Commonly Asked C/C++ Questions

#include <iostream>
#include <string>

using namespace std;

int main()
{
    string name;
    cout<<"Enter a name:\n";
    getline(cin, name);
    cout<<name<<endl;

    return 0;
}

#include <stdlib.h>
int main(void)
{
     char *buf1 = "42";
     char buf2[] = "69.00";
     int i;
     double d;
     long l;
     i = atoi(buf1);
     l = atol(buf1);
     d = atof(buf2);
     return 0;
}

#include <stdio.h>
int main(void)
{
    int i = 42;
    float f = 69.0;
    double d = 105.24;
    long l = 23;
    char buf[50];
 
    sprintf(buf, "%d", i);
    sprintf(buf, "%f", f);
    sprintf(buf, "%f", d);
    sprintf(buf, "%ld", l);
    sprintf(buf, "%d %f %ld", i, f, l);
    return 0;
}    

#include <sstream>
#include <string>

int main(void) {
    std::stringstream ss;
    int i = 42;
    double d = 105.24;

    ss << i << " " << d;

    // Convert to string or char array
    std::string s = ss.str();
    char buf[50];
    sprintf(buf, ss.str().c_str());
}

#include <stdlib.h>
#include <stdio.h>
int main(void)
{
  long l; int i;
  unsigned int ui;
  char *hexstr = "12FC3";
  char *octstr = "1245";
  char *binarystr = "1101";

  l = strtol(hexstr, NULL, 16);
  l = strtol(octstr, NULL, 8);
  l = strtol(binarystr, NULL, 2);
  sscanf("12", "%d", &i);
  sscanf("14", "%ld", &l);
  sscanf(hexstr, "%x", &ui);
  sscanf(hexstr, "%o", &ui);

  return 0;
}

#include <sstream>
#include <string>

int main(void)
{
  long l; int i;
  char *hexstr = "12FC3";
  char *octstr = "1245";
  std::stringstream ss;

  ss << std::hex << hexstr;
  ss >> l;
  ss.clear();
  ss << std::oct << octstr;
  ss >> i;

  return 0;
}

#include <stdio.h>
int main(void)
{
    int i = 42;
    char buf[50];
    sprintf(buf, "%x", i); /* convert to hex */
    sprintf(buf, "%o", i); /* convert to octal */
    return 0;
}

#include <sstream>
#include <string>

int main(void)
{
  int i = 42;
  char buf[50];
  std::stringstream ss;
  ss << std::hex << i;
  ss >> buf;

  ss.clear();
  ss << std::oct << i;
  ss >> buf;
  return 0;
}

char ch;
int i;

ch = 'A';
i = ch; /* Assigns the ASCII value of 'A' (i.e.) 65 to i */
printf("%d\n", i); /* Prints 65 */

int i;
char ch;

i = 65;
ch = (char) i;
/* some compilers don't warn you if you do:
ch = i;
and some will, hence the explicit cast to char type. The cast
assures the compilers that warn you, that you know what you're doing. */
printf("%c\n", ch); /* prints 'A' */

char ConvertToUpperCase(char ch) {
   if (ch >= 'a' && ch <= 'z')
      ch = ch - 'a' + 'A';
   return ch;
}

#include <stdio.h>

int main(void)
{
  int i = 23; long l = 42;
  float f = 105.23; double d = 69.05;
  long double ld = 55.23; long long ll = 92;
  char c='a'; char s[] = "This is a string";

  printf("int = %d\nlong = %ld\n", i, l);
  printf("float = %f\ndouble = %fd\n", f, d);
  printf("long double = %Lf\nlong long = %lld\n", ld, ll);
  printf("char = "%c\nString=%s\n", c, s);

  return 0;
}

#include <stdio.h>

int main(void)
{
  float f = 105.2345;
  char buf[] = "This is a string";

  /* Limit to 2 decimals */
  printf("float = %.2f\n", f);
  /* Print 10 chars wide, limit to 2 decimals */
  printf("float = %10.2f\n", f);
  /* Print 10 chars wide, limit to 2 decimals and left-justify */
  printf("float = %-10.2f\n", f);
  /* Limit to 10 characters */
  printf("%.10s\n", buf);

  return 0;
}

#include <iostream>
#include <iomanip>

int main(void)
{
  float f = 105.2345;
  char buf[] = "This is a string";

  std::cout.setf(std::ios::fixed);
  /* Limit to 2 decimals */
  std::cout << std::setprecision(2) << f << endl; 
  /* Print 10 chars wide, limit to 2 decimals */
  std::cout << std::setw(10) << std::setprecision(2) << f << endl; 
  /* Right justify to 30 chars */
  std::cout << std::setw(30) << buf << endl; 

  return 0;
}

char buf[25];
scanf("%20s", buf);

std::cin >> std::setw(20) >> buf;
std::cin.get(buf, 20);

   // Example variables
   string sFileName = "test.txt";
   string sContents = "File data\n";
   string sToken;
   char   lineBuffer [256];
   int    nCount = 0;

   // Construct an fstream object and attempt to open
   // for output
   fstream ooFile;
   ooFile.open (sFileName.c_str (), ios::binary | ios::out);

   // Test for success
   if (!ooFile.is_open ()) return badHappenstance ("Couldn't open file for writing");

   // Write example
   ooFile << sContents;

   // Test the write for success
   if (!ooFile.good ()) return badHappenstance ("Write failed");

   // Close the file.  Note that the file will be closed automatically when
   // the object is destroyed (goes out of scope).
   ooFile.close ();

