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      std::scanf,std::fscanf,std::sscanf

      From cppreference.com
      <cpp‎ |io‎ |c
       
       
       
       
      Defined in header<cstdio>
      int scanf(constchar* format, ...);
      		 (1)
      int fscanf(std::FILE* stream,constchar* format, ...);
      		 (2)
      int sscanf(constchar* buffer,constchar* format, ...);
      		 (3)

      Reads data from a variety of sources, interprets it according toformat and stores the results into given locations.

      1) Reads the data fromstdin.
      2) Reads the data from file streamstream.
      3) Reads the data from null-terminated character stringbuffer.

      Contents

      [edit]Parameters

      stream - input file stream to read from
      buffer - pointer to a null-terminated character string to read from
      format - pointer to a null-terminated character string specifying how to read the input
      ... - receiving arguments

      Theformat string consists of

      • non-whitespace multibyte characters except%: each such character in the format string consumes exactly one identical character from the input stream, or causes the function to fail if the next character on the stream does not compare equal.
      • whitespace characters: any single whitespace character in the format string consumes all available consecutive whitespace characters from the input (determined as if by callingstd::isspace in a loop). Note that there is no difference between"\n"," ","\t\t", or other whitespace in the format string.
      • conversion specifications. Each conversion specification has the following format:
      • introductory% character.
      • (optional) assignment-suppressing character*. If this option is present, the function does not assign the result of the conversion to any receiving argument.
      • (optional) integer number (greater than zero) that specifiesmaximum field width, that is, the maximum number of characters that the function is allowed to consume when doing the conversion specified by the current conversion specification. Note that%s and%[ may lead to buffer overflow if the width is not provided.
      • (optional)length modifier that specifies the size of the receiving argument, that is, the actual destination type. This affects the conversion accuracy and overflow rules. The default destination type is different for each conversion type (see table below).
      • conversion format specifier.

      The following format specifiers are available:

      Conversion
      specifier      		ExplanationExpected
      Argument type
      Length Modifier→hhhnonellljztL
      Only available since C++11→YesYesYesYesYes
      %
      Matches literal%.
      		N/AN/AN/AN/AN/AN/AN/AN/AN/A
      c

      Matches acharacter or a sequence ofcharacters.

      • If a width specifier is used, matches exactlywidth characters (the argument must be a pointer to an array with sufficient room).
      • Unlike %s and %[, does not append the null character to the array.
      		N/AN/A
      char*
      wchar_t*
      		N/AN/AN/AN/AN/A
      s

      Matches a sequence of non-whitespace characters (astring).

      • If width specifier is used, matches up towidth or until the first whitespace character, whichever appears first.
      • Always stores a null character in addition to the characters matched (so the argument array must have room for at leastwidth+1 characters).
      [set ]

      Matches a non-empty sequence of character fromset of characters.

      • If the first character of the set is^, then all characters not in the set are matched.
      • If the set begins with] or^] then the] character is also included into the set.
      • It is implementation-defined whether the character- in the non-initial position in the scanset may be indicating a range, as in[0-9].
      • If width specifier is used, matches only up towidth.
      • Always stores a null character in addition to the characters matched (so the argument array must have room for at leastwidth+1 characters).
      d

      Matches adecimal integer.

      • The format of the number is the same as expected bystd::strtol with the value10 for thebase argument.
      signedchar* orunsignedchar*
      signedshort* orunsignedshort*
      signedint* orunsignedint*
      signedlong* orunsignedlong*
      signedlonglong* orunsignedlonglong*
      		N/A
      i

      Matches aninteger.

      • The format of the number is the same as expected bystd::strtol with the value​0​ for thebase argument (base is determined by the first characters parsed).
      u

      Matches an unsigneddecimal integer.

      • The format of the number is the same as expected bystd::strtoul with the value10 for thebase argument.
      o

      Matches an unsignedoctal integer.

      • The format of the number is the same as expected bystd::strtoul with the value8 for thebase argument.
      x
      X

      Matches an unsignedhexadecimal integer.

      • The format of the number is the same as expected bystd::strtoul with the value16 for thebase argument.
      n

      Returns thenumber of characters read so far.

      • No input is consumed. Does not increment the assignment count.
      • If the specifier has assignment-suppressing operator defined, the behavior is undefined.
      a(C++11)
      A(C++11)
      e
      E
      f
      F(C++11)
      g
      G

      Matches afloating-point number.

      • The format of the number is the same as expected bystd::strtof.
      		N/AN/A
      float*
      double*
      		N/AN/AN/AN/A
      longdouble*
      p

      Matches implementation defined character sequence defining apointer.

