How to check if a string is a number

Question

I want to check if a string is a number with this code  I must check that all the chars in the string are integer  but the while returns always isDigit   1  I don t know why that if doesn t work   char tmp 16   scanf   s   tmp    int isDigit   0  int j 0  while j lt strlen tmp   amp  amp  isDigit    0     if tmp j   gt  57  amp  amp  tmp j   lt  48      isDigit   0    else     isDigit   1    j

User · Answer

I need to do the same thing for a project I am currently working on  Here is how I solved things      Prompt user for input    printf  Enter a number          Read user input    char input 255     Of course  you can choose a different input size fgets input  sizeof input   stdin       Strip trailing newline    size t ln   strlen input  - 1  if  input ln       n    input ln      0       Ensure that input is a number    for  size t i   0  i  lt  ln  i         if   isdigit input i              fprintf stderr    c is not a number  Try again  n   input i            getInput      Assuming this is the name of the function you are using         return

User · Answer

if   strlen str     strlen  itoa atoi str               its an integer     As atoi converts string to number skipping letters other than digits  if there was no other than digits its string length has to be the same as the original  This solution is better than innumber   if the check is for integer

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The problem is that the result of your code  quot isDigit quot  is reflecting only the last digit test  As far as I understand your qustion  you want to return isDigit   0 whenever you have any character that is not a number in your string  Following your logic  you should code it like this  char tmp 16   scanf  quot  s quot   tmp    int isDigit   0  int j 0  isDigit   1      Initialised it here    while j lt strlen tmp   amp  amp  isDigit    0     if tmp j   gt  57    tmp j   lt  48     changed it to OR           isDigit   0    j       To get a more understandable code  I d also change the test  if tmp j   gt  57    tmp j   lt  48    to the following  if tmp j   gt   9     tmp j   lt   0

User · Answer

Forget about ASCII code checks  use isdigit or isnumber  see man isnumber   The first function checks whether the character is 0   9  the second one also accepts various other number characters depending on the current locale   There may even be better functions to do the check     the important lesson is that this is a  bit more complex than it looks  because the precise definition of a    number string    depends on the particular locale and the string encoding

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rewrite the whole function as below  bool IsValidNumber char   string       for int i   0  i  lt  strlen  string    i                  ASCII value of 0   48  9   57  So if value is outside of numeric range then fail         Checking for negative sign  quot - quot  could be added  ASCII value 45        if  string i   lt  48    string i   gt  57           return FALSE           return TRUE

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include  lt stdio h gt   include  lt string h gt  char isNumber char  text        int j      j   strlen text       while j--                if text j   gt  47  amp  amp  text j   lt  58              continue           return 0            return 1    int main        char tmp 16       scanf   s   tmp        if isNumber tmp           return printf  is a number n         return printf  is not a number n        You can also check its stringfied value  which could also work with non Ascii  char isNumber char  text        int j      j   strlen text       while j--                if text j   gt    0   amp  amp  text j   lt    9               continue           return 0            return 1

User · Answer

More obvious and simple  thread safe example    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   int main int argc  char   argv        if  argc  lt  2           printf   Dont  forget to pass arguments  n            return -1              printf   You have executed the program    s n   argv 0         for int i   1  i  lt  argc  i             if strcmp argv i   --some definite parameter      0               printf  You have passed some definite parameter as an argument  And it is    s    n  argv i                      else if strspn argv i    0123456789      strlen argv i                  size t big digit   0              sscanf argv i     zu  c   amp big digit               printf  Your  d nd argument contains only digits  and it is a number    zu    n  i big digit                     else if strspn argv i    0123456789abcdefghijklmnopqrstuvwxyz        strlen argv i                          printf   s - this string might contain digits  small letters and path symbols  It could be used for passing a file name or a path  for example  n  argv i                      else if strspn argv i    ABCDEFGHIJKLMNOPQRSTUVWXYZ      strlen argv i                          printf  The string    s   contains only capital letters  n  argv i

User · Answer

if tmp j   gt    0   amp  amp  tmp j   lt    9      should do the trick

User · Answer

In this part of your code   if tmp j   gt  57  amp  amp  tmp j   lt  48    isDigit   0  else   isDigit   1    Your if condition will always be false  resulting in isDigit always being set to 1   You are probably wanting   if tmp j   gt   9     tmp j   lt   0     isDigit   0  else   isDigit   1    But  this can be simplified to   isDigit   isdigit tmp j      However  the logic of your loop seems kind of misguided   int isDigit   0  int j 0  while j lt strlen tmp   amp  amp  isDigit    0     isDigit   isdigit tmp j      j        As tmp is not a constant  it is uncertain whether the compiler will optimize the length calculation out of each iteration   As  andlrc suggests in a comment  you can instead just check for digits  since the terminating NUL will fail the check anyway   while  isdigit tmp j      j

User · Answer

Your condition says if X is greater than 57 AND smaller than 48  X cannot be both greater than 57 and smaller than 48 at the same time   if tmp j   gt  57  amp  amp  tmp j   lt  48    It should be if X is greater than 57 OR smaller than 48   if tmp j   gt  57    tmp j   lt  48

[c] How to check if a string is a number?

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