Reversing a string in C

Question

I have developed a reverse-string program  I am wondering if there is a better way to do this  and if my code has any potential problems  I am looking to practice some advanced features of C    char  reverse string char  str        char temp      size t len   strlen str  - 1      size t i      size t k   len       for i   0  i  lt  len  i                  temp   str k           str k    str i           str i    temp          k--              As 2 characters are changing place for each cycle of the loop            only traverse half the array of characters            if k     len   2                         break

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That s a good question ant2009  You can use a standalone function to reverse the string  The code is      include  lt stdio h gt   define MAX CHARACTERS 99  int main  void    int strlen  char   str     int main         char  str  MAX CHARACTERS        char  new string  MAX CHARACTERS        int i  j       printf   enter string           gets   str         for  i   0  j     strlen   str   - 1    i  lt  strlen   str    j  gt  -1  i    j--              str  i      new string  j              printf   Reverse string is   s    new string         return   0       int strlen  char   str           int count      for  int i   0    str  i        0   i                  count            return   count

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Author  Siken Dongol     include  lt stdio h gt   int strLength char  input        int i   0      while input i       0        return --i     int main         char input      Siken Man Singh Dongol        int len   strLength input       char output len        int index   0      while len  gt   0            output index      input --len              printf   s n  input       printf   s n  output       return 0

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include  lt stdio h gt   include  lt string h gt   int main          char  data    hello world       int length strlen data       char bytes length       int n 0      while n lt  length               bytes n    data length-n-1          n              printf   s n   bytes       return 0

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This complete program shows how I would do it  Keep in mind I was writing C when most of you whippersnappers were a glint in your mothers eyes so it s old-school  do-the-job  long-var-names-are-for-wimps  Fix that if you wish  I m more interested in the correctness of the code   It handles NULLs  empty strings and all string sizes  I haven t tested it with strings of maximum size  max size t   but it should work  and if you re handling strings that big  you re insane anyway  -    include  lt stdio h gt   include  lt string h gt   char  revStr  char  str        char tmp   src   dst      size t len      if  str    NULL                len   strlen  str           if  len  gt  1                src   str              dst   src   len - 1              while  src  lt  dst                    tmp    src                   src      dst                   dst--   tmp                                    return str     char  str           a    ab    abc    abcd    abcde     int main int argc  char  argv          int i      char s 10000       for  i 0  i  lt  sizeof str  sizeof str 0    i              strcpy  s  str i            printf     s  - gt    s  n   str i   revStr s              return 0      The output of that is      - gt      a  - gt   a   ab  - gt   ba   abc  - gt   cba   abcd  - gt   dcba   abcde  - gt   edcba

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My two cents      Reverses n characters of a string and adds a   0  at the end    void strnrev  char  txt  size t len        size t idx      for  idx   len  gt  gt  1  idx  gt  0  idx--            txt len    txt idx - 1           txt idx - 1    txt len - idx           txt len - idx    txt len             txt len      0         Reverses a null-terminated string    void strrev  char  txt        size t len   0      while  txt len          strnrev txt  --len       Test  1     strrev     char string      Hello world    strrev string   printf   s n   string      Displays   dlrow olleH    Test  2     strnrev     char string      Hello world    strnrev string  5   printf   s n   string      Displays  olleH

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I don t see a return statement  and you are changing the input string  which may be a problem for the programmer  You may want the input string to be immutable   Also  this may be picky  but len 2 should be calculated only one time  IMO   Other than that  it will work  as long as you take care of the problem cases mentioned by rossfabricant

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void reverse char  s      char  end temp    end   s    while  end      0        end          end--     end points to last letter now   for  s lt end s   end--       temp    end       end    s       s   temp

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You can actually do something like this   include  lt string h gt   void reverse char      int main void    char name 7     quot walter quot    reverse name    printf  quot  s quot   name      void reverse char  s      size t len   strlen s     char  a   s    char  b    amp s  int len - 1     char tmp    for    a  lt  b    a  --b        tmp    a       a    b       b   tmp

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Since you say you want to get fancy  perhaps you ll want to exchange your characters using an XOR swap

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easy and simple code xD  void strrev  char s       int i  int dim   strlen  s   char l   for  i   0  i  lt  dim   2  i          l   s i       s i    s dim-i-1       s dim-i-1    l

