How to check if C string is empty

Question

I m writing a very small program in C that needs to check if a certain string is empty  For the sake of this question  I ve simplified my code    include  lt stdio h gt   include  lt string gt   int main       char url 63       0      do       printf  Enter a URL          scanf   s   url       printf   s   url       while    what should I put in here         return 0       I want the program to stop looping if the user just presses enter without entering anything

User · Answer

strlen url   Returns the length of the string  It counts all characters until a null-byte is found  In your case  check it against 0   Or just check it manually with    url      0

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It is very simple  check for string empty condition in while condition   You can use strlen function to check for the string length   include lt stdio h gt   include  lt string h gt      int main         char url 63       0        do               printf  quot Enter a URL   quot            scanf  quot  s quot   url           printf  quot  s quot   url         while  strlen url  lt  0       return 0       check first character is   0   include  lt stdio h gt       include  lt string h gt   int main                 char url 63       0         do               printf  quot Enter a URL   quot            scanf  quot  s quot   url           printf  quot  s quot   url         while  url 0     0         return 0        For your reference  C arrays  https   www javatpoint com c-array https   scholarsoul com arrays-in-c  C strings  https   www programiz com c-programming c-strings https   scholarsoul com string-in-c  https   en wikipedia org wiki C string handling

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Typically speaking  you re going to have a hard time getting an empty string here  considering  s ignores white space  spaces  tabs  newlines     but regardless  scanf   actually returns the number of successful matches      From the man page       the number of input items successfully matched and assigned  which can be fewer than provided for  or even zero in the event of an early matching failure    so if somehow they managed to get by with an empty string  ctrl z for example  you can just check the return result   int count   0  do           count   scanf   62s   url       You should check return values and limit the                                    input length         while  count  lt   0    Note you have to check less than because in the example I gave  you d get back -1  again detailed in the man page      The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs  EOF is also returned if a read error occurs  in which case the error indicator for the stream  see ferror 3   is set  and errno is set indicate the error

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You can try like this -  if  string 0       0         In your case it can be like -  do            while  url 0       0

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The shortest way to do that would be   do          Something   while   url     Basically   url will return the char at the first position in the array  since C strings are null-terminated  if the string is empty  its first position will be the character   0   whose ASCII value is 0  since C logical statements treat every zero value as false  this loop will keep going while the first position of the string is non-null  that is  while the string is not empty   Recommended readings if you want to understand this better    C strings  https   www tutorialspoint com cprogramming c strings htm C arrays  https   www tutorialspoint com cprogramming c arrays htm Relation between arrays and pointers  https   www programiz com c-programming c-pointers-arrays Logical operators  https   www tutorialspoint com cprogramming c logical operators htm

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With strtok    it can be done in just one line   if  strtok s    t    NULL    For example    include  lt stdio h gt   include  lt string h gt   int is whitespace char  s        if  strtok s    t    NULL            return 1        else           return 0           void demo void        char s1 128       char s2 128       strcpy s1     abc   t         strcpy s2       t           printf  s1      s   n   s1       printf  s2      s   n   s2       printf  is whitespace s1   d n  is whitespace s1        printf  is whitespace s2   d n  is whitespace s2       int main         char url 63       0        do           printf  Enter a URL              scanf   s   url           printf  url   s  n   url         while  is whitespace url        return 0

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You can check the return value from scanf  This code will just sit there until it receives a string     int a   do        other code   a   scanf   s   url      while  a  lt   0

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Since C-style strings are always terminated with the null character   0   you can check whether the string is empty by writing  do            while  url 0       0      Alternatively  you could use the strcmp function  which is overkill but might be easier to read   do            while  strcmp url          Note that strcmp returns a nonzero value if the strings are different and 0 if they re the same  so this loop continues to loop until the string is nonempty   Hope this helps

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If you want to check if a string is empty   if  str 0       0            your code here

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First replace the scanf   with fgets        do       if   fgets url  sizeof url  stdin      error                     while   url      n

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If the first character happens to be   0   then you have an empty string   This is what you should do   do                   Resetting first character before getting input             url 0      0           code   while  url 0       0

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I ve written down this macro   define IS EMPTY STR X     1    sizeof X 0      1    type check    amp  amp    X 0    content check      so it would be  while    IS EMPTY STR url      The benefit in this macro it that it s type-safe  You ll get a compilation error if put in something other than a pointer to char

[c] How to check if C string is empty

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