How to scanf only integer

Question

I want the code to run until the user enters an integer value   The code works for char and char arrays   I have done the following      include lt stdio h gt  int main         int n      printf  Please enter an integer          while scanf   d   amp n     1                printf  Please enter an integer              while getchar        n              printf  You entered   d n  n       return 0      The problem is if the user inputs a float value scanf will accept it   Please enter an integer  abcd Please enter an integer  a Please enter an integer  5 9 You entered  5   How can that be avoided

User · Answer

You take scanf    You throw it in the bin  You use fgets   to get an entire line  You use strtol   to parse the line as an integer  checking if it consumed the entire line      char  end  char buf LINE MAX    do        if   fgets buf  sizeof buf  stdin           break           remove  n      buf strlen buf  - 1    0        int n   strtol buf   amp end  10     while  end    buf   strlen buf

User · Answer

A possible solution is to think about it backwards  Accept a float as input and reject the input if the float is not an integer   int n  float f  printf  Please enter an integer      while scanf   f   amp f   1     int f    f              n   f    Though this does allow the user to enter something like 12 0  or 12e0  etc

User · Answer

Use fgets and strtol   A pointer to the first character following the integer representation in s is stored in the object pointed by p  if  p is different to  n then you have a bad input    include  lt stdio h gt   include  lt stdlib h gt   int main void         char  p  s 100       long n       while  fgets s  sizeof s   stdin             n   strtol s   amp p  10           if  p    s     p      n                 printf  Please enter an integer                else break            printf  You entered   ld n   n       return 0

User · Answer

If you re set on using scanf  you can do something like the following   int val  char follow    int read   scanf    d c    amp val   amp follow     if   read    2       if   isspace  follow                input is an integer followed by whitespace  accept       else            input is an integer followed by non-whitespace  reject       else if   read    1          input is an integer followed by EOF  accept   else        input is not an integer  reject

User · Answer

I know how this can be done using fgets and strtol  I would like to know how this can be done using scanf    if possible      As the other answers say  scanf isn t really suitable for this  fgets and strtol is an alternative  though fgets has the drawback that it s hard to detect a 0-byte in the input and impossible to tell what has been input after a 0-byte  if any    For sake of completeness  and assuming valid input is an integer followed by a newline    while scanf   d 1  n     amp n   char  2    0     lt  2    Alternatively  use  n before and after   1  n  with assignment-suppression  Note  however  from the Debian manpage       This is not a conversion  although it can be suppressed with the   assignment-suppression character    The C standard says   Execution of a  n directive does not increment the assignment count returned at the completion of execution  but the Corrigendum seems to contradict this    Probably it is wise not to make any assumptions on the effect of  n conversions on the return value

User · Answer

Try using the following pattern in scanf  It will read until the end of the line   scanf   d n    amp n    You won t need the getchar   inside the loop since scanf will read the whole line  The floats won t match the scanf pattern and the prompt will ask for an integer again

User · Answer

Using fgets   is better     To solve only using scanf   for input  scan for an int and the following char   int ReadUntilEOL void      char ch    int count    while   count   scanf   c    amp ch      1  amp  amp  ch      n            Consume char until  n or EOF or IO error   return count      include lt stdio h gt  int main void      int n     for            printf  Please enter an integer          char NextChar     n       int count   scanf   d c    amp n   amp NextChar       if  count  gt   1  amp  amp  NextChar      n          break      if  ReadUntilEOL      EOF         return 1      No valid input ever found       printf  You entered   d n   n     return 0      This approach does not re-prompt if user only enters white-space such as only Enter

[c] How to scanf only integer?

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