C - C++ IO Tips

Header files to include when using C++ I/O.

#include<iostream>

Include this file whenever using C++ I/O.

#include<iomanip>

This file must be included for most C++ manipulators.

#include<fstream>

Include this file whenever working with files.

By default, leading whitespace (carriage returns, tabs, spaces) is ignored by cin.

Given:

int i;
float fl;
std::cin >> fl;
std::cin >> i;

And you type: 3.14<return>42<return>

1. 3.14 is read into fl . The carriage return (newline) following the 3.14 is still sitting on the input buffer.
2. Since std::cin ignores whitespace, the first return is “eaten” by std::cin » i. Then the integer 42 is read into i and the second return is left on the input buffer.

std::cin.getline() can run into problems when used with std::cin » var.

getline can be provided a third argument; a “stop” character. This character ends getline's input. The character is eaten and the string is terminated.

Example:

std::cin.getline(str, 100, '|')

If std::cin.getline() is not provided a “stop” character as a third argument, it will stop when it reaches a newline.

Given:

  float fl;
  std::cin >> fl;
  char str[101]
  std::cin.getline(str, 101);

And you type: 3.14<return>

1. 3.14 is read into fl . The newline following the 3.14 is still sitting on the input buffer.
2. std::cin.getline(str, 101) immediately processes the newline that is still on the input buffer. str becomes an empty string.
3. The illusion is that the application “skipped” the std::cin.getline() statement.

The solution is to add std::cin.ignore(); immediately after the first std::cin statement. This will grab a character off of the input buffer (in this case, newline) and discard it.

std::cin.ignore() can be called three different ways:

• No arguments: A single character is taken from the input buffer and discarded:

std::cin.ignore(); //discard 1 character

• One argument: The number of characters specified are taken from the input buffer and discarded:

std::cin.ignore(33); //discard 33 characters

• Two arguments: discard the number of characters specified, or discard characters up to and including the specified delimiter (whichever comes first):

std::cin.ignore(26, '\n'); //ignore 26 characters or to a newline, whichever comes first

If std::cin is presented with input it cannot process, std::cin goes into a “fail” state.

The input it cannot process is left on the input stream.

All input will be ignored by std::cin until the “fail” state is cleared: std::cin.clear().

A routine that reads a number directly should:

• Read in the number.
• Check to see that the input stream is still valid.
• If the input stream is not good (!std::cin)
  • Call std::cin.clear() to take the stream out of the “fail” state.
  • Remove from the stream the input that caused the problem: std::cin.ignore(…)
  • Get the input again if appropriate or otherwise handle the error

Inputting numbers directly, version 1:

  #include<limits> //for numeric_limits
  float fl;
  int bad_input;
  do{
    bad_input=0;
    std::cin >> fl;
    if(!std::cin)
    {
      bad_input=1;
      std::cin.clear();
      std::cin.ignore(std::numeric_limits<streamsize>::max(),'\n');
    }
 
  }while(bad_input);

Inputting numbers directly, version 2:

  #include<limits> //for numeric_limits
  float fl;
  while(!(std::cin >> fl))
  {
    std::cin.clear();
    std::cin.ignore(std::numeric_limits<streamsize>::max(),'\n');
  }

NOTE: A note on limits. If your compiler doesn't support std::numeric_limits<streamsize>::max(), an alternative is to use the c-style method for determining the maximum integer allowed:

#include<climits>
...
std::cin.ignore(INT_MAX, '\n');

 #include <cstdlib>
...
int i;
float fl;
char temp[100];
 
std::cin.getline(temp, 100);
fl=strtof(temp,0);
std::cin.getline(temp, 100);
i=strtol(temp,0,10);

• getline will read both strings and numbers without going into a “fail” state.
• Include cstdlib to use the converter functions: string-to-long-integer (strtol), string-to-double (strtod), string-to-float (strtof), and string-to-long-double (strtold). Refer to a reference for more information on the second (and third for strtol) arguments to these converter functions.

std::ifstream someVarName("data.txt");
float fl;
char temp[100];
someVarName.getline(temp, 100);
fl=strtof(temp);
int i;
someVarName >> i;

std::ifstream inf("data.txt");
char temp[100];
while(!inf.getline(temp, 100).eof())
{
  //process the line
}

• the loop will exit once the end of the file is reached

If the expression in the while() loop above is confusing, I've provided an explanation: UnderstandingComplexExpressions.pdf.

char temp[100];
std::cin.getline(temp, 100, '|');

• If only two arguments are supplied to getline, getline will stop at the end of the line (at the newline character).
• If three arguments are supplied to getline, getline will stop at the character designated by the third argument.
• The stop character is not copied to the string.
• The stop character is “eaten” (removed from the input stream).

Given data file:

  John|83|52.2
  swimming|Jefferson
  Jane|26|10.09
  sprinting|San Marin

Process using:

std::ifstream inf("data.txt");
char name[30];
while(!inf.getline(name, 30, '|').eof())
{
  Athlete* ap;
  char jersey_number[10];
  char best_time[10];
  char sport[40];
  char high_school[40];
  inf.getline(jersey_number, 10, '|'); #read thru pipe
  inf.getline(best_time, 10);          #read thru newline
  inf.getline(sport, 40, '|');         #read thru pipe
  inf.getline(high_school, 40);        #read thru newline
  ap = new Athlete(name, strtod(number), strtof(best_time), sport, high_school);
  //do something with ap
}

• In a delimited file, only the first field should be in the while loop
• For each field: If the field is the last field in the line or the only field in the line, be sure that getline stops at a newline and not some other delimiter

All of the previous examples have assumed that C-style strings (null-terminated character arrays) were being used. C++ provides a string class that, when combined with a particular “getline” function, can dynamically resize to accommodate user input. In general, C++ strings are preferred over C strings.

Given data file:

  John|83|52.2
  swimming|Jefferson
  Jane|26|10.09
  sprinting|San Marin

Here is the same code shown above, this time using C++ strings:

#include <string>
std::ifstream inf("data.txt");
string name;
while(!std::getline(inf, name, '|').eof())
{
  Athlete* ap;
  std::string jersey_number;
  std::string best_time;
  std::string sport;
  string high_school;
  std::getline(inf, jersey_number, '|'); #read thru pipe
  std::getline(inf, best_time);          #read thru newline
  std::getline(inf, sport, '|');         #read thru pipe
  std::getline(inf, high_school);        #read thru newline
  ap = new Athlete(name, strtod(number.c_str()), 
       strtof(best_time.c_str()), sport, high_school);
  //do something with ap
}

How to prepare the output stream to print fixed precision numbers (3.40 instead of 3.4)

  std::cout.setf(std::ios::fixed, std::ios::floatfield);
  std::cout.setf(std::ios::showpoint);
  std::cout.precision(2);

Given: int one=4, two=44;

  std::cout << one << std::endl.; 
  //output:  "4"
 
  std::cout << setw(2) << one << std::endl.;
  //output:  " 4"
 
  std::cout.fill('X');
  std::cout << setw(2) << one << std::endl.;
  //output:  "X4"
 
  std::cout.fill('X');
  std::cout << setw(2) << two << std::endl.;
  //output:  "44"

• The default fill character is a space.
• A common fill character when printing numbers is zero “0”.