I am a beginner with C++. I have a new project at work where I have to learn it, so I'm trying some things just to test my understanding. For this problem, I'm trying to read a file and then print it on screen. Super simple, just trying to get good at it and understand the functions that I'm using. I copied some text from a MS Word document into a notepad (*.txt) file, and I'm trying to read this *.txt file. All of the text in the word document is bolded, but other than that there are no 'unusual' characters. Everything prints out on the screen as it appears in the document except the bolded " - " symbol. This character is printed as the "u" with a hat character ("so called extended ASCII" code 150). I try to print out the integer value of this character in my array (which should be 150) but I get -106. I realize this signed integer has the same bits as the unsigned integer 150. My question is how to get the output to say 150? Here's my code:
#include <iostream>
#include <fstream>
using namespace std;
int main() {
unsigned char* input1;
int input1size = 57;
ifstream file("hello_world2.txt",ios::binary | ios::ate);
if (file.is_open()){
int size;
size = (int) file.tellg();
cout <<"This file is " << size << " bytes." << endl;
file.seekg(0,ios::beg);
input1 = new unsigned char[input1size];
file.read(input1, input1size);
cout << "The first " << input1size <<" characters of this file are:" << endl<<endl;
for (int i=0; i<input1size; i++) {
cout << input1[i];
}
cout<<endl;
}
else {
cout <<"Unable to open file" << endl;
int paus;
cin>>paus;
return 0;
}
file.close();
int charcheck = 25;
int a=0;
int a1=0;
int a2=0;
unsigned int a3=0;
unsigned short int a4=0;
short int a5=0;
a = input1[charcheck];
a1 = input1[charcheck-1];
a2 = input1[charcheck+1];
a3 = input1[charcheck];
a4 = input1[charcheck];
a5 = input1[charcheck];
cout <<endl<<"ASCII code for char in input1[" << charcheck-1 <<"] is: " << a1 << endl;
cout <<endl<<"ASCII code for char in input1[" << charcheck <<"] is: " << a << endl;
cout <<endl<<"ASCII code for char in input1[" << charcheck+1 <<"] is: " << a2 << endl;
cout <<endl<<"ASCII code for char in input1[" << charcheck <<"] as unsigned int: " << a3 << endl;
cout <<endl<<"ASCII code for char in input1[" << charcheck <<"] as unsigned short int: " << a4 << endl;
cout <<endl<<"ASCII code for char in input1[" << charcheck <<"] as short int: " << a5 << endl;
int paus;
cin>>paus;
return 0;
}
Output for all this looks like:
This file is 80 bytes.
The first 57 characters of this file are:
STATUS REPORT
PERIOD 01 u 31 JUL 09
TASK 310: APPLIC
ASCII code for char in input1[24] is: 32
ASCII code for char in input1[25] is: -106
ASCII code for char in input1[26] is: 32
ASCII code for char in input1[25] as unsigned int: 4294967190
ASCII code for char in input1[25] as unsigned short int: 65430
ASCII code for char in input1[25] as short int: -106
So it appears "int a" is always read as signed. When I try to make "a" unsigned, it turns all the bits left of the eight bits for the char to 1's. Why is this? Sorry for the length of the question, just trying to be detailed. Thanks!
What you're dealing with is the sign-extension that takes place when the char is promoted to int when you assign it to one of your a? variables.
All the higher order bits must be set to 1 to keep it the same negative value as was in the smaller storage of the char.
Related
In a small utility I'm writing, I want to read a file record and:
look for the presence of an XOR checksum in the form *XX, where XX are hex digits
replace it if it's incorrect
add one if it's not present
So far, I'm only to the point of reading the file and looking for the checksum. The problem I'm up against is that std::string::find is not finding the * I know to be present; it returns npos every time.
