How can I convert a std string to int

Question

Just have a quick question  I ve looked around the internet quite a bit  and I ve found a few solutions but none of them have worked yet  Looking at converting a string to an int and I don t mean ASCII codes   For a quick run-down  we are passed in an equation as a string  We are to break it down  format it correctly and solve the linear equations  Now  in saying that  I m not able to convert a string to an int   I know that the string will be in either the format  -5  or  25  etc  so it s definitely an int  But how do we extract that from a string   One way I was thinking is running a for while loop through the string  check for a digit  extract all the digits after that and then look to see if there was a leading  -   if there is  multiply the int by -1   It seems a bit over complicated for such a small problem though  Any ideas

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atoi is a built-in function that converts a string to an integer  assuming that the string begins with an integer representation   http   www cplusplus com reference clibrary cstdlib atoi

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The possible options are described below   1  First option  sscanf         include  lt cstdio gt       include  lt string gt           int i          float f          double d          std  string str              string - gt  integer         if sscanf str c str      d    amp i     1                 error management             string - gt  float         if sscanf str c str      f    amp f     1                 error management             string - gt  double          if sscanf str c str      lf    amp d     1                 error management   This is an error  also shown by cppcheck  because  scanf without field width limits can crash with huge input data on some versions of libc   see here  and here    2  Second option  std  sto          include  lt iostream gt       include  lt string gt           int i          float f          double d          std  string str           try                  string - gt  integer             int i   std  stoi str                   string - gt  float             float f   std  stof str                   string - gt  double              double d   std  stod str             catch                        error management                This solution is short and elegant  but it is available only on on C  11 compliant compilers   3  Third option  sstreams       include  lt string gt       include  lt sstream gt           int i          float f          double d          std  string str              string - gt  integer         std  istringstream   str    gt  gt  i              string - gt  float         std  istringstream   str    gt  gt  f              string - gt  double          std  istringstream   str    gt  gt  d              error management      However  with this solution is hard to distinguish between bad input  see here    4  Fourth option  Boost s lexical cast       include  lt boost lexical cast hpp gt       include  lt string gt           std  string str           try               int i   boost  lexical cast lt int gt   str c str                 float f   boost  lexical cast lt int gt   str c str                 double d   boost  lexical cast lt int gt   str c str                   catch  boost  bad lexical cast const amp                         Error management             However  this is just a wrapper of sstream  and the documentation suggests to use sstream for better error management  see here    5  Fifth option  strto     This solution is very long  due to error management  and it is described here  Since no function returns a plain int  a conversion is needed in case of integer  see here for how this conversion can be achieved    6  Sixth option  Qt       include  lt QString gt       include  lt string gt           bool ok          std  string           int i   QString  fromStdString str  toInt  amp ok           if   ok                 Error management          float f   QString  fromStdString str  toFloat  amp ok           if   ok                 Error management           double d   QString  fromStdString str  toDouble  amp ok           if   ok         Error management        Conclusions  Summing up  the best solution is C  11 std  stoi   or  as a second option  the use of Qt libraries  All other solutions are discouraged or buggy

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From http   www cplusplus com reference string stoi      stoi example  include  lt iostream gt       std  cout  include  lt string gt         std  string  std  stoi  int main        std  string str dec    2001  A Space Odyssey     std  string str hex    40c3     std  string str bin    -10010110001     std  string str auto    0x7f      std  string  size type sz       alias of size t    int i dec   std  stoi  str dec  amp sz     int i hex   std  stoi  str hex nullptr 16     int i bin   std  stoi  str bin nullptr 2     int i auto   std  stoi  str auto nullptr 0      std  cout  lt  lt  str dec  lt  lt        lt  lt  i dec  lt  lt    and     lt  lt  str dec substr sz   lt  lt     n     std  cout  lt  lt  str hex  lt  lt        lt  lt  i hex  lt  lt    n     std  cout  lt  lt  str bin  lt  lt        lt  lt  i bin  lt  lt    n     std  cout  lt  lt  str auto  lt  lt        lt  lt  i auto  lt  lt    n      return 0      Output   2001  A Space Odyssey  2001 and    A Space Odyssey   40c3   16579  -10010110001  -1201  0x7f  127

