How do I check if a C std string starts with a certain string and convert a substring to an int

Question

How do I implement the following  Python pseudocode  in C     if argv 1  startswith  --foo         foo value   int argv 1  len  --foo          For example  if argv 1  is --foo 98  then foo value is 98    Update  I m hesitant to look into Boost  since I m just looking at making a very small change to a simple little command-line tool  I d rather not have to learn how to link in and use Boost for a minor change

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text substr 0  start length       start

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With C  11 or higher you can use find   and find first of     Example using find to find a single char    include  lt string gt  std  string name    Aaah   size t found index   name find  a    if  found index    std  string  npos           Found string containing  a      Example using find to find a full string  amp  starting from position 5   std  string name    Aaah   size t found index   name find  h   3   if  found index    std  string  npos           Found string containing  h      Example using the find first of   and only the first char  to search at the start only   std  string name     hidden  di r   size t found index   name find first of       if  found index    0           Found     at first position in string      More about find More about find first of   Good luck

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You can also use strstr   if  strstr str  substr     substr            str  starts with  substr      but I think it s good only for short strings because it has to loop through the whole string when the string doesn t actually start with  substr

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Ok why the complicated use of libraries and stuff  C   String objects overload the    operator  so you can just compare chars   Like what I just did  because I want to list all files in a directory and ignore invisible files and the    and   pseudofiles    while   ep   readdir dp          string s ep- gt d name       if    s 0              Omit invisible files and    or           files push back s       It s that simple

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In C  20 now there is starts with available as a member function of std  string defined as  constexpr bool starts with string view sv  const noexcept   constexpr bool starts with CharT c  const noexcept   constexpr bool starts with const CharT  s  const   So your code could be something like this  std  string s argv 1     if  s starts with  quot --foo  quot

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In case you need C  11 compatibility and cannot use boost  here is a boost-compatible drop-in with an example of usage    include  lt iostream gt   include  lt string gt   static bool starts with const std  string str  const std  string prefix        return   prefix size    lt   str size     amp  amp  std  equal prefix begin    prefix end    str begin         int main int argc  char  argv          bool usage   false      unsigned int foos   0     default number of foos if no parameter was supplied      if  argc  gt  1                const std  string fParamPrefix    -f       shorthand for foo         const std  string fooParamPrefix    --foo             for  unsigned int i   1  i  lt  argc    i                        const std  string arg   argv i                try                               if   arg     -h       arg     --help                                          usage   true                    else if  starts with arg  fParamPrefix                         foos   std  stoul arg substr fParamPrefix size                        else if  starts with arg  fooParamPrefix                         foos   std  stoul arg substr fooParamPrefix size                                      catch  std  exception amp  e                    std  cerr  lt  lt   Invalid parameter     lt  lt  argv i   lt  lt  std  endl  lt  lt  std  endl                  usage   true                                     if  usage                std  cerr  lt  lt   Usage     lt  lt  argv 0   lt  lt     OPTION       lt  lt  std  endl          std  cerr  lt  lt   Example program for parameter parsing    lt  lt  std  endl  lt  lt  std  endl          std  cerr  lt  lt     -f  --foo N   use N foos  optional    lt  lt  std  endl          return 1             std  cerr  lt  lt   number of foos given     lt  lt  foos  lt  lt  std  endl

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Just for completeness  I will mention the C way to do it      If str is your original string  substr is the substring you want to   check  then       strncmp str  substr  strlen substr        will return 0 if str   starts with substr  The functions strncmp and strlen are in the C   header file  lt string h gt     originally posted by Yaseen Rauf here  markup added   For a case-insensitive comparison  use strnicmp instead of strncmp   This is the C way to do it  for C   strings you can use the same function like this   strncmp str c str    substr c str    substr size

