Check if input is integer type in C

Question

The catch is that I cannot use atoi or any other function like that  I m pretty sure we re supposed to rely on mathematical operations     int num    scanf   d   amp num    if    num is not integer         printf  enter integer      return       I ve tried    num 2  2    num num 1  0 if scanf   d   amp num   1    but none of these worked   Any ideas

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printf  type a number     int converted   scanf   d    amp a   printf   n     if  converted    0         printf  enter integer        system  PAUSE  n        return 0      scanf   returns the number of format specifiers that match  so will return zero if the text entered cannot be interpreted as a decimal integer

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You need to read your input as a string first  then parse the string to see if it contains valid numeric characters  If it does then you can convert it to an integer   char s MAX LINE    valid   FALSE  fgets s  sizeof s   stdin   len   strlen s   while  len  gt  0  amp  amp  isspace s len - 1        len--         strip trailing newline or other white space if  len  gt  0        valid   TRUE      for  i   0  i  lt  len    i                if   isdigit s i                          valid   FALSE              break

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I looked over everyone s input above  which was very useful  and made a function which was appropriate for my own application  The function is really only evaluating that the user s input is not a  0   but it was good enough for my purpose  Hope this helps    include lt stdio h gt   int iFunctErrorCheck int iLowerBound  int iUpperBound    int iUserInput 0  while  iUserInput  0       scanf   i    amp iUserInput       if  iUserInput  0           printf  Please enter an integer   i- i   n   iLowerBound  iUpperBound           getchar              if   iUserInput  0   amp  amp   iUserInput lt iLowerBound    iUserInput gt iUpperBound            printf  Please make a valid selection   i- i   n   iLowerBound  iUpperBound           iUserInput 0          return iUserInput

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I developed this logic using gets and away from scanf hassle    void readValidateInput          char str 10        0          readStdin  fgets str  10  stdin         printf  fgets is returning  s n   str        int numerical   1      int i   0       for  i   0  i  lt  10  i                printf  Digit at str  d  is  c n   i  str i              printf  numerical    d n   numerical           if  isdigit str i      0                if  str i       n  break              numerical   0                printf  numerical changed   d n   numerical               break                      if   numerical            printf  This is not a valid number of tasks  you need to enter at least 1 task n            goto readStdin            else if  str i       n             str i      0           numOfTasks   atoi str             printf  Captured Number of tasks from stdin is  d n   numOfTasks

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I ve been searching for a simpler solution using only loops and if statements  and this is what I came up with  The program also works with negative integers and correctly rejects any mixed inputs that may contain both integers and other characters      include  lt stdio h gt   include  lt stdlib h gt     Used for atoi   function  include  lt string h gt     Used for strlen   function   define TRUE 1  define FALSE 0  int main void        char n 10      Limits characters to the equivalent of the 32 bits integers limit  10 digits      int intTest      printf  Give me an int           do                       scanf    s   n            intTest   TRUE     Sets the default for the integer test variable to TRUE          int i   0  l   strlen n           if  n 0      -      Tests for the negative sign to correctly handle negative integer values             i            while  i  lt  l                                    if  n i   lt   0     n i   gt   9      Tests the string characters for non-integer values                                             intTest   FALSE     Changes intTest variable from TRUE to FALSE and breaks the loop early                 break                            i                      if  intTest    TRUE              printf   i n   atoi n       Converts the string to an integer and prints the integer value         else             printf  Retry         Prints  Retry   if tested FALSE           while  intTest    FALSE      Continues to ask the user to input a valid integer value     return 0

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This is a more user-friendly one I guess     include lt stdio h gt      This program checks if the entered input is an integer    or provides an option for the user to re-enter       int getint       int x    char c    printf   nEnter an integer  say -1 or 26 or so          while  scanf   d   amp x     1           c getchar         printf  You have entered         putchar c       printf   in the input which is not an integer         while   getchar        n             wasting the buffer till the next new line      printf   nEnter an integer  say -1 or 26 or so              return x      int main void      int x    x getint       printf  Main Function   gt  n      printf  Integer    d n  x     return 0

