C Compare char array with string

Question

I m trying to compare a character array against a string like so   const char  var1        var1   getenv  myEnvVar     if var1     dev           do stuff     This if statement never validates as true    when I output var1 it is  dev   I was thinking maybe it has something to do with a null terminated string  but the strlen of  dev  and var1 are equal    I also thought maybe var1     dev  was comparing  dev  against the memory location of var1 instead of the value   var1     dev  results in an error     tried many things  probably a simple solution for the saavy c   developer  I havent coded c   in a looong time    edit  we ve tried  if strcmp var1   dev      0    and   if strncmp var1   dev   3     0    Thanks  edit  After testing at home I m just going to suggest my co-worker changes the datatype to a string  I believe he was comparing a char array of a large size against a string  I put together a program that outputs sizeof  strlen  etc to help us work through it  Thanks to everyone for the help

User · Accepted Answer

Use strcmp   to compare the contents of strings   if  strcmp var1   dev      0        Explanation  in C  a string is a pointer to a memory location which contains bytes  Comparing a char  to a char  using the equality operator won t work as expected  because you are comparing the memory locations of the strings rather than their byte contents  A function such as strcmp   will iterate through both strings  checking their bytes to see if they are equal  strcmp   will return 0 if they are equal  and a non-zero value if they differ  For more details  see the manpage

User · Answer

There is more stable function  also gets rid of string folding      Add to C   source bool string equal  const char  arg0  const char  arg1                  This function wraps string comparison with string pointers         and also works around  string folding   as I said          Converts pointers to std  string        for make use of string equality operator              Parameters use  const  for prevent possible object corruption              std  string var0    std  string  arg0      std  string var1    std  string  arg1      if  var0    var1                return true            else               return false            And add declaration to header     Parameters use  const  for prevent possible object corruption  bool string equal  const char  arg0  const char  arg1     For usage  just place an  string equal  call as condition of if  or ternary  statement block   if  string equal  var1   dev             It is equal  do what needed here    else          It is not equal  do what needed here  optional       Source  sinatramultimedia fl32 codec  it s written by myself

User · Answer

In this code you are not comparing string values  you are comparing pointer values   If you want to compare string values you need to use a string comparison function such as strcmp   if   0    strcmp var1   dev

User · Answer

You re not working with strings  You re working with pointers  var1 is a char pointer  const char    It is not a string  If it is null-terminated  then certain C functions will treat it as a string  but it is fundamentally just a pointer   So when you compare it to a char array  the array decays to a pointer as well  and the compiler then tries to find an operator     const char   const char     Such an operator does exist  It takes two pointers and returns true if they point to the same address  So the compiler invokes that  and your code breaks   IF you want to do string comparisons  you have to tell the compiler that you want to deal with strings  not pointers   The C way of doing this is to use the strcmp function   strcmp var1   dev      This will return zero if the two strings are equal   It will return a value greater than zero if the left-hand side is lexicographically greater than the right hand side  and a value less than zero otherwise    So to compare for equality you need to do one of these   if   strcmp var1   dev         if  strcmp var1   dev      0          However  C   has a very useful string class  If we use that your code becomes a fair bit simpler  Of course we could create strings from both arguments  but we only need to do it with one of them   std  string var1   getenv  myEnvVar     if var1     dev           do stuff     Now the compiler encounters a comparison between string and char pointer  It can handle that  because a char pointer can be implicitly converted to a string  yielding a string string comparison  And those behave exactly as you d expect

User · Answer

dev  is not a string it is a const char   like var1  Thus you are indeed comparing the memory adresses  Being that var1 is a char pointer   var1 is a single char  the first character of the pointed to character sequence to be precise   You can t compare a char against a char pointer  which is why that did not work   Being that this is tagged as c    it would be sensible to use std  string instead of char pointers  which would make    work as expected   You would just need to do const std  string var1 instead of const char  var1

User · Answer

your thinking about this program below    include  lt stdio h gt   include  lt string h gt   int main      char str   5       R2D2     C3PO     R2A6     int n  puts   Looking for R2 astromech droids       for  n 0   n lt 3   n    if  strncmp  str n   R2xx  2     0      printf   found  s n  str n      return 0      outputs       Looking for R2 astromech droids      found R2D2   found R2A6   when you should be thinking about inputting something into an array  amp  then use strcmp functions like the program above     check out a modified program below    include  lt iostream gt   include lt cctype gt   include  lt string h gt   include  lt string gt  using namespace std   int main     int Students 2  int Projects 3  Avg2 0  Sum2 0  SumT2 0  AvgT2 0  i 0  j 0  int Grades Students  Projects    for int j 0  j lt  Projects-1  j       for int i 0  i lt  Students  i       cout  lt  lt  Please give grade of student   lt  lt  j  lt  lt  in project   lt  lt  i   lt  lt          cin  gt  gt  Grades j  i          Sum2   Sum2   Grades i  j        Avg2   Sum2 Students    SumT2   SumT2   Avg2  AvgT2   SumT2 Projects  cout  lt  lt   avg is       lt  lt  AvgT2  lt  lt    and sum      lt  lt  SumT2  lt  lt       return 0      change to string except it only reads 1 input and throws the rest out  maybe need two for loops and two pointers    include  lt cstring gt   include  lt iostream gt   include  lt string gt   include  lt stdio h gt  using namespace std  int main     char name 100     string userInput 26   int i 0  n 0  m 0  cout lt  lt  your name     cin gt  gt name  cout lt  lt  Hello   lt  lt name lt  lt  endl   char  ptr name  for  i   0  i  lt  20  i      cout lt  lt i lt  lt     lt  lt ptr i  lt  lt     lt  lt  int ptr i  lt  lt endl       int length   0  while name length       0     length                          for n 0  n lt 4  n                                                  if  strncmp ptr   snit   4     0                                            cout  lt  lt   you found the snitch       lt  lt         ptr i                                                   cout lt  lt name  lt  lt  is  lt  lt length lt  lt  chars long

[c++] C++ Compare char array with string

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