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Nobody used the STL algorithm mismatch function yet  If this returns true  prefix is a prefix of  toCheck    std  mismatch prefix begin    prefix end    toCheck begin    first    prefix end     Full example prog    include  lt algorithm gt   include  lt string gt   include  lt iostream gt   int main int argc  char   argv        if  argc    3            std  cerr  lt  lt   Usage     lt  lt  argv 0   lt  lt    prefix string   lt  lt  std  endl                    lt  lt   Will print true if  prefix  is a prefix of string   lt  lt  std  endl          return -1            std  string prefix argv 1        std  string toCheck argv 2        if  prefix length    gt  toCheck length              std  cerr  lt  lt   Usage     lt  lt  argv 0   lt  lt    prefix string   lt  lt  std  endl                    lt  lt    prefix  is longer than  string    lt  lt   std  endl          return 2            if  std  mismatch prefix begin    prefix end    toCheck begin    first    prefix end              std  cout  lt  lt       lt  lt  prefix  lt  lt       lt  lt    is a prefix of    lt  lt       lt  lt  toCheck  lt  lt       lt  lt  std  endl          return 0        else           std  cout  lt  lt       lt  lt  prefix  lt  lt       lt  lt    is NOT a prefix of    lt  lt       lt  lt  toCheck  lt  lt       lt  lt  std  endl          return 1              Edit   As  James T  Huggett suggests  std  equal is a better fit for the question  Is A a prefix of B  and is slight shorter code   std  equal prefix begin    prefix end    toCheck begin      Full example prog    include  lt algorithm gt   include  lt string gt   include  lt iostream gt   int main int argc  char   argv      if  argc    3        std  cerr  lt  lt   Usage     lt  lt  argv 0   lt  lt    prefix string   lt  lt  std  endl                lt  lt   Will print true if  prefix  is a prefix of string                 lt  lt  std  endl      return -1        std  string prefix argv 1      std  string toCheck argv 2      if  prefix length    gt  toCheck length          std  cerr  lt  lt   Usage     lt  lt  argv 0   lt  lt    prefix string   lt  lt  std  endl                lt  lt    prefix  is longer than  string    lt  lt  std  endl      return 2        if  std  equal prefix begin    prefix end    toCheck begin           std  cout  lt  lt       lt  lt  prefix  lt  lt       lt  lt    is a prefix of    lt  lt       lt  lt  toCheck                lt  lt       lt  lt  std  endl      return 0      else       std  cout  lt  lt       lt  lt  prefix  lt  lt       lt  lt    is NOT a prefix of    lt  lt                     lt  lt  toCheck  lt  lt       lt  lt  std  endl      return 1

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Starting with C  20  you can use the starts with method   std  string s    abcd   if  s starts with  abc

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Code I use myself   std  string prefix    -param    std  string argument   argv 1   if argument substr 0  prefix size       prefix        std  string argumentValue   argument substr prefix size

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if boost  starts with string to search  string to look for       intval   boost  lexical cast lt int gt  string to search substr string to look for length        This is completely untested  The principle is the same as the Python one  Requires Boost StringAlgo and Boost LexicalCast   Check if the string starts with the other string  and then get the substring   slice   of the first string and convert it using lexical cast

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You would do it like this  std  string prefix  quot --foo  quot    if   arg compare 0  prefix size    prefix       foo value   std  stoi arg substr prefix size       Looking for a lib such as Boost ProgramOptions that does this for you is also a good idea

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std  string text    --foo 98   std  string start    --foo     if  text find start     0        int n   stoi text substr start length          std  cout  lt  lt  n  lt  lt  std  endl

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With C  17 you can use std  basic string view  amp  with C  20 std  basic string  starts with or std  basic string view  starts with    The benefit of std  string view in comparison to std  string - regarding memory management - is that it only holds a pointer to a  string   contiguous sequence of char-like objects  and knows its size  Example without moving copying the source strings just to get the integer value    include  lt exception gt   include  lt iostream gt   include  lt string gt   include  lt string view gt   int main         constexpr auto argument    --foo 42      Emulating command argument      constexpr auto prefix    --foo        auto inputValue   0       constexpr auto argumentView   std  string view argument       if  argumentView starts with prefix                 constexpr auto prefixSize   std  string view prefix  size            try                          The underlying data of argumentView is nul-terminated  therefore we can use data                inputValue   std  stoi argumentView substr prefixSize  data                       catch  std  exception  amp  e                        std  cerr  lt  lt  e what                        std  cout  lt  lt  inputValue     42

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Use an overload of rfind which has the pos parameter   std  string s    tititoto   if  s rfind  titi   0     0         s starts with prefix     Who needs anything else  Pure STL   Many have misread this to mean  search backwards through the whole string looking for the prefix   That would give the wrong result  e g  string  tititito   rfind  titi   returns 2 so when compared against    0 would return false  and it would be inefficient  looking through the whole string instead of just the start   But it does not do that because it passes the pos parameter as 0  which limits the search to only match at that position or earlier  For example   std  string test    0123123   size t match1   test rfind  123          returns 4  rightmost match  size t match2   test rfind  123   2      returns 1  skipped over later match  size t match3   test rfind  123   0      returns std  string  npos  i e  not found