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There are several problems with using scanf with the  d conversion specifier to do this    If the input string starts with a valid integer  such as  12abc    then the  12  will be read from the input stream and converted and assigned to num  and scanf will return 1  so you ll indicate success when you  probably  shouldn t  If the input string doesn t start with a digit  then scanf will not read any characters from the input stream  num will not be changed  and the return value will be 0  You don t specify if you need to handle non-decimal formats  but this won t work if you have to handle integer values in octal or hexadecimal formats  0x1a    The  i conversion specifier handles decimal  octal  and hexadecimal formats  but you still have the first two problems      First of all  you ll need to read the input as a string  preferably using fgets    If you aren t allowed to use atoi  you probably aren t allowed to use strtol either   So you ll need to examine each character in the string   The safe way to check for digit values is to use the isdigit library function  there are also the isodigit and isxdigit functions for checking octal and hexadecimal digits  respectively   such as  while   input  amp  amp  isdigit  input      input           if you re not even allowed to use isdigit  isodigit  or isxdigit  then slap your teacher professor for making the assignment harder than it really needs to be      If you need to be able to handle octal or hex formats  then it gets a little more complicated   The C convention is for octal formats to have a leading 0 digit and for hex formats to have a leading 0x   So  if the first non-whitespace character is a 0  you have to check the next character before you can know which non-decimal format to use     The basic outline is    If the first non-whitespace character is not a  -         0   or non-zero decimal digit  then this is not a valid integer string  If the first non-whitespace character is  -   then this is a negative value  otherwise we assume a positive value  If the first character is      then this is a positive value  If the first non-whitespace and non-sign character is a non-zero decimal digit  then the input is in decimal format  and you will use isdigit to check the remaining characters  If the first non-whitespace and non-sign character is a  0   then the input is in either octal or hexadecimal format  If the first non-whitespace and non-sign character was a  0  and the next character is a digit from  0  to  7   then the input is in octal format  and you will use isodigit to check the remaining characters  If the first non-whitespace and non-sign character was a 0 and the second character is x or X  then the input is in hexadecimal format and you will use isxdigit to check the remaining characters  If any of the remaining characters do not satisfy the check function specified above  then this is not a valid integer string

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First ask yourself how you would ever expect this code to NOT return an integer   int num   scanf   d   amp num     You specified the variable as type integer  then you scanf  but only for an integer   d    What else could it possibly contain at this point

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This method works for everything  integers and even doubles  except zero  it calls it invalid    The while loop is just for the repetitive user input  Basically it checks if the integer x x   1  If it does  as it would with a number   its an integer double  If it doesn t  it obviously it isn t  Zero fails the test though    include  lt stdio h gt    include  lt math h gt   void main          double x      int notDouble      int true   1      while true            printf  Input an integer   n            scanf   lf    amp x           if  x x    1                notDouble   1              fflush stdin                     if  notDouble    1                printf  Input is valid n                      else               printf  Input is invalid n                      notDouble   0

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I was having the same problem  finally figured out what to do    include  lt stdio h gt   include  lt conio h gt   int main          int x      float check      reprocess      printf   enter a integer number         scanf    f    amp check       x check      if  x  check      printf   nYour number is  d   x       else                 printf   nThis is not an integer number  please insert an integer  n n             goto reprocess             getch        return 0

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I found a way to check whether the input given is an integer or not using atoi   function    Read the input as a string  and use atoi   function to convert the string in to an integer   atoi   function returns the integer number if the input string contains integer  else it will return 0  You can check the return value of the atoi   function to know whether the input given is an integer or not   There are lot more functions to convert a string into long  double etc   Check the standard library  stdlib h  for more   Note   It works only for non-zero numbers    include lt stdio h gt   include lt stdlib h gt   int main         char  string      int number       printf  Enter a number          string   scanf   s   string        number   atoi string        if number    0          printf  The number is  d n   number       else         printf  Not a number     n        return 0

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Try this      include  lt stdio h gt   int main  void        float a      int q       printf   nInsert number t        scanf   f   amp a        q  int a        q       if  q - a     1          printf   nThe number is not an integer n n        else         printf   nThe number is an integer n n         return 0

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num will always contain an integer because it s an int  The real problem with your code is that you don t check the scanf return value  scanf returns the number of successfully read items  so in this case it must return 1 for valid values  If not  an invalid integer value was entered and the num variable did probably not get changed  i e  still has an arbitrary value because you didn t initialize it    As of your comment  you only want to allow the user to enter an integer followed by the enter key  Unfortunately  this can t be simply achieved by scanf   d n    but here s a trick to do it   int num  char term  if scanf   d c    amp num   amp term     2    term      n       printf  failure n    else     printf  valid integer followed by enter key n

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Just check is your number has any difference with float version of it  or not  float num   scanf  quot  f quot   amp num    if num     int num        printf  quot it s not an integer quot        return

[c] Check if input is integer type in C

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