The find() is on line 37. The first line read into strInput is:
$GPGGA,14240.99,2732.581,S,15301.947,E,1,06,3,65,M,37,M,-1.0,0006*6E\n
Here's the code:
#include <iostream>
#include <string>
int main()
{
std::cout << "Enter input file name:\n";
std::string strFileName = "";
std::getline(std::cin, strFileName);
std::cout << "Filename entered: " << strFileName << '\n';
FILE* fp;
int nErrCode = EXIT_FAILURE;
fopen_s(&fp, strFileName.c_str(), "r+");
if (!fp)
{
std::string strErr = "Failed to open " + strFileName;
perror(strErr.c_str());
return nErrCode;
}
rewind(fp);
std::string strInput;
strInput.reserve(100);
std::string::size_type n;
do
{
fgets(&strInput[0], 99, fp);
std::cout << strInput.c_str();
//n = 0;
n = strInput.find('*');
if (n != std::string::npos)
std::cout << "Found checksum at position " << n;
else
std::cout << "Did not find checksum";
} while (!feof(fp));
}
Thanks in advance.
I've debugged this and all is well up to the find(). At that point, I can see that the return value is npos, even though I can see the value of strInput and verify that it contains an asterisk.
The code successfully compiles it but I can't understand why, for certain values of number, the program crashes and for other values it doesn't. Could someone explain the behavior of adding a long int with a char* that the compiler uses?
#include <iostream>
int main()
{
long int number=255;
std::cout<< "Value 1 : " << std::flush << ("" + number) << std::flush << std::endl;
number=15155;
std::cout<< "Value 2 : " << std::flush << ("" + number) << std::flush << std::endl;
return 0;
}
Test results:
Value 1 : >
Value 2 : Segmentation fault
Note: I'm not looking for a solution on how to add a string with a number.
In C++, "" is a const char[1] array, which decays into a const char* pointer to the first element of the array (in this case, the string literal's '\0' nul terminator).
Adding an integer to a pointer performs pointer arithmetic, which will advance the memory address in the pointer by the specified number of elements of the type the pointer is declared as (in this case, char).
So, in your example, ... << ("" + number) << ... is equivalent to ... << &""[number] << ..., or more generically:
const char *ptr = &""[0];
ptr = reinterpret_cast<const char*>(
reinterpret_cast<const uintptr_t>(ptr)
+ (number * sizeof(char))
);
... << ptr << ...
Which means you are going out of bounds of the array when number is any value other than 0, thus your code has undefined behavior and anything could happen when operator<< tries to dereference the invalid pointer you give it.
Unlike in many scripting languages, ("" + number) is not the correct way to convert an integer to a string in C++. You need to use an explicit conversion function instead, such as std::to_string(), eg:
#include <iostream>
#include <string>
int main()
{
long int number = 255;
std::cout << "Value 1 : " << std::flush << std::to_string(number) << std::flush << std::endl;
number = 15155;
std::cout << "Value 2 : " << std::flush << std::to_string(number) << std::flush << std::endl;
return 0;
}
Or, you can simply let std::ostream::operator<< handle that conversion for you, eg:
#include <iostream>
int main()
{
long int number = 255;
std::cout<< "Value 1 : " << std::flush << number << std::flush << std::endl;
number = 15155;
std::cout<< "Value 2 : " << std::flush << number << std::flush << std::endl;
return 0;
}
Pointer arithmetic is the culprit.
A const char* is accepted by operator<<, but will not point to a valid memory address in your example.
If you switch on -Wall, you will see a compiler warning about that:
main.cpp: In function 'int main()':
main.cpp:6:59: warning: array subscript 255 is outside array bounds of 'const char [1]' [-Warray-bounds]
6 | std::cout<< "Value 1 : " << std::flush << ("" + number) << std::flush << std::endl;
| ^
main.cpp:8:59: warning: array subscript 15155 is outside array bounds of 'const char [1]' [-Warray-bounds]
8 | std::cout<< "Value 2 : " << std::flush << ("" + number) << std::flush << std::endl;
| ^
Value 1 : q
Live Demo
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How would you be able to replace digits in a given number using basic c++. Example if the number is 23444 and you want to take the old digit 4 and replace is with a new digit 5 to get a new number 23555.
I have some work done below but when I enter the inputs, it ends up giving me an incorrect result.
cout << "Enter the number: " << endl;
cin >> number;
cout << "Enter the old digit: " << endl;
cin >> oldDigit;
cout << "Enter the newDigit: " << endl;
cin >> newDigit;
newDigit=oldDigit;
cout << Newnum << endl;
You can convert int to char* using itoa() and iterate over it to
check does it contain number. If it does, get 4's position and replace
it with 5.
