how to use strcmp in g++ - c++

I compiled .cc file whith g++ on linux ubuntu, I want to use srtcmp() function to compare two strings. the strings are not constant. user will give both of them, but I get this error:
error: invalid conversion from ‘char’ to ‘const char*’ [-fpermissive]
and this is my code:
if (!strcmp(a[i].personalNo,pcode)){
#some code
}
which function can I use instead of strcmp() to compare two strings?

The problem isn't on the function but on the way that you're using it.
int strcmp ( const char * str1, const char * str2 );
strcmp takes two const char * arguments.
The error tells you that you are giving the function a char so the problem is on the types of personalNo and/or pcode. Your mistake is probably on the declaration of the type of those two variables. You would want to change their type to char * as char only stores one character while char * is an array of characters.
Also, an another way to compare two strings in C++ is to use std::string. Then you can just do the following (provided that both personalNo and pcode are std::string:
if (a[i].personalNo != pcode){
#some code
}

Related

why it cannot convert char* to char

In c++ we can write
1 char *s="hello"
but the below lines of program produces an error ( cannot convert char* to char)
2 char *s;
*s="hello";
I am confused here, what is difference between 1 and 2
why this error is coming?
In C++, a string literal is a constant array of characters, not just an array of characters like in C. Anyways, to assign to such a variable (Which is best avoided), you do not have to dereference the pointer. Dereferencing it accesses the first element, which is just a char. A char cannot hold an array of characters inside it, causing an error. This is more the reason why you should be using std::string.
Some compilers such as GCC provide extensions to make such code possible since it is not standards compliant code, and it would look like:
char* s = "hello";
s = "new string";
This generates the following warning in GCC (But still gets the expected result):
warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
Clang also has the same behavior with the same output (Also generating a warning)
A string is an array of characters. The start of a string therefore is const char *.
Therefore to reference a string, you can use const char * s = "hello";
However if you dereference a const char*, you get a const char. This isn't a string i.e. *s gives you 'h'.
In your code *s="hello";, you are saying "assign at the dereferened s the value "hello"". Dereferencing s is a character only, to which you are trying to assign a string.
The problem is the second asterisk in your second example.
The first code is this
char *s="hello";
The equivalent code is this
char *s;
s="hello";
No * before s in the second line.
Now as everyone is pointing out neither of these are legal C++. The correct code is
const char *s="hello";
or
const char *s;
s="hello";
Because string literals are constant, and so you need a pointer to const char.
I am confused here, what is difference between 1 and 2 why this error is coming?
As many others * in C++ means different things in different context:
char *s; // * here means that s type is a pointer to char, not just char
*s; // in this context * means dereference s, result of exression is char
int a = 5 * 2; // in this context * means multiply
so case 1 and 2 may look similar to you but they mean very different things hence the error.

When calling the function this error [Error] incompatible types in assignment of 'char' to 'char [40]'

I am returning the *res (which is value resultant string after concatenating strings) in the function. When I call this function and store the result in the char array then it gives me error [Error] incompatible types in assignment of 'char' to 'char [40]'
.I want to concatenate two strings in the function and return the concatenating string from function.Kindly help to solve to this problem
#include<iostream>
#include<stdio.h>
using namespace std;
int main(){
char strConcat(char *,char *);
char input1[10],input2[12],resultantString[40];
cout<<"Enter the 1st String=";
cin.getline(input1,10);
cout<<"Enter the 2nd String=";
cin.getline(input2,10);
//faulty code
resultantString=strConcat(input1,input2);
cout<<resultantString;
}
// st1 means String1 and St2 means string2 which we want to concat.
char strConcat(char *st1,char *st2)
{
char *res;
while(*st1!='\0')
{
*res=*st1;
*st1++;
*res++;
}
while(*st2!='\0')
{
*res=*st2;
*st2++;
*res++;
}
*res='\0';
return *res;
}
First off, remove using namespace std;, it's a bad habit.
Next up, move the function declaration void strConcat(char *,char *); out of main and make it the same type as the definition, this is your error, you declare strConcat to return void first but then you define it to return char, the compiler still thinks it returns void and thus when you're trying to assign something to it the compiler complains.
Next up, make main return int, your current definition isn't valid.
Next, indent your code so not only the compiler can read it but other humans too.
And the most important tip here, remove all of those static-sized arrays and that homebrewed strCat function and use std::string and it's operator+ for concatenation.