   // Open it for input
   ooFile.open (sFileName.c_str (), ios::binary | ios::in);
   if (!ooFile.is_open ()) return badHappenstance ("Couldn't open file for reading");

   // Read tokens until end of file or error
   while (ooFile.good ())
   {
      ooFile >> sToken;
      cout << "Token " << ++nCount << ": ";
      if (sToken.size ()) cout << sToken; else cout << "(null token)";
      cout << endl;
   }
   // Check some status for grins
   if (ooFile.eof ()) cout << "Reached EOF" << endl;
   else if (ooFile.fail ()) return badHappenstance ("Read failed for some reason");

   // Alternatively, read entire line
   ooFile.getline (lineBuffer, sizeof (lineBuffer));

   // Always the status
   if (!(ooFile.good () || ooFile.eof ()))
      return badHappenstance ("Failure reading line");

   cout << "This is the line: " << lineBuffer << endl;

   // Position to beginning after clearing EOF or fail bits
   ooFile.clear ();
   ooFile.seekg (0, ios::beg);
   if (!ooFile.good ()) return badHappenstance ("Seek failed");

   ooFile.close ();

   // Example variables
   FILE *ooFile;
   char  sFileName [] = "test.txt";
   char  sContents [] = "File data\n";
   char  sToken [32];
   char  lineBuffer [256];
   int   nCount = 0;
   int   status;

   // Attempt to open file for output
   ooFile = fopen (sFileName, "wb");

   // Test for success
   if (!ooFile) return badHappenstance ("Couldn't open file for writing");

   // Write the contents
   status = fputs (sContents, ooFile);

   // Test for success
   if (status == EOF) return badHappenstance ("Write failed");

   // Close the file
   fclose (ooFile);

   ooFile = fopen (sFileName, "rb");
   if (!ooFile) return badHappenstance ("Couldn't open file for reading");

   // Read tokens until end of file or error -- one method: fscanf
   // fscanf consumes whitespace, unlike some C++ methods
   // so there won't be a null token at the end.
   while (1)
   {
      if (fscanf (ooFile, "%s", sToken) != 1)
      {
         if (feof (ooFile)) 
         {
            printf ("Reached EOF\n");
            break;
         }
         else return badHappenstance ("Failed to read token");
      }
      printf ("Token %d: %s\n", ++nCount, sToken);
   }
// REPOSITIONING EXAMPLE

   rewind (ooFile);

   // Read a line all in one swell foop
   // We'll either fill the buffer or have a newline
   // at the end of the buffer, or both.  If you don't
   // want the newline:
   //
   //    index = strlen (lineBuffer) - 1;
   //    if (lineBuffer [index] == '\n')
   //        lineBuffer [index] = '\0';"
   //
   if (fgets (lineBuffer, sizeof (lineBuffer), ooFile) == NULL)
      // Always the status; EOF is okay, error is not
      if (ferror (ooFile)) return badHappenstance ("Failed to read line");

   printf ("This is the line: %s", lineBuffer);
   fclose (ooFile);

typedef int fileHandle;
extern int errno;

   // Example variables
   fileHandle ooFile;
   char  sFileName [] = "test.txt";
   char  sContents [] = "File data\n";
   char  sToken [32];
   char  sTrash [32];
   char  lineBuffer [256];
   int   nChars, nTokens;
   int   status;

   // Attempt to open file for input/output
   ooFile = _open ("test.txt", _O_CREAT | _O_RDWR | _O_TRUNC | _O_BINARY, _S_IWRITE);

   // Test for success
   if (ooFile == EOF) return badHappenstance (strerror (errno));

   // Write the contents
   status = _write (ooFile, sContents, strlen (sContents));

   // Test for success
   if (status == EOF) return badHappenstance ("Write failed");

   // Return to the beginning for input
   if (_lseek (ooFile, 0, SEEK_SET) == -1L)
      return badHappenstance ("Seek to beginning failed");

   // Read tokens until end of file or error
   nTokens = 0;
   while (1)
   {
      // Skip any whitespace
      while (status = _read (ooFile, sTrash, 1) == 1)
         if (!isspace (sTrash [0])) break;
      // Check for EOF or failure
      if (status < 0) return badHappenstance ("Failure to read data");
      else if (status == 0) break;
      sToken [0] = sTrash [0];
      for (nChars = 1; nChars < sizeof (sToken) - 1; nChars++)
      {
         status = _read (ooFile, &sToken [nChars], 1);
         // Check for EOF or failure
         if (status < 0) return badHappenstance ("Failure in reading data");
         else if ((status == 0) || (isspace (sToken [nChars])))
         {
            sToken [nChars] = '\0';
            break;
         }
      }
      // The token could be too long...
      if (nChars >= sizeof (sToken) - 1)
      {
         sTrash [0] = ' ';
         // Skip any remaining non-whitespace characters
         while (status = _read (ooFile, sTrash, 1) == 1)
            if (!isspace (sTrash [0])) break;
         // Check for EOF or failure
         if (status < 0) return badHappenstance ("Failure in reading data");
         else if (status == 0) break;
      }
      // Or we could have completed the token
      if (strlen (sToken) > 0) printf ("Token %d: %s\n", ++nTokens, sToken);
   }
   printf ("End of file\n");

   // Reposition to beginning
   if (_lseek (ooFile, 0, SEEK_SET) == -1L)
      return badHappenstance ("Seek to beginning failed");

   // Read one buffer's worth
   status = _read (ooFile, lineBuffer, sizeof (lineBuffer) - 1);
   if (status > 0) 
      lineBuffer [status] = '\0';

   printf ("This is the line: %s", lineBuffer);
   _close (ooFile);

   return 0;
}