how to check if the input is a number or not in C

Question

In the main function of C   void main int argc  char   argv          do something here     In the command line  we will type any number for example 1 or 2 as input  but it will be treated as char array for the parameter of argv  but how to make sure the input is a number  in case people typed hello or c

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Using scanf is very easy  this is an example   if  scanf  quot  d quot    amp val a tester     1               it s an integer

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Using fairly simple code   int i  int value  int n  char ch      Skip i  0 because that will be the program name    for  i 1  i lt argc  i          n   sscanf argv i     d c    amp value   amp ch        if  n    1               sscanf didn t find a number to convert  so it wasn t a number              else              It was

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You can use a function like strtol   which will convert a character array to a long   It has a parameter which is a way to detect the first character that didn t convert properly  If this is anything other than the end of the string  then you have a problem   See the following program for an example    include  lt stdio h gt   include  lt stdlib h gt   int main  int argc  char  argv          int i      long val      char  next          Process each argument given       for  i   1  i  lt  argc  i                 Get value with failure detection           val   strtol  argv i    amp next  10               Check for empty string and characters left after conversion           if   next    argv i        next      0                  printf     s  is not valid n   argv i              else               printf     s  gives  ld n   argv i   val                        return 0      Running this  you can see it in operation   pax gt  testprog hello    42 12 2 77x   hello  is not valid    is not valid  42  gives 42  12 2  is not valid  77x  is not valid

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A self-made solution   bool isNumeric const char  str         while  str      0                 if  str  lt   0      str  gt   9               return false          str              return true      Note that this solution should not be used in production-code  because it has severe limitations  But I like it for understanding C-Strings and ASCII

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I was struggling with this for awhile  so I thought I d just add my two cents   1  Create a separate function to check if an fgets input consists entirely of numbers    int integerCheck    char myInput 4   fgets myInput  sizeof myInput   stdin       int counter   0      int i      for  i 0  myInput i      0   i             if  isalpha myInput i      0               counter                if counter  gt  0                   printf  Input error  Please try again   n                     return main                                       return atoi myInput        The above starts a loop through every unit of an fgets input until the ending NULL value  If it comes across a letter or an operator  it adds  1  to the int  counter  which is initially set to 0  Once the counter becomes greater than 0  the nested if statement instructs the loop to print an error message  amp  then restart the program  When the loops completes  if int  counter  is still the value of 0  it returns the initially inputted integer to be used in the main function       2  the main function would be   int main void   unsigned int numberOne  unsigned int numberTwo  numberOne   integerCheck    numberTwo   integerCheck    return numberOne numberTwo       Assuming both integers are inputted correctly  the example provided will yield the result of int  numberOne  multiplied by int  numberTwo   The program will repeat for however long it takes to get two properly inputted integers

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if  sscanf command level 2     f c    amp check f   amp check c   1            is num false    else           is num true        if sscanf command level 2    f   amp check f     1         is num false      how about this

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The sscanf   solution is better in terms of code lines  My answer here is a user-build function that does almost the same as sscanf    Stores the converted number in a pointer and returns a value called  val   If val comes out as zero  then the input is in unsupported format  hence conversion failed  Hence  use the pointer value only when val is non-zero   It works only if the input is in base-10 form    include  lt stdio h gt   include  lt string h gt  int CONVERT 3 double  Amt        char number 100           Input the Data     printf   nPlease enter the amount  integer only            fgets number sizeof number  stdin           Detection-Conversion begins     int iters   strlen number -2      int val   1      int pos      double Amount   0       Amt   0      for int i   0   i  lt   iters   i              switch i               case 0                  if number i        break                   if number i    -   val   2  break                   if number i        val   val   10  pos   0  break                   if number i    0   Amount   0  break                   if number i    1   Amount   1  break                   if number i    2   Amount   2  break                   if number i    3   Amount   3  break                   if number i    4   Amount   4  break                   if number i    5   Amount   5  break                   if number i    6   Amount   6  break                   if number i    7   Amount   7  break                   if number i    8   Amount   8  break                   if number i    9   Amount   9  break               default                  switch number i                        case                              val   val   10                          pos   i                          break                      case  0                           Amount    Amount  10                          break                      case  1                           Amount    Amount  10   1                          break                      case  2                           Amount    Amount  10   2                          break                      case  3                           Amount    Amount  10   3                          break                      case  4                           Amount    Amount  10   4                          break                      case  5                           Amount    Amount  10   5                          break                      case  6                           Amount    Amount  10   6                          break                      case  7                           Amount    Amount  10   7                          break                      case  8                           Amount    Amount  10   8                          break                      case  9                           Amount    Amount  10   9                          break                      default                          val   0                                      if    val     val gt 20    val   0  break     val    0            if val  1   Amt   Amount       if val  2   Amt   0 - Amount       if val  11           int exp   iters - pos          long den   1          for    exp--                  den   den 10                     Amt   Amount den            if val  12           int exp   iters - pos          long den   1          for    exp--                  den   den 10                     Amt   0 -  Amount den              return val      int main void        double AM   0      int c   CONVERT 3  amp AM       printf   n n d     lf n  c AM        return 0

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Another way of doing it is by using isdigit function  Below is the code for it     include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   include  lt ctype h gt   define MAXINPUT 100 int main         char input MAXINPUT            int length i        scanf    s   input       length   strlen  input       for  i 0 i lt length  i            if   isdigit input i                          printf   Entered input is not a number n                exit 1                 printf   Given input is a number n

[c] how to check if the input is a number or not in C?

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