C++ error: cannot convert ‘std::basic_string<char>’ to ‘const char*’

I'm using a function to download a file.
void downloadFile(const char* url, const char* fname) {
//..
}
This is called like :
downloadFile("http://servera.com/file.txt", "/user/tmp/file.txt");
This working fine.
But I want to change the URL to be a value from an array. The array stores encrypted values which when decrypted are strings, so I get the issue error: cannot convert ‘std::basic_string<char>’ to ‘const char*’
I've tried:
string test = decode(foo[5]);
const char* t1= test.c_str();
downloadFile(t1 "filename.txt", "/user/tmp/file.txt");
downloadFile(t1 + "filename.txt", "/user/tmp/file.txt");
and
downloadFile((decode(foo[5]).c_str()) + "filename.txt", "/user/tmp/file.txt");
which gives:
error: invalid operands of types ‘const char*’ and ‘const char [17]’ to binary ‘operator+’
What am I doing wrong ?
Thanks
C-strings can't be concatenated with +.
Use std::string::+ instead:
downloadFile((test + "filename.txt").c_str(), "/user/tmp/file.txt");
Note that c_str only returns a pointer to the std::string's internal character array, so it's valid only during the execution of the downloadFile function.
Try this:
downloadFile((decode(foo[5]) + "filename.txt").c_str(), "/user/tmp/file.txt");
The operator+ is not defined for char arrays.
The main problem in your code is that you are trying to use operator+ to concatenate raw C strings (i.e. raw const char* pointers, or raw char [] arrays), which doesn't work.
In C, you should use proper library functions (like strncat or safer variants) to do that; but since you are using C++, you can do better, and write easier code: just use a C++ string class, like std::string.
In fact, the C++ standard library offers convenient overloads for operator+ that work with std::string, so you can concatenate C++ strings in an easy, intuitive and safe way; for example:
// Build your URL string
std::string test = decode(foo[5]);
std::string url = test + "filename.txt";
// Use std::string::c_str() to convert from C++ string
// to C raw string pointer const char*
downloadFile(url.c_str(), "/user/tmp/file.txt");

Warning generated due wrong strcmp parameter handling

So I have an
unsigned char * pMyPtr
assigned to something.
Then I want to compare this to an arbitrary string with
strcmp(const char* , const char* )
But when I do that, clang compiler tells me
warning: passing (aka 'unsigned char *') to parameter of type 'const char *' converts between pointers to integer types with different sign
How do I remove this warning?
With all the comments to the question, I feel like I'm missing something.
I know that casts are unfashionable, but isn't the following a simple workaround?
strcmp((const char*) pMyPtr , whatever_is_being_compared)
It isn't even unsigned. Behind it, is a struct.
This means that you cannot use strcmp here. You can use strcmp when the input data are null-terminated strings. That's not the case when the input data are structs. Perhaps you should consider memcmp instead, or perhaps you need to compare the structs as structs.
Clang can't convert from unsigned char* to const char*.
That because unsigned char* is different then char*.
By adding unsiged you make the range of char 0to255 instead of -127to127.
On the line where strcmp goes, you can typecast the unsigned char value with (const char*) which will work, because there it will be threated as a const char*. instead of unsigned char
If you feel that warning does not have any side effect. you can ignore the warning like this:
#pragma warning( disable : 4507 34 )

Opening directory function

This is a basic understanding concepts related question.
Working using: Embarcadero C++ Builder
What is the difference between:
opendir("C:\\XYZ")
and
String file = "C:\\XYZ";
opendir(file);
Aren't both strings?
The first one works but the sexond gives me error:
E2034 Cannot convert Unicode String to ' const char*'
In a case where I take input from the user I can only pass a string. How do i pass the whole path?
first one is a const char*, second one is a std::string. The opendir function accepts only const char* in your case and thus cannot convert std::string to const char* on its own. you can get the function to work by opendir(file.c_str()); .
No. A String is not a char array. opendir needs a char array.
opendir() expects an 8bit narrow const char* as input. When you pass a narrow literal to opendir(), you are passing it a const char[], which implicitly degrades to const char*, and all is fine.
String is System::String, which is a typedef for System::UnicodeString, which is Embarcadero's UTF-16 encoded string class (similar to std::wstring, but with different semantics). When you pass a String to opendir(), you get a conversion error.
To pass a String value to opendir() (or any other function that expects char*), you need to first convert it to a System::AnsiString, and then use AnsiString::c_str() to get a char* from it, eg:
String file = "C:\\XYZ";
opendir(AnsiString(file).c_str());