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At the risk of being flamed for using C constructs  I do think this sscanf example is more elegant than most Boost solutions   And you don t have to worry about linkage if you re running anywhere that has a Python interpreter    include  lt stdio h gt   include  lt string h gt   int main int argc  char   argv        for  int i   1  i    argc    i            int number   0          int size   0          sscanf argv i    --foo  d n    amp number   amp size           if  size    strlen argv i                  printf  number   d n   number                     else               printf  not-a-number n                        return 0      Here s some example output that demonstrates the solution handles leading trailing garbage as correctly as the equivalent Python code  and more correctly than anything using atoi  which will erroneously ignore a non-numeric suffix        scan --foo 2 --foo 2d --foo  2     --foo 2  number  2 not-a-number not-a-number not-a-number

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I use std  string  compare wrapped in utility method like below   static bool startsWith const string amp  s  const string amp  prefix        return s size    gt   prefix size    amp  amp  s compare 0  prefix size    prefix     0

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Given that both strings     argv 1  and  --foo      are C strings   FelixDombek s answer is hands-down the best solution   Seeing the other answers  however  I thought it worth noting that  if your text is already available as a std  string  then a simple  zero-copy  maximally efficient solution exists that hasn t been mentioned so far   const char   foo    --foo   if  text rfind foo  0     0      foo value   text substr strlen foo      And if foo is already a string   std  string foo  --foo    if  text rfind foo  0     0      foo value   text substr foo length

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Why not use gnu getopts  Here s a basic example  without safety checks     include  lt getopt h gt   include  lt stdio h gt   int main int argc  char   argv      option long options             foo   required argument  0  0        0 0 0 0          getopt long argc  argv   f    long options  0      printf   s n   optarg       For the following command       a out --foo 33   You will get  33

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If you re already using Boost  you can do it with boost string algorithms   boost lexical cast    include  lt boost algorithm string predicate hpp gt   include  lt boost lexical cast hpp gt   try           if  boost  starts with argv 1    --foo             foo value   boost  lexical cast lt int gt  argv 1  6     catch  boost  bad lexical cast           bad parameter     This kind of approach  like many of the other answers provided here is ok for very simple tasks  but in the long run you are usually better off using a command line parsing library  Boost has one  Boost Program options   which may make sense if you happen to be using Boost already   Otherwise a search for  c   command line parser  will yield a number of options

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Using STL this could look like   std  string prefix    --foo    std  string arg   argv 1   if  prefix size   lt  arg size    amp  amp  std  equal prefix begin    prefix end    arg begin         std  istringstream iss arg substr prefix size        iss  gt  gt  foo value

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Since C  11 std  regex search can also be used to provide even more complex expressions matching  The following example handles also floating numbers thorugh std  stof and a subsequent cast to int   However the parseInt method shown below could throw a std  invalid argument exception if the prefix is not matched  this can be easily adapted depending on the given application    include  lt iostream gt   include  lt regex gt   int parseInt const std  string  amp str  const std  string  amp prefix      std  smatch match    std  regex search str  match  std  regex       prefix       -           d   d         d       Ee   -    d            return std  stof match 1       int main         std  cout  lt  lt  parseInt  foo 13 3    foo     lt  lt  std  endl      std  cout  lt  lt  parseInt  foo - 9    foo     lt  lt  std  endl      std  cout  lt  lt  parseInt  foo  13 3    foo     lt  lt  std  endl      std  cout  lt  lt  parseInt  foo -0 133    foo     lt  lt  std  endl      std  cout  lt  lt  parseInt  foo  00123456    foo     lt  lt  std  endl      std  cout  lt  lt  parseInt  foo -06 12e 3    foo     lt  lt  std  endl         throw std  invalid argument       std  cout  lt  lt  parseInt  foo 1    bar     lt  lt  std  endl       return 0      The kind of magic of the regex pattern is well detailed in the following answer   EDIT  the previous answer did not performed the conversion to integer

[c++] How do I check if a C++ std::string starts with a certain string, and convert a substring to an int?

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