String Padding in C

Question

I wrote this function that s supposed to do StringPadRight  Hello   10   0   -   Hello00000    char  StringPadRight char  string  int padded len  char  pad        int len    int  strlen string       if  len  gt   padded len            return string            int i      for  i   0  i  lt  padded len - len  i              strcat string  pad             return string      It works but has some weird side effects    some of the other variables get changed   How can I fix this

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include lt stdio h gt   include  lt string h gt    void padLeft int length  char pad  char  inStr char  outStr        int minLength   length   sizeof char       if  minLength  lt  sizeof outStr             return             int padLen   length - strlen inStr       padLen   padLen  lt  0   0   padLen       memset outStr  0  sizeof outStr        memset outStr  pad padLen       memcpy outStr padLen  inStr  minLength - padLen

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include  lt iostream gt   include lt stdio h gt   include lt stdlib h gt   include lt string h gt   using namespace std   int main            your code goes here     int pi length 11    Total length      char  str1      const char  padding  0000000000000000000000000000000000000000       const char  myString  Monkey        int padLen   pi length - strlen myString     length of padding to apply      if padLen  lt  0  padLen   0          str1   char   malloc 100 sizeof char         sprintf str1      s s   padLen  padLen  padding  myString        printf   s -- gt   d  n  str1 strlen str1         return 0

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It might be helpful to know that printf does padding for you  using  -10s as the format string will pad the input right in a field 10 characters long  printf    -10s     Hello      will output   Hello         In this case the - symbol means  Left align   the 10 means  Ten characters in field  and the s means you are aligning a string   Printf style formatting is available in many languages and has plenty of references on the web  Here is one of many pages explaining the formatting flags   As usual WikiPedia s printf page is of help too  mostly a history lesson of how widely printf has spread

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Oh okay  makes sense   So I did this       char foo 10     hello       char padded 16       strcpy padded  foo       printf   s   StringPadRight padded  15           Thanks

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The argument you passed  Hello  is on the constant data area  Unless you ve allocated enough memory to char   string  it s overrunning to other variables   char buffer 1024   memset buffer  0  sizeof buffer    strncpy buffer   Hello   sizeof buffer    StringPadRight buffer  10   0      Edit  Corrected from stack to constant data area

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The argument you passed  Hello  is on the constant data area  Unless you ve allocated enough memory to char   string  it s overrunning to other variables   char buffer 1024   memset buffer  0  sizeof buffer    strncpy buffer   Hello   sizeof buffer    StringPadRight buffer  10   0      Edit  Corrected from stack to constant data area

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You must make sure that the input string has enough space to hold all the padding characters  Try this   char hello 11     Hello   StringPadRight hello  10   0      Note that I allocated 11 bytes for the hello string to account for the null terminator at the end

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The function itself looks fine to me  The problem could be that you aren t allocating enough space for your string to pad that many characters onto it  You could avoid this problem in the future by passing a size of string argument to the function and make sure you don t pad the string when the length is about to be greater than the size

User · Answer

Oh okay  makes sense   So I did this       char foo 10     hello       char padded 16       strcpy padded  foo       printf   s   StringPadRight padded  15           Thanks

User · Answer

The argument you passed  Hello  is on the constant data area  Unless you ve allocated enough memory to char   string  it s overrunning to other variables   char buffer 1024   memset buffer  0  sizeof buffer    strncpy buffer   Hello   sizeof buffer    StringPadRight buffer  10   0      Edit  Corrected from stack to constant data area

User · Answer

It might be helpful to know that printf does padding for you  using  -10s as the format string will pad the input right in a field 10 characters long  printf    -10s     Hello      will output   Hello         In this case the - symbol means  Left align   the 10 means  Ten characters in field  and the s means you are aligning a string   Printf style formatting is available in many languages and has plenty of references on the web  Here is one of many pages explaining the formatting flags   As usual WikiPedia s printf page is of help too  mostly a history lesson of how widely printf has spread

User · Answer

It might be helpful to know that printf does padding for you  using  -10s as the format string will pad the input right in a field 10 characters long  printf    -10s     Hello      will output   Hello         In this case the - symbol means  Left align   the 10 means  Ten characters in field  and the s means you are aligning a string   Printf style formatting is available in many languages and has plenty of references on the web  Here is one of many pages explaining the formatting flags   As usual WikiPedia s printf page is of help too  mostly a history lesson of how widely printf has spread

User · Answer

Oh okay  makes sense   So I did this       char foo 10     hello       char padded 16       strcpy padded  foo       printf   s   StringPadRight padded  15           Thanks

User · Answer

include  lt stdio h gt   include  lt string h gt   int main void        char buf BUFSIZ      0        char str      Hello       char fill            int width   20     or whatever you need but less than BUFSIZ            printf   s s n    char  memset buf  fill  width - strlen str    str        return 0      Output     gcc -Wall -ansi -pedantic padding c     a out                 Hello

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You must make sure that the input string has enough space to hold all the padding characters  Try this   char hello 11     Hello   StringPadRight hello  10   0      Note that I allocated 11 bytes for the hello string to account for the null terminator at the end

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include  lt iostream gt   include lt stdio h gt   include lt stdlib h gt   include lt string h gt   using namespace std   int main            your code goes here     int pi length 11    Total length      char  str1      const char  padding  0000000000000000000000000000000000000000       const char  myString  Monkey        int padLen   pi length - strlen myString     length of padding to apply      if padLen  lt  0  padLen   0          str1   char   malloc 100 sizeof char         sprintf str1      s s   padLen  padLen  padding  myString        printf   s -- gt   d  n  str1 strlen str1         return 0