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ll toll string a       ll ret 0      bool minus false      for auto i a           if i   -    minus true  continue            ret  10          ret   i- 0          if minus  ret  -1      return ret        ll is defined as   define ll long long int       usage  ll a   toll string  -1234

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Well  lot of answers  lot of possibilities  What I am missing here is some universal method that converts a string to different C   integral types  short  int  long  bool        I came up with following solution    include lt sstream gt   include lt exception gt   include lt string gt   include lt type traits gt   using namespace std   template lt typename T gt  T toIntegralType const string  amp str        static assert is integral lt T gt   value   Integral type required         T ret      stringstream ss str       ss  gt  gt  ret      if   to string ret     str          throw invalid argument  Can t convert     str       return ret      Here are examples of usage   string str    123   int x   toIntegralType lt int gt  str      x   123  str    123a   x   toIntegralType lt int gt  str      throws exception  because  123a  is not int  str    1   bool y   toIntegralType lt bool gt  str      y is true str    0   y   toIntegralType lt bool gt  str      y is false str    00   y   toIntegralType lt bool gt  str      throws exception   Why not just use stringstream output operator to convert a string into an integral type  Here is the answer  Let s say a string contains a value that exceeds the limit for intended integral type  For examle  on Wndows 64 max int is 2147483647  Let s assign to a string a value max int   1  string str    2147483648    Now  when converting the string to an int   stringstream ss str   int x  ss  gt  gt  x    x becomes 2147483647  what is definitely an error  string  2147483648  was not supposed to be converted to the int 2147483647  The provided function toIntegralType spots such errors and throws exception

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I know this question is really old but I think there s a better way of doing this  include  lt string gt   include  lt sstream gt   bool string to int std  string value  int   result      std  stringstream stream1  stream2    std  string stringednumber    int tempnumber    stream1  lt  lt  value    stream1  gt  gt  tempnumber    stream2  lt  lt  tempnumber    stream2  gt  gt  stringednumber    if   value compare stringednumber          result   tempnumber      return true        else return false     If I wrote the code right  this will return a boolean value that tells you if the string was a valid number  if false  it wasn t a number  if true it was a number and that number is now result  you would call this this way  std  string input  std  cin  gt  gt  input  bool worked   string to int input   amp result

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there is another easy way   suppose you have a character like c  4  therefore you can do one of these steps    1st   int q   q  int  c    q is now 52 in ascii table     q q-48  remember that adding 48 to digits is their ascii code    the second way     q c- 0   the same   character  0  means 48

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To convert from string representation to integer value  we can use std  stringstream   if the value converted is out of range for integer data type  it returns INT MIN or INT MAX   Also if the string value can   t be represented as an valid int data type  then 0 is returned    include   include   include   int main          std  string x    50       int y      std  istringstream x   gt  gt  y      std  cout  lt  lt  y  lt  lt    n       return 0      Output  50  As per the above output  we can see it converted from string numbers to integer number   Source and more at string to int c

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To be more exhaustive  and as it has been requested in comments   I add the solution given by C  17 using std  from chars  std  string str    quot 10 quot   int number  std  from chars str data    str data   str size    number    If you want to check whether the conversion was successful  std  string str    quot 10 quot   int number  auto  ptr  ec    std  from chars str data    str data   str size    number   assert ec    std  errc        ptr points to chars after read number  Moreover  to compare the performance of all these solutions  see the following quick-bench link  https   quick-bench com q GBzK53Gc-YSWpEA9XskSZLU963Y  std  from chars is the fastest and std  istringstream is the slowest

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Admittedly  my solution wouldn t work for negative integers  but it will extract all positive integers from input text containing integers  It makes use of numeric only locale   int main             int num          std  cin imbue std  locale std  locale    new numeric only              while   std  cin  gt  gt  num               std  cout  lt  lt  num  lt  lt  std  endl          return 0      Input text    the format  -5  or  25  etc    some text   and then   7987   78hjh hhjg9878   Output integers    5 25 7987 78 9878   The class numeric only is defined as   struct numeric only  std  ctype lt char gt         numeric only    std  ctype lt char gt  get table            static std  ctype base  mask const  get table                 static std  vector lt std  ctype base  mask gt               rc std  ctype lt char gt   table size std  ctype base  space            std  fill  amp rc  0     amp rc       std  ctype base  digit           return  amp rc 0              Complete online demo   http   ideone com dRWSj