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Made a tiny program that accomplishes that   include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   int main         char str 8192     quot string quot      string     size t len   strlen str -1     get string length and reduce 1      while len 1  gt  0     Loop for every character on string               printf  quot  c quot  str len--       Print string reversed and reducing len by one           return 0     Quit program      Explanation  We take the length of the string  and then we start looping by the last position until we arrive to index 0 quit program

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Rather than breaking half-way through  you should simply shorten your loop   size t length   strlen str   size t i   for  i   0  i  lt   length   2   i          char temp   str length - i - 1       str length - i - 1    str i       str i    temp

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Does nobody use pointers anymore   void inplace rev  char   s       char t   e   s   strlen s     while   --e  gt  s     t    s  s    e  e t        EDIT  Sorry  just noticed the above XOR example

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Here is my shot which will handle all the cases  char  p   KDLAKDADKADAD  char p      lammdlamldaldladadada  also empty string    include lt stdio h gt   include lt string h gt enter code here  include lt stdlib h gt  char  string reverse char  p   int main              char  p     Deepak klkaldkaldkakdoroorerr           char  temp   string reverse p           printf   s   temp     char    string reverse  char  p              if  p      0                             printf  No charecters are present  n                    return 0                    int count   strlen p  1          int mid   strlen p  2          char  q     char   malloc count   sizeof char            if  q                             strcpy q p                   char  begin  end temp                  begin   q                   end   q strlen p -1                    int i   0                  while  i  lt  mid 2                                             temp    end                           end    begin                           begin   temp                          begin                            end--                          i                                      return q                    else                            printf  Memory Not allocated                       free q

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Just a rearrangement  and safety check  I also removed your non-used return type  I think this is a safe and clean as it gets    include  lt stdio h gt   include  lt string h gt   void reverse string char  str           skip null        if  str    0                return                skip empty string        if   str    0                return                get range        char  start   str      char  end   start   strlen str  - 1     -1 for  0        char temp          reverse        while  end  gt  start                   swap            temp    start           start    end           end   temp              move              start          --end            int main void        char s1      Reverse me        char s2      abc       char s3      ab       char s4      a       char s5              reverse string 0        reverse string s1       reverse string s2       reverse string s3       reverse string s4       reverse string s5        printf   s n   s1       printf   s n   s2       printf   s n   s3       printf   s n   s4       printf   s n   s5        return 0      Edited so that end will not point to a possibly bad memory location when strlen is 0

