Checking cin input stream produces an integer

Question

I was typing this and it asks the user to input two integers which will then become variables   From there it will carry out simple operations    How do I get the computer to check if what is entered is an integer or not  And if not  ask the user to type an integer in   For example  if someone inputs  a  instead of 2  then it will tell them to reenter a number   Thanks    include  lt iostream gt  using namespace std   int main           int firstvariable      int secondvariable      float float1      float float2       cout  lt  lt   Please enter two integers and then press Enter    lt  lt  endl      cin  gt  gt  firstvariable      cin  gt  gt  secondvariable       cout  lt  lt   Time for some simple mathematical operations  n   lt  lt  endl       cout  lt  lt   The sum  n    lt  lt  firstvariable  lt  lt       lt  lt  secondvariable           lt  lt     lt  lt  firstvariable   secondvariable  lt  lt    n    lt  lt  endl

User · Answer

You can use the variables name itself to check if a value is an integer  for example    include  lt iostream gt  using namespace std   int main      int firstvariable  int secondvariable  float float1  float float2   cout  lt  lt   Please enter two integers and then press Enter    lt  lt  endl  cin  gt  gt  firstvariable  cin  gt  gt  secondvariable   if firstvariable  amp  amp  secondvariable       cout  lt  lt   Time for some simple mathematical operations  n   lt  lt  endl       cout  lt  lt   The sum  n    lt  lt  firstvariable  lt  lt       lt  lt  secondvariable       lt  lt     lt  lt  firstvariable   secondvariable  lt  lt    n    lt  lt  endl   else      cout  lt  lt    n ERROR tINVALID INPUT  n        return 1      return 0

User · Answer

You can check like this   int x  cin  gt  gt  x   if  cin fail            Not an int      Furthermore  you can continue to get input until you get an int via    include  lt iostream gt     int main          int x      std  cin  gt  gt  x      while std  cin fail              std  cout  lt  lt   Error   lt  lt  std  endl          std  cin clear            std  cin ignore 256   n            std  cin  gt  gt  x            std  cout  lt  lt  x  lt  lt  std  endl       return 0      EDIT   To address the comment below regarding input like 10abc  one could modify the loop to accept a string as an input   Then check the string for any character not a number and handle that situation accordingly   One needs not clear ignore the input stream in that situation   Verifying the string is just numbers  convert the string back to an integer   I mean  this was just off the cuff   There might be a better way   This won t work if you re accepting floats doubles  would have to add     in the search string     include  lt iostream gt   include  lt string gt   int main          std  string theInput      int inputAsInt       std  getline std  cin  theInput        while std  cin fail      std  cin eof      theInput find first not of  0123456789      std  string  npos             std  cout  lt  lt   Error   lt  lt  std  endl           if  theInput find first not of  0123456789      std  string  npos                std  cin clear                std  cin ignore 256   n                       std  getline std  cin  theInput              std  string  size type st      inputAsInt   std  stoi theInput  amp st       std  cout  lt  lt  inputAsInt  lt  lt  std  endl      return 0

User · Answer

If istream fails to insert  it will set the fail bit   int i   0  std  cin  gt  gt  i     type a and press enter if  std  cin fail          std  cout  lt  lt   I failed  try again       lt  lt  std  endl     std  cin clear       reset the failed state     You can set this up in a do-while loop to get the correct type  int in this case  propertly inserted   For more information   http   augustcouncil com  tgibson tutorial iotips html directly

User · Answer

You could use    int a   12  if  a gt 0    a lt 0   cout  lt  lt   Your text  lt  lt endl      I m pretty sure it works

User · Answer

Heh  this is an old question that could use a better answer   User input should be obtained as a string and then attempt-converted to the data type you desire  Conveniently  this also allows you to answer questions like    what type of data is my input      Here is a function I use a lot  Other options exist  such as in Boost  but the basic premise is the same  attempt to perform the string type conversion and observe the success or failure   template  lt typename T gt  std  optional  lt T gt  string to  const std  string amp  s       std  istringstream ss  s      T result    ss  gt  gt  result  gt  gt  std  ws          attempt the conversion   if  ss eof    return result      success   return                           failure     Using the optional type is just one way  You could also throw an exception or return a default value on failure  Whatever works for your situation   Here is an example of using it   int n  std  cout  lt  lt   n         std  string s    getline  std  cin  s      auto x   string to  lt int gt    s      if   x  return complain      n    x    std  cout  lt  lt   Multiply that by seven to get    lt  lt   7   n   lt  lt     n     limitations and type identification  In order for this to work  of course  there must exist a method to unambiguously extract your data type from a stream  This is the natural order of things in C       that is  business as usual  So no surprises here   The next caveat is that some types subsume others  For example  if you are trying to distinguish between int and double  check for int first  since anything that converts to an int is also a double

User · Answer

There is a function in c called isdigit    That will suit you just fine  Example   int var1    h   int var2    2    if  isdigit var1         printf  var1     c  is a digit n   var1      else      printf  var1     c  is not a digit n   var1      if  isdigit var2        printf  var2     c  is a digit n   var2      else      printf  var2     c  is not a digit n   var2        From here

User · Answer

I prefer to use  lt limits gt  to check for an int until it is passed   include  lt iostream gt   include  lt limits gt    std  numeric limits  using std  cout  std  endl  std  cin   int main         int num      while   cin  gt  gt  num       check the Input format for integer the right way         cin clear            cin ignore std  numeric limits lt std  streamsize gt   max      n            cout  lt  lt   quot Invalid input   Reenter the number   quot                 cout  lt  lt   quot output   quot   lt  lt  num  lt  lt  endl       return 0

[c++] Checking cin input stream produces an integer

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