why does this C++ code print a after every line? - c++

Question

#include <iostream>
using namespace std;
int main() {
for (;;) {
char ch ;
if (cin.fail()) break;
cin.read((char*)&ch, sizeof(unsigned char));
cout<<hex<<(unsigned int)(unsigned char)ch<<endl;
cin.clear();
}
}
Why does this code always print a after every line? I just used any char as standard input. Added: I am trying to read unformatted input with read.

Answer 1

This code is reading a character at a time and writing out the value of the character in hexadecimal.
What you might not be expecting is that the pressing Enter also sends a character, which is read by your call to cin.read.
The a is the hexadecimal value of that character. So if you type hello and press Enter, the following will result from the cout statements:
68
65
6c
6c
6f
a
If you stop displaying the value in hexadecimal, you'll notice that it prints 10 after each entry.

Answer 2

The a that you see is simply the hex value of the '\n' character at the end of the line of input.
If you don't want to see that character, simply wrap the output line in an if statement that checks for that character and doesn't bother to do any output when it's seen:
if (ch != '\n') {
cout<<hex<<(unsigned int)(unsigned char)ch<<endl;
}

Answer 3

I don't even know what you're trying to accomplish with this. Don't use cin.read to read a single character. This loop should look more like this:
char ch;
while (std::cin.get(ch)) {
std::cout << std::hex << static_cast<unsigned>(ch) << std::endl;
}
As to why it prints something, are you sure it's not the character you're actually inputting?

Answer 4

GargantuChet has correctly explained why you get 'a's.
More generally, there are many other issues
1 for (;;)
2 {
3 char ch;
4 if (cin.fail()) break;
5 cin.read((char*)&ch, sizeof(unsigned char));
6 cout << hex << (unsigned int)(unsigned char)ch < <endl;
7 cin.clear();
8 }
On line 4, you see if cin.fail() is set, but with streams they will never start in a failed state - you have to attempt to do something for that to fail. In other words, you should do the read() then have a look at cin.fail(). In general, you should also use gcount() to check how many bytes could actually be read (e.g. despite asking for say 4 you might only get 2, which wouldn't be considered a failure), but here you're only requesting 1 character so it can be simpler.
Cleaning it up a bit but keeping the same basic approach:
1 for (char ch; cin.read(&ch, sizeof ch); )
2 cout << hex << (unsigned)(unsigned char)ch < <endl;
This works because read() returns a reference to cin, and evaluating the "truth" of cin is a shorthand for asking if the input it's performed so far has been error-free (more strictly, at least since the last clear() if you're using that).
Still, std::istream - of which std::cin is an instance - also has a function designed for getting characters, allowing the loop to be simplified to:
for (char ch; std::cin.get(ch); )
...
Aside
Remember that a for( ; ; ) control statement has three parts:
the initialisation code on the left which can also create new variables
the test: this happens before the 1st and every subsequent execution of the loop's statement(s)
code to be executed only after the each execution of the statement(s) and before repeating the test.
Because of this, tests like std::cin.get(ch) are called and evaluated for success as a condition for each iteration. The last solution listed above is equivalent to:
{
char ch;
while (std::cin.get(ch))
...
}