Write a program that allows the user to enter an unsigned integer (the maximum v

Question

Write a program that allows the user to enter an unsigned integer (the maximum value of an unsigned 4-byte int is 232 = 4,294,967,296) and reverses its format (from little to big endian, or vice versa). Print out the user-entered number in hexadecimal and binary, reverse the endianness, and print the reverse in hexadecimal and binary.

Integers in most machine architectures are represented in little endian format: the least significant byte is stored in the smallest address; for instance, the decimal number 23456789 is 0165EC15 in hex. In little endian format the number is stored in main memory as follows:

Byte (in hex)

15

EC

65

01

Byte Address

a

a+1

a+2

a+3

In big endian format the 4-byte integer is stored with the most significant byte is stored in the least address:

Byte (in hex)

01

65

EC

15

Byte Address

a

a+1

a+2

a+3

You may NOT use the C++ hex formatting capability of the extraction operator (<<). Write your own routines to print a number in hexadecimal and binary using bit operators: <<, >>, &, |, and/or ~.

It is recommended that you use the following constants, a union to represent 4-byte integers, and print prototypes as shown here:

const int INTSIZE = 4; // in bytes

const int BYTESIZE = 8; // in bits

const int NIBSIZE = 4; // nibble, in bits

union integer4 {

   unsigned int intrep;

   unsigned char byterep[INTSIZE];

};

void prHex (unsigned char);

void prBin (unsigned char);

You will find that is very important that you use unsigned chars. In most machine architectures, negative integers are represented in 2s complement format with the most significant bit a 1. To convert a binary (byte) 1, or 00000001, to negative -1 in binary, use the following procedure:

1)find the 1s complement; that is, flip all the 1s and 0s: 11111110;

2)add 1 to the result: 11111110 + 00000001 = 11111111, or 0xFF.

Hence a 0xFF as an unsigned char is 255 (decimal), and 0xFF as a (signed) char is -1 (decimal).

Here is a sample run:

Enter an unsigned integer in base 10 => 23456789

In hex:

15 EC 65 01

In binary:

00010101 11101100 01100101 00000001

Reverse endian:

In hex:

01 65 EC 15

In binary:

00000001 01100101 11101100 00010101

Would be appreciated if you add comments in each line of code and try to use the constants / code given! :)

Byte (in hex)

15

EC

65

01

Byte Address

a

a+1

a+2

a+3

Explanation / Answer

#include <bits/stdc++.h>
using namespace std;

const int INTSIZE = 4; // in bytes
const int BYTESIZE = 8; // in bits
const int NIBSIZE = 4; // nibble, in bits

union integer4 {
unsigned int intrep;
unsigned char byterep[INTSIZE];
};

void prHex (unsigned char);
void prBin (unsigned char);

// helper to get hexadecimal char
char getHexChar(unsigned char);

int main() {

// prompt user to take input in base 10
unsigned int userInput;
cout << "Enter an unsigned integer in base 10 => ";
cin >> userInput;

// declare integer4 representation of number
integer4 num;

// set its integer representation as current number
num.intrep = userInput;

// iterate and grab 8-bits (byte) and add it to byterep of representation
for (int i = 0; i < INTSIZE; ++i) {

// grab last 8 bits using 11111111 with prefixed with 0s
num.byterep[i] = (userInput & ((((unsigned int) 1) << BYTESIZE) - 1));

// chop of last 8 bits (a byte)
userInput >>= BYTESIZE;
}

///////////////////////////////////////////////////////////
cout << "Hexadecimal: ";
for (int i = 0; i < INTSIZE; ++i) {
// print hexadecimal
prHex(num.byterep[i]);

// separate bytes with space
cout << ' ';
}
cout << endl;

cout << "Binary: ";
for (int i = 0; i < INTSIZE; ++i) {
// print binary
prBin(num.byterep[i]);

// separate bytes with space
cout << ' ';
}
cout << endl;

////////////////////////////////////////////////////////////
cout << "Reverse Endian Representation: " << endl;
cout << "Hexadecimal: ";
for (int i = INTSIZE - 1; i >= 0; --i) {
// print hexadecimal
prHex(num.byterep[i]);

// separate bytes with space
cout << ' ';
}
cout << endl;

cout << "Binary: ";
for (int i = INTSIZE - 1; i >= 0; --i) {
// print binary
prBin(num.byterep[i]);

// separate bytes with space
cout << ' ';
}
cout << endl;

return 0;
}

void prHex(unsigned char byte) {
// get last 4 bits
unsigned char firstNib = (byte & ((((unsigned int) 1) << NIBSIZE) - 1));

// get first 4 bits
unsigned char secondNib = (byte >> NIBSIZE);

// print it!
cout << getHexChar(secondNib) << getHexChar(firstNib);
}

void prBin(unsigned char byte) {
// iterate from 8th to 1st bit and print it
for (int i = BYTESIZE - 1; i >= 0; --i) {
// check bit at ith position and print it
if ((byte & (1 << i)) != 0) cout << '1';
else cout << '0';
}
}

char getHexChar(unsigned char x) {
// by definition of hexadecimal chars
if (x >= 0 && x <= 9) return char('0' + x);
return char('A' + (x - 10));
}

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