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If you want to practice advanced features of C  how about pointers  We can toss in macros and xor-swap for fun too    include  lt string h gt     for strlen       reverse the given null-terminated string in place void inplace reverse char   str      if  str          char   end   str   strlen str  - 1          swap the values in the two given variables        XXX  fails when a and b refer to same memory location     define XOR SWAP a b  do               a    b         b    a         a    b         while  0          walk inwards from both ends of the string          swapping until we get to the middle     while  str  lt  end              XOR SWAP  str   end         str          end--            undef XOR SWAP         A pointer  e g  char    read from right-to-left as a pointer to a char  is a data type in C that is used to refer to location in memory of another value   In this case  the location where a char is stored   We can dereference pointers by prefixing them with an    which gives us the value stored at that location   So the value stored at str is  str   We can do simple arithmetic with pointers   When we increment  or decrement  a pointer  we simply move it to refer to the next  or previous  memory location for that type of value   Incrementing pointers of different types may move the pointer by a different number of bytes because different values have different byte sizes in C   Here  we use one pointer to refer to the first unprocessed char of the string  str  and another to refer to the last  end   We swap their values   str and  end   and move the pointers inwards to the middle of the string   Once str  gt   end  either they both point to the same char  which means our original string had an odd length  and the middle char doesn t need to be reversed   or we ve processed everything   To do the swapping  I ve defined a macro   Macros are text substitution done by the C preprocessor   They are very different from functions  and it s important to know the difference   When you call a function  the function operates on a copy of the values you give it   When you call a macro  it simply does a textual substitution - so the arguments you give it are used directly   Since I only used the XOR SWAP macro once  it was probably overkill to define it  but it made more clear what I was doing   After the C preprocessor expands the macro  the while loop looks like this       while  str  lt  end              do    str     end   end     str   str     end    while  0         str          end--          Note that the macro arguments show up once for each time they re used in the  macro definition    This can be very useful - but can also break your code if used incorrectly   For example  if I had compressed the increment decrement instructions and the macro call into a single line  like        XOR SWAP  str     end--     Then this would expand to        do    str       end--   end--     str     str       end--    while  0     Which has triple the increment decrement operations  and doesn t actually do the swap it s supposed to do   While we re on the subject  you should know what xor     means   It s a basic arithmetic operation - like addition  subtraction  multiplication  division  except it s not usually taught in elementary school   It combines two integers bit by bit - like addition  but we don t care about the carry-overs   1 1   0  1 0   1  0 1   1  0 0   0   A well known trick is to use xor to swap two values   This works because of three basic properties of xor  x   0   x  x   x   0 and x   y   y   x for all values x and y   So say we have two variables a and b that are initially storing two values va and vb         initially       a    va      b    vb   a    b       now  a    va   vb   b    a       now  b    vb    va   vb                 va    vb   vb                 va   0                va   a    b       now  a     va   vb    va                 va   va    vb                0   vb                vb   So the values are swapped   This does have one bug - when a and b are the same variable         initially       a    va   a    a       now  a    va   va                0   a    a       now  a    0   0                0   a    a       now  a    0   0                0   Since we str  lt  end  this never happens in the above code  so we re okay   While we re concerned about correctness we should check our edge cases   The if  str  line should make sure we weren t given a NULL pointer for string   What about the empty string      Well strlen        0  so we ll initialize end as str - 1  which means that the while  str  lt  end  condition is never true  so we don t do anything   Which is correct   There s a bunch of C to explore   Have fun with it   Update  mmw brings up a good point  which is you do have to be slightly careful how you invoke this  as it does operate in-place    char stack string      This string is copied onto the stack     inplace reverse stack string     This works fine  since stack string is an array  whose contents are initialized to the given string constant   However   char   string literal    This string is part of the executable     inplace reverse string literal     Will cause your code to flame and die at runtime   That s because string literal merely points to the string that is stored as part of your executable - which is normally memory that you are not allowed to edit by the OS   In a happier world  your compiler would know this  and cough an error when you tried to compile  telling you that string literal needs to be of type char const   since you can t modify the contents   However  this is not the world my compiler lives in   There are some hacks you could try to make sure that some memory is on the stack or in the heap  and is therefore editable   but they re not necessarily portable  and it could be pretty ugly   However  I m more than happy to throw responsibility for this to the function invoker   I ve told them that this function does in place memory manipulation  it s their responsibility to give me an argument that allows that

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rev   int len   strlen str -1  for   int i  0  i lt  len 2   i               char t   str i           str i    str len-i           str len-i    t

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You could change your for loop declaration to make the code shorter   char  reverse string char  str        char temp      size t len   strlen str  - 1      size t stop   len 2      size t i k       for i   0  k   len  i  lt  stop  i    k--                temp   str k           str k    str i           str i    temp            return str

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You can try this pointer arithmetic   void revString char  s      char  e   s  while  e   e      e--    while e  gt  s    s     e   e     s   s       e--

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You can put your  len 2  test in the for loop   for i   0 k len-1   i  lt   len 2   i   k--            temp   str k           str k    str i           str i    temp

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The code looks unnecessarily complicated  Here is my version    void strrev char  str         size t len   strlen str       char buf len         for  size t i   0  i  lt  len  i               buf i    str len - 1 - i                 for  size t i   0  i  lt  len  i               str i    buf i

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Here is my shot  I avoid swapping just by using the standard strcpy pattern   char  string reverse char  dst  const char  src        if  src    NULL  return NULL       const char  src start   src      char  dst end   dst   strlen src        dst end     0        while    --dst end    src start                return dst      and here a running example

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include  lt stdio h gt   int main              char string 100       int i       printf  Enter a string  n        gets string       printf   n        for i strlen string -1 i gt -1 i--       printf   c  string i

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bool reverse string char  str        if str    NULL           return false            if strlen str   lt  2           return false                 char  first   str      char  last   str   strlen str  - 1     Minus 1 accounts for Index offset     char temp       do          temp    first           first    last           last   temp            while    first  lt  --last      Update Pointer Addresses and check for equality      return true     This solution is based on GManNickG s post with a few modifications  The initial logical statement may be dangerous if  str is not evaluated before the strlen operation  For a NULL ptr   This wasn t the case with my compiler  I thought I would add this code because its a nice example of a do-while loop

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Try this   reverse string NULL   reverse string

[c] Reversing a string in C

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