I know you didnt work with strings, but it can be helpful in your case.
Simple code:
#include <string.h>
#include <iostream>
#include <stdlib.h>
int main(){
int numer;
std::cin>>numer;
char* str;
itoa(numer, str, 10);
for(int i = 0; i < strlen(str); i++){
if(str[i] == '4') str[i]='5';
}
}
If I understand you correctly, you don't just want to simply add 111, you want to treat the number as a string, then change elements in the array. Is that correct?
This may get you on the right track:
Convert an int to ASCII character
if you really want to use only int to do this, here is a working example (base on some of your code)
#include <iostream>
using namespace std;
int replaceDig( int num, int oldDigit, int newDigit)
{
if(num==0)return 0;
int digit = num%10;
if(digit==oldDigit)digit = newDigit;
return replaceDig(num/10,oldDigit,newDigit)*10+digit;
}
int main()
{
int num, newnum, oldDigit, newDigit;
cout << "Enter the number: " << endl;
cin >> num;
cout << "Enter the old digit: " << endl;
cin >> oldDigit;
cout << "Enter the newDigit: " << endl;
cin >> newDigit;
newnum = replaceDig(num, oldDigit, newDigit);
cout << newnum << endl;
return newnum; //do you really want to return this?
}
I have come up with a solution . Don't know if it contains bug or not. Please let me know.
int num = 23444 ,new_num = 0;
int mod;
int exponent = 0;
/**
* Now form the new number
*/
while ( num > 0 ) {
mod = num % 10;
num /= 10;
if ( mod == 4 ) // check whether this is the old digit or not
new_num += 5 * pow( 10 , exp); // replace with new digit
else
new_num += mod * pow(10 , exp); // otherwise no change
exp++;
}
num = new_num;
std::cout << num;
I hope this works for you -
std::string s = std::to_string(23444);
std::replace( s.begin(), s.end(), '4', '5');
int num = std::stoi(s);
int replaceDig( int num, int oldDigit, int newDigit) // replacing the old digits in a number with a new digit
{
int position = numDigits(num);
int remainder = num;
int currentDigit;
while (remainder >0)
{
currentDigit=(num/pow(10,position))%10;
if(currentDigit==oldDigit)
{
num = num - oldDigit*pow(10,position);
num = num + newDigit*pow(10,position);
}
remainder = remainder/10;
position--;
}
}
This is the general idea, I guess. I didn't try to compile it though. And of course, this version isn't really optimized and we could find some more efficient ways of doing it. Oh, and it doesn't work with negative numbers, but this should be quite easy to adapt.
I am trying to write a loop that validates user input, and then repeats if the input is bad. The input must be either a binary number (as a string) or a decimal number (as an int). I have seperate functions to validate this input, but they are not causing any trouble.
The problem arises when I select 1 or 2, and then willingly enter an invalid binary or decimal number. At this point, the do-while loop repeats successfully. The program prints another request for user input to cout, But when it comes time for the user to enter input, the program thinks that there is input in the console before I even enter anything. I believe this is a problem with whitespace/control characters in the buffer, but I am not sure how to fix it. I have tried using std::cin >> std::ws to clear any straggling white space, but no luck.