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std  istringstream ss thestring   ss  gt  gt  thevalue    To be fully correct you ll want to check the error flags

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One line version  long n   strtol s c str    NULL  base        s is the string  and base is an int such as 2  8  10  16    You can refer to this link for more details of strtol     The core idea is to use strtol function  which is included in cstdlib   Since strtol only handles with char array  we need to convert string to char array  You can refer to this link   An example    include  lt iostream gt   include  lt string gt       string type  include  lt bitset gt       bitset type used in the output  int main        s    1111000001011010       long t   strtol s c str    NULL  2      2 is the base which parse the string      cout  lt  lt  s  lt  lt  endl      cout  lt  lt  t  lt  lt  endl      cout  lt  lt  hex  lt  lt  t  lt  lt  endl      cout  lt  lt  bitset lt 16 gt   t   lt  lt  endl       return 0      which will output   1111000001011010 61530 f05a 1111000001011010

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What about Boost Lexical cast   Here is their example      The following example treats command line arguments as a sequence of numeric data    int main int argc  char   argv          using boost  lexical cast      using boost  bad lexical cast       std  vector lt short gt  args       while    argv                try                       args push back lexical cast lt short gt   argv                      catch bad lexical cast  amp                         args push back 0

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In Windows  you could use   const std  wstring hex   L 0x13   const std  wstring dec   L 19    int ret  if  StrToIntEx hex c str    STIF SUPPORT HEX   amp ret         std  cout  lt  lt  ret  lt  lt    n     if  StrToIntEx dec c str    STIF SUPPORT HEX   amp ret         std  cout  lt  lt  ret  lt  lt    n       strtol stringstream need to specify the base if you need to interpret hexdecimal

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int stringToInt std  string value         if value length      0   return 0    tu zmiana     if  value find   std  string  quot NULL quot        std  string  npos         return 0          if  value find   std  string  quot null quot        std  string  npos         return 0          int i  std  stringstream stream1  stream1 clear    stream1 str value   stream1  gt  gt  i  return i

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My Code    include  lt iostream gt  using namespace std   int main         string s  32      String     int n stoi s      Convert to int     cout  lt  lt  n   1  lt  lt  endl       return 0

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If you wot hard code    bool strCanBeInt std  string string       for  char n   string            if  n     0   amp  amp  n     1   amp  amp  n     2   amp  amp  n     3   amp  amp  n     4   amp  amp  n     5               amp  amp  n     6   amp  amp  n     7   amp  amp  n     8   amp  amp  n     9                 return false                      return true     int strToInt std  string string        int integer   0      int numInt      for  char n   string            if n     0   numInt   0          if n     1   numInt   1          if n     2   numInt   2          if n     3   numInt   3          if n     4   numInt   4          if n     5   numInt   5          if n     6   numInt   6          if n     7   numInt   7          if n     8   numInt   8          if n     9   numInt   9          if  integer               integer    10                    integer    numInt            return integer

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1  std  stoi std  string str    quot 10 quot     int number   std  stoi str     2  string streams std  string str    quot 10 quot     int number    std  istringstream str   gt  gt  number  3  boost  lexical cast  include  lt boost lexical cast hpp gt  std  string str    quot 10 quot     int number       try        number   boost  lexical cast lt int gt  str       std  cout  lt  lt  number  lt  lt  std  endl    catch  boost  bad lexical cast const  amp e     bad input       std  cout  lt  lt   quot error quot   lt  lt  std  endl     4  std  atoi std  string str    quot 10 quot   int number   std  atoi str c str       5  sscanf    std  string str    quot 10 quot    int number   if  sscanf str  c str     quot  d quot    amp number     1           std  cout  lt  lt  number  lt  lt    n        else          std  cout  lt  lt   quot Bad Input quot

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In C  11 there are some nice new convert functions from std  string to a number type   So instead of   atoi  str c str       you can use  std  stoi  str     where str is your number as std  string   There are version for all flavours of numbers  long stol string   float stof string   double stod string      see http   en cppreference com w cpp string basic string stol

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use the atoi function to convert the string to an integer   string a    25    int b   atoi a c str       http   www cplusplus com reference clibrary cstdlib atoi

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It s probably a bit of overkill  but  boost  lexical cast lt int gt   theString   should to the job quite well

[c++] How can I convert a std::string to int?

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