Table of Contents
C - C++ IO Tips
C++ IO Header Files
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.
Leading whitespace
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>
- 3.14 is read into fl . The carriage return (newline) following the 3.14 is still sitting on the input buffer.
- 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.
Read in an entire line
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>
- 3.14 is read into fl . The newline following the 3.14 is still sitting on the input buffer.
- std::cin.getline(str, 101) immediately processes the newline that is still on the input buffer. str becomes an empty string.
- 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
Reading in numbers directly is problematic
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');
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);
- 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.
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 }
- 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.
Getline can be told to stop grabbing input at any designated character
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).
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 }
- 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
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"
- The default fill character is a space.
- A common fill character when printing numbers is zero “0”.