How to check if input is numeric in C

Question

I want to create a program that takes in integer input from the user and then terminates when the user doesn t enter anything at all  ie  just presses enter   However  I m having trouble validating the input  making sure that the user is inputting integers  not strings  atoi   won t work  since the integer inputs can be more than one digit   What is the best way of validating this input  I tried something like the following  but I m not sure how to complete it   char input  while  cin gt  gt input      n            some way to check if input is a valid number      while  inputIsNumeric                  cin gt  gt input

User · Answer

I find myself using boost  lexical cast for this sort of thing all the time these days   Example   std  string input  std  getline std  cin input   int input value  try     input value boost  lexical cast lt int gt  input      catch boost  bad lexical cast  amp          Deal with bad input here     The pattern works just as well for your own classes too  provided they meet some simple requirements  streamability in the necessary direction  and default and copy constructors

User · Answer

The problem with the usage of   cin gt  gt number variable    is that when you input 123abc value  it will pass and your variable will contain 123   You can use regex  something like this  double inputNumber         string str      regex regex pattern  -  0-9     0-9          do               cout  lt  lt   Input a positive number             cin  gt  gt  str       while  regex match str regex pattern         return stod str       Or you can change the regex pattern to validate anything that you would like

User · Answer

If you already have the string  you can use this function   bool isNumber  const string amp  s       bool hitDecimal 0    for  char c   s           if  c       amp  amp   hitDecimal      2     in string mean invalid       hitDecimal 1     first hit here  we forgive and skip     else if   isdigit  c            return 0      not    not        return 1

User · Answer

Why not just using scanf   i   and check its return

User · Answer

I guess ctype h is the header file that you need to look at  it has numerous functions for handling digits as well as characters   isdigit or iswdigit is something that will help you in this case   Here is a reference  http   docs embarcadero com products rad studio delphiAndcpp2009 HelpUpdate2 EN html devwin32 isdigit xml html

User · Answer

Check out std  isdigit   function

User · Answer

When cin gets input it can t use  it sets failbit   int n  cin  gt  gt  n  if  cin     or if cin fail             user didn t input a number     cin clear       reset failbit     cin ignore std  numeric limits lt std  streamsize gt   max      n      skip bad input        next  request user reinput     When cin s failbit is set  use cin clear   to reset the state of the stream  then cin ignore   to expunge the remaining input  and then request that the user re-input  The stream will misbehave so long as the failure state is set and the stream contains bad input

[c++] How to check if input is numeric in C++

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