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One thing that s definitely wrong in the function which forms the original question in this thread  which I haven t seen anyone mention  is that it is concatenating extra characters onto the end of the string literal that has been passed in as a parameter  This will give unpredictable results  In the example call of the function  the string literal  Hello  will be hard-coded into the program  so presumably concatenating onto the end of it will dangerously write over code  If you want to return a string which is bigger than the original then you need to make sure you allocate it dynamically and then delete it in the calling code when you re done

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include lt stdio h gt   include  lt string h gt    void padLeft int length  char pad  char  inStr char  outStr        int minLength   length   sizeof char       if  minLength  lt  sizeof outStr             return             int padLen   length - strlen inStr       padLen   padLen  lt  0   0   padLen       memset outStr  0  sizeof outStr        memset outStr  pad padLen       memcpy outStr padLen  inStr  minLength - padLen

User · Answer

include  lt stdio h gt   include  lt string h gt   int main void        char buf BUFSIZ      0        char str      Hello       char fill            int width   20     or whatever you need but less than BUFSIZ            printf   s s n    char  memset buf  fill  width - strlen str    str        return 0      Output     gcc -Wall -ansi -pedantic padding c     a out                 Hello

User · Answer

It might be helpful to know that printf does padding for you  using  -10s as the format string will pad the input right in a field 10 characters long  printf    -10s     Hello      will output   Hello         In this case the - symbol means  Left align   the 10 means  Ten characters in field  and the s means you are aligning a string   Printf style formatting is available in many languages and has plenty of references on the web  Here is one of many pages explaining the formatting flags   As usual WikiPedia s printf page is of help too  mostly a history lesson of how widely printf has spread

User · Answer

The function itself looks fine to me  The problem could be that you aren t allocating enough space for your string to pad that many characters onto it  You could avoid this problem in the future by passing a size of string argument to the function and make sure you don t pad the string when the length is about to be greater than the size

User · Answer

You must make sure that the input string has enough space to hold all the padding characters  Try this   char hello 11     Hello   StringPadRight hello  10   0      Note that I allocated 11 bytes for the hello string to account for the null terminator at the end

User · Answer

The function itself looks fine to me  The problem could be that you aren t allocating enough space for your string to pad that many characters onto it  You could avoid this problem in the future by passing a size of string argument to the function and make sure you don t pad the string when the length is about to be greater than the size

User · Answer

For  C  there is alternative  more complex  use of  s printf that does not require any malloc   or pre-formatting  when custom padding is desired   The trick is to use     length specifiers  min and max  for  s  plus a string filled with your padding character to the maximum potential length     int targetStrLen   10               Target output length   const char  myString  Monkey        String for output  const char  padding                                                           int padLen   targetStrLen - strlen myString      Calc Padding length if padLen  lt  0  padLen   0        Avoid negative length  printf       s s    padLen  padLen  padding  myString       LEFT Padding  printf    s    s    myString  padLen  padLen  padding       RIGHT Padding    The      s  can be placed before OR after your   s   depending desire for LEFT or RIGHT padding           Monkey     lt -- Left padded        s s        Monkey         lt -- Right padded    s    s    I found that the PHP printf  here  does support the ability to give a custom padding character  using the single quote     followed by your custom padding character  within the  s format        printf      10s  n     s      use the custom padding character     produces             monkey

User · Answer

Oh okay  makes sense   So I did this       char foo 10     hello       char padded 16       strcpy padded  foo       printf   s   StringPadRight padded  15           Thanks

User · Answer

The argument you passed  Hello  is on the constant data area  Unless you ve allocated enough memory to char   string  it s overrunning to other variables   char buffer 1024   memset buffer  0  sizeof buffer    strncpy buffer   Hello   sizeof buffer    StringPadRight buffer  10   0      Edit  Corrected from stack to constant data area

User · Answer

For  C  there is alternative  more complex  use of  s printf that does not require any malloc   or pre-formatting  when custom padding is desired   The trick is to use     length specifiers  min and max  for  s  plus a string filled with your padding character to the maximum potential length     int targetStrLen   10               Target output length   const char  myString  Monkey        String for output  const char  padding                                                           int padLen   targetStrLen - strlen myString      Calc Padding length if padLen  lt  0  padLen   0        Avoid negative length  printf       s s    padLen  padLen  padding  myString       LEFT Padding  printf    s    s    myString  padLen  padLen  padding       RIGHT Padding    The      s  can be placed before OR after your   s   depending desire for LEFT or RIGHT padding           Monkey     lt -- Left padded        s s        Monkey         lt -- Right padded    s    s    I found that the PHP printf  here  does support the ability to give a custom padding character  using the single quote     followed by your custom padding character  within the  s format        printf      10s  n     s      use the custom padding character     produces             monkey

User · Answer

One thing that s definitely wrong in the function which forms the original question in this thread  which I haven t seen anyone mention  is that it is concatenating extra characters onto the end of the string literal that has been passed in as a parameter  This will give unpredictable results  In the example call of the function  the string literal  Hello  will be hard-coded into the program  so presumably concatenating onto the end of it will dangerously write over code  If you want to return a string which is bigger than the original then you need to make sure you allocate it dynamically and then delete it in the calling code when you re done

User · Answer

The function itself looks fine to me  The problem could be that you aren t allocating enough space for your string to pad that many characters onto it  You could avoid this problem in the future by passing a size of string argument to the function and make sure you don t pad the string when the length is about to be greater than the size

User · Answer

You must make sure that the input string has enough space to hold all the padding characters  Try this   char hello 11     Hello   StringPadRight hello  10   0      Note that I allocated 11 bytes for the hello string to account for the null terminator at the end

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