#include <iostream>
#include <string>
#include <limits>
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
using std::cout;
using std::cin;
using std::endl;
using std::numeric_limits;
using std::max;
using std::streamsize;
using std::string;
//int toDecimal;
//true is is binary
bool validateBinary(const string &binaryNumber){
for(int i = 0; i < binaryNumber.length(); i++){
if((binaryNumber[i] != 1) && (binaryNumber[i] != 0)){
return false;
}
}
return true;
}
//true if is decimal
bool validateDecimal(){
return cin;
}
int main() {
int conversionType = 0; //we initialize conversionType to a default value of 0 to ensure the copiler it will always have a value
bool isBinary = false;
bool isDecimal = false;
string binaryNumberInput;
int decimalNumberInput;
do {
if(conversionType == 0){
cout << "Enter 1 to convert binary to decimal," << endl;
cout << "2 to convert decimal to binary, " << endl;
cout << "or 3 to exit the program: ";
std::cin >> std::ws; //to clear any whitespace fron cin
cin >> conversionType; //upon a second iteration, this value is read in before a user input is given
}
if(!cin || (conversionType != 1 && conversionType != 2)){
cout << "Incorrect input." << endl;
cin.clear(); //clear the fail bit
cin.ignore(numeric_limits<streamsize>::max(), '\n'); //used to ignore not-numeric input
}
cout << "You have selected option " << conversionType << "." << endl;
if(conversionType == 1){
cout << "Please enter a binary number: ";
cin >> binaryNumberInput;
isBinary = validateBinary(binaryNumberInput);
if(!isBinary){
cout << "The numbered you entered is not a binary number!" << endl;
conversionType = 0;
}
}
if(conversionType == 2){
cout << "Please enter a decimal number: ";
cin >> decimalNumberInput;
isDecimal = validateDecimal(); //true if succeeded, meaning is a number
if(!isDecimal){
cout << "The numbered you entered is not a decimal number!" << endl;
conversionType = 0;
}
}
}
while((conversionType != 1 && conversionType != 2) || (isBinary == isDecimal));
return 0;
}
Rather than debug your current program you might want to consider using the standard library to simply things
#include <iostream>
#include <string>
#include <bitset>
#include <climits>
#include <limits>
template<typename T>
void get(T& value)
{
while (!(std::cin >> value)) {
std::cout << "Invalid input\n";
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}
int main()
{
std::cout << "Enter 1 to convert binary to decimal,\n" <<
"2 to convert decimal to binary\n";
int option;
if (std::cin >> option) {
switch (option) {
case 1: {
std::bitset<CHAR_BIT * sizeof(unsigned long long)> bits;
get(bits);
std::cout << bits.to_ullong() << '\n';
break;
}
case 2: {
unsigned long long i;
get(i);
std::cout << std::bitset<CHAR_BIT * sizeof i>(i) << '\n';
break;
}
}
}
}
If you want this to loop you should be able to add it back in again easily enough.
I have problem converting String^ containing 8 bytes as characters (as ascii) to double.
I want to take those 8 characters and convert them binary to double.
What would you recommend to do this conversion in C++/cli?
I was trying to use Marshal::Copy, Double::TryParse, etc.
Maybe I use wrong specifications of parameters, but I really lost my last hopes.
There must be something easy to do this conversion.
Thanks.
Well, the bad news is that the System.String class uses only Unicode encoding internally.
So if you give it bytes it will map them to its internal encoding hiding the original value.
The good news is that you can play with the System.Text.Encoding class to retrieve 8bits values corresponding to the unicode characters.
Here is a sample :
#include <iostream>
using namespace System;
using namespace System::Text;
int main()
{
int n = 123456;
double d = 123.456;
std::cout << n << std::endl;
std::cout << d << std::endl;
char* n_as_bytes = (char*)&n;
char* d_as_bytes = (char*)&d;
String^ n_as_string = gcnew String(n_as_bytes, 0, sizeof(n));
String^ d_as_string = gcnew String(d_as_bytes, 0, sizeof(d));
Encoding^ ascii = Encoding::GetEncoding("iso-8859-1");
array<Byte>^ n_as_array = ascii->GetBytes(n_as_string);
array<Byte>^ d_as_array = ascii->GetBytes(d_as_string);
cli::pin_ptr<unsigned char> pin_ptr_n = &n_as_array[0];
cli::pin_ptr<unsigned char> pin_ptr_d = &d_as_array[0];
unsigned char* ptr_n = pin_ptr_n;
unsigned char* ptr_d = pin_ptr_d;
int n_out = *(int*)ptr_n;
double d_out = *(double*)ptr_d;
std::cout << n_out << std::endl;
std::cout << d_out << std::endl;
return 0;
}
This should give you :
123456
123.456
123456
123.456
Not sure it is completely safe, but trying it in your context should be a good start to ensure it is viable. :)