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Tips and tricks for using C++ I/O (input/output)

Table of Contents

  1. There are three header files to include when using C++ I/O
  2. How to set the width of a printing field
  3. By default, leading whitespace (carriage returns, tabs, spaces) is ignored by cin.
  4. cin.getline() can run into problems when used with cin >> var.
  5. Reading in numbers directly is problematic
  6. Using getline to input numbers is a more robust alternate to reading numbers directly
  7. Once a file is opened, it may be used exactly as cin is used.
  8. When reading an entire file, embed the file input inside of the loop condition
  9. Getline can be told to stop grabbing input at any designated character
  10. Delimited files can easily be read using a while loop and getline.
  11. Using C++-style strings
  12. How to prepare the output stream to print fixed precision numbers (3.40 instead of 3.4)

There are three 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. If you don't know what a manipulator is, don't worry. Just include this file along with iostream and you can't go wrong
#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.

Given:
  float fl;
  std::cin >> fl;
  char str[101]
  std::cin.getline(str, 101);
  1. And you type: 3.14<return>
  2. 3.14 is read into fl . The newline following the 3.14 is still sitting on the input buffer.
  3. std::cin.getline(str, 101) immediately processes the newline that is still on the input buffer. str becomes an empty string.
  4. 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:

  1. No arguments: A single character is taken from the input buffer and discarded:
    std::cin.ignore(); //discard 1 character
  2. One argument: The number of characters specified are taken from the input buffer and discarded:
    std::cin.ignore(33); //discard 33 characters
  3. 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

Reading in numbers directly is problematic

Inputing 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);
Inputing 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');
  }

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');

Using getline to input numbers is a more robust alternate to reading numbers directly

#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);

Once a file is opened, it may be used exactly as std::cin is used.

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

When reading an entire file, embed the file input inside of the loop condition

std::ifstream inf("data.txt");
char temp[100];
while(!inf.getline(temp, 100).eof())
{
  //process the line
}
If the expression in the while() loop above is confusing, I've provided an explanation:
UnderstandingComplexExpressions.pdf.

Getline can be told to stop grabbing input at any designated character

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

Delimited files can easily be read using a while loop and getline.

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

}

Using C++-style strings

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);

How to set the width of a printing field

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"
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Revision History

2012 May 02 Added a link to a 2-page PDF document: "Understanding Complex Expressions".
2012 April 18 Added std:: to ios flags
2007 March 1 Corrected spelling error
2010 February 18 Corrected HTML error