}
For each index i you are copying in all 20 elements from str4, leaving just the value at str4[19] which could be anything
Just increment i and m by one together
int m = 0;
for(int i=j;i<40;i++)
{
str5[i]=str4[m++];
}
You are copying the entire 20 characters, 40 characters in the loop into the variables. stop copying when you find a '\0' character.
But using the std::string will make life simpler :)
Using std::string is nice and all but here's a few tips for working with char*:
1) You shouldn't copy strings to separate shorter and longer string, just use pointers and then work with these pointers, something along these lines:
const char *longer_string = 0, *shorter_string = 0;
if(strlen(str1)<=strlen(str2))
{
shorter_string = str1;
longer_string = str2;
}
else
{
shorter_string = str2;
storter_string = str1;
}
2) Using strcpy and strcat to combine strings could make life a lot easier:
char *combined_string = new char [strlen (shorter_string) + strlen (longer_string) + 1];
strcpy (combined_string, longer_string);
strcat (combined_string, shorter_string);
Some compilers would say that these functions aren't safe and you have to stick to _s versions, but I guess it's entirely up to you.
Since this is obviously homework: I'll just point out the existence of the function strcat, and the fact that you can use char* to the arrays, and just swap them, without having to recopy anything between the initial read and the concatenation (which means that you only need two arrays: one for each of the inputs, and one for the final value).
And also, when calculating sizes, etc. do not forget that C style strings have an extra '\0' at the end, and make allowances for it.
As #David Sykes has pointed out, the problem is with your for loop. So when you read input from cin ,it is not necessary that your input string contains 20 character. But in you form loop you are looping through those string beyond their length which may contains garbage characters. Example
char str1[20]
cin.getline(str1,20);
cout << str1[19] << endl;
Suppose your input for above code is "ABCD" which contains only 4 characters but your array has capacity of 20. So the remaining space has junk characters and when you will try to print any thing beyond actual length you will get wild character as you are getting in your code.

How can a conversion from string to char[] make it longer?

I have a problem I cannot really understand how it could exist.
I have a bunch of files ordered by time and containing a bunch of objects. The result should be one file per time ordered in a directory per object.
It works quite fine but at the point where I convert the Outputstring to a char[] to use fstream.open(), the array has 3 characters more than the string has.
#include <iostream>
#include <stdio.h>
#include <string.h>
using namespace std;
int main()
{
string strOutput;
char *OutputFile;
short z;
strOutput = "/home/.../2046001_2013-02-25T0959.txt";
cout << strOutput << endl;
OutputFile = new char[strOutput.length()];
z = 0;
while (z < strOutput.length())
{
OutputFile[z] = strOutput[z];
z++;
}
cout << OutputFile << endl;
return 0;
}
The first output is always correct but the second sometimes has the end .txt60A, .txt5.a or .txt9.A.
When it occurs its always the same object and time and it happens every try. But not every object does that.
For obvious reasons I cannot reproduce this error in this minimal code snippet, but I also don't want to post the whole 390 lines of code.
Do you have any suggestions?
You are missing terminating null at the end of C string. To fix:
OutputFile = new char[strOutput.length() + 1]; // notice +1
z = 0;
while (z < strOutput.length())
{
OutputFile[z] = strOutput[z];
z++;
}
OutputFile[z] = 0; // add terminating 0 byte
Of course there are better ways to do the whole thing... you don't really need to copy at all, just get rid of OutputFile and the whole loop, and use the char array inside std::string:
cout << strOutput.c_str() << endl;
I assume the real code wants a C string. std::cout can print std::string directly, of course:
cout << strOutput << endl;
If you actually want to create a copy, it's best to just copy std::string and store that, and use c_str-method to get the C buffer when you need it:
string OutputFile = strOutput;
If you know you really do need a raw char array allocated from heap, you should use std::unique_ptr (or possibly some other C++ smart pointer class) to wrap the pointer, so you do not need to delete manually and avoid memory leaks, and also use standard library function to do copying:
#include <memory>
#include <cstring>
...
unique_ptr<char[]> OutputFile(new char[strOutput.length() + 1];
::strcpy(OutputFile, strOutput.c_str()); // :: means top level namespace
Char arrays need an extra null character or \0 appended to the end, otherwise the code reading the string will run past the end of the array until it finds one.
OutputFile = new char[strOutput.length() + 1];
z = 0;
while (z < strOutput.length())
{
OutputFile[z] = strOutput[z];
z++;
}
OutputFile[z] = '\0';
It may appear to work if the next byte after the array happens to be a null, but that's just a coincidence. I'm sure that's why your code works on the first pass.
at the point where I convert the Outputstring to a char[] to use fstream.open()
You don't have to do that. Do something like this instead:
outfile.open(Outputstring.c_str(), std::fstream::out)
Of course, if you have a C++11-compliant compiler, you can just do:
outfile.open(Outputstring, std::fstream::out)

How to get the size of the used space in an array? (NOT sizeof); c++

#include<iostream>
using namespace std;
int main()
{
char arr[200];
while(1) {
cin >> arr;
int i = sizeof(arr);
cout << "The arr input is "<< arr
<< " and the size of the array is "<< i << endl;
}
return 0;
}
For the input of 34,
This code outputs :The arr input is 34 and the size of the array is 200
while I want it to get the size of the used space of the array . So for The last input i want it to output :The arr input is 34 and the size of the array is 2
Can someone tell me how?
Maybe you want strlen(arr) here. It must be null terminated, otherwise the cout << arr would not have worked.
You would need to #include <cstring>
There's no automatic way to do what you want in the general case - you'll need to keep track somehow, either with your own counter, or by seeding the array with an 'invalid' value (that you define) and search for to find the end of the used elements (that's what the '\0' terminator character in a C-style string is).
In the example code you posted, the array should receive a null terminated C-style string, you can use that knowledge to count the number of valid elements.
If you're using C++ or some other library that has some more advanced data structures, you may be able to use one that keeps track of this kind of thing for you (like std::vector<>).
the size of the used space of the array
There is no such thing. If you have an array of 200 chars, then you have 200 chars. Arrays have no concept of "used" and "unused" space. It only works with C-strings because of the convention that those are terminated by a 0 character. But then again, the array itself cannot know if it is holding a C-string.
in a less involved manner, you can just count through each character till you hit a null with just a while loop. It will do the exact same thing strlen() does. Also, in practice, you should do type checking with cin, but i'll assume this was just a test.
#include <iostream>
using namespace std;
int main()
{
char arr[200];
int i;
while(1) {
cin >> arr;
i=0;
while (arr[i] != '\0' && i<sizeof(arr))
i++;
cout << "The arr input is "<< arr
<< " and the size of the array is "<< i << endl;
}
return 0;
}
Just for completeness, here is a much more C++ like solution that is using std::string instead of a raw char array.
#include <iostream>
#include <string>
int
main()
{
while (std::cin.good()) {
std::string s;
if (std::cin >> s) {
std::cout
<< "The input is " << s
<< " and the size is " << s.length()
<< std::endl;
}
}
return 0;
}
It doesn't use an array, but it is the preferable solution for this kind of problem. In general, you should try to replace raw arrays with std::string and std::vector as appropriate, raw pointers with shared_ptr (scoped_ptr, or shared_array, whatever is most appropriate), and snprintf with std::stringstream. This is the first step to simply writing better C++. You will thank yourself in the future. I wish that I had followed this advice a few years ago.
Try it
template < typename T, unsigned N >
unsigned sizeOfArray( T const (&array)[ N ] )
{
return N;
}