      • printf family of functions should produce the same sequence using%p format specifier.
      		N/AN/A
      void**
      		N/AN/AN/AN/AN/AN/A
      Notes

      For every conversion specifier other thann, the longest sequence of input characters which does not exceed any specified field width and which either is exactly what the conversion specifier expects or is a prefix of a sequence it would expect, is what's consumed from the stream. The first character, if any, after this consumed sequence remains unread. If the consumed sequence has length zero or if the consumed sequence cannot be converted as specified above, the matching failure occurs unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

      All conversion specifiers other than[,c, andn consume and discard all leading whitespace characters (determined as if by callingstd::isspace) before attempting to parse the input. These consumed characters do not count towards the specified maximum field width.

      The conversion specifierslc,ls, andl[ perform multibyte-to-wide character conversion as if by callingstd::mbrtowc with anstd::mbstate_t object initialized to zero before the first character is converted.

      The conversion specifierss and[ always store the null terminator in addition to the matched characters. The size of the destination array must be at least one greater than the specified field width. The use of%s or%[, without specifying the destination array size, is as unsafe asstd::gets.

      The correct conversion specifications for thefixed-width integer types (std::int8_t, etc) are defined in the header<cinttypes> (althoughSCNdMAX,SCNuMAX, etc is synonymous with%jd,%ju, etc).

      There is asequence point after the action of each conversion specifier; this permits storing multiple fields in the same “sink” variable.

      When parsing an incomplete floating-point value that ends in the exponent with no digits, such as parsing"100er" with the conversion specifier%f, the sequence"100e" (the longest prefix of a possibly valid floating-point number) is consumed, resulting in a matching error (the consumed sequence cannot be converted to a floating-point number), with"r" remaining. Some existing implementations do not follow this rule and roll back to consume only"100", leaving"er", e.g.,glibc bug 1765.

      If a conversion specification is invalid, the behavior is undefined.

      [edit]Return value

      Number of receiving arguments successfully assigned (which may be zero in case a matching failure occurred before the first receiving argument was assigned), orEOF if input failure occurs before the first receiving argument was assigned.

      [edit]Complexity

      Not guaranteed. Notably, some implementations ofstd::sscanf areO(N), whereN=std::strlen(buffer)[1]. For performant string parsing, seestd::from_chars.

      [edit]Notes

      Because most conversion specifiers first consume all consecutive whitespace, code such as

      std::scanf("%d",&a);std::scanf("%d",&b);

      will read two integers that are entered on different lines (second%d will consume the newline left over by the first) or on the same line, separated by spaces or tabs (second%d will consume the spaces or tabs).

      The conversion specifiers that do not consume leading whitespace, such as%c, can be made to do so by using a whitespace character in the format string:
      std::scanf("%d",&a);std::scanf(" %c",&c);// ignore the endline after %d, then read a char

      Note that some implementations ofstd::sscanf involve a call tostd::strlen, which makes their runtime linear on the length of the entire string. This means that ifstd::sscanf is called in a loop to repeatedly parse values from the front of a string, your code might run in quadratic time (example).

      [edit]Example

      Run this code
      #include <clocale>#include <cstdio>#include <iostream> int main(){int i, j;float x, y;char str1[10], str2[4];wchar_t warr[2];std::setlocale(LC_ALL,"en_US.utf8"); char input[]="25 54.32E-1 Thompson 56789 0123 56ß水";// parse as follows:// %d: an integer// %f: a floating-point value// %9s: a string of at most 9 non-whitespace characters// %2d: two-digit integer (digits 5 and 6)// %f: a floating-point value (digits 7, 8, 9)// %*d an integer which isn't stored anywhere// ' ': all consecutive whitespace// %3[0-9]: a string of at most 3 digits (digits 5 and 6)// %2lc: two wide characters, using multibyte to wide conversionconstint ret= std::sscanf(input,"%d%f%9s%2d%f%*d %3[0-9]%2lc",&i,&x, str1,&j,&y, str2, warr); std::cout<<"Converted "<< ret<<" fields:\n""i = "<< i<<"\n""x = "<< x<<"\n""str1 = "<< str1<<"\n""j = "<< j<<"\n""y = "<< y<<"\n""str2 = "<< str2<<std::hex<<"\n""warr[0] = U+"<<(int)warr[0]<<"\n""warr[1] = U+"<<(int)warr[1]<<'\n';}

      Output:

      Converted 7 fields:i = 25x = 5.432str1 = Thompsonj = 56y = 789str2 = 56warr[0] = U+dfwarr[1] = U+6c34

      [edit]See also

      (C++11)(C++11)(C++11)
      		 reads formatted input fromstdin, a file stream or a buffer
      using variable argument list
      (function)[edit]
      gets a character string from a file stream
      (function)[edit]
      prints formatted output tostdout, a file stream or a buffer
      (function)[edit]
      (C++17)
      		 converts a character sequence to an integer or floating-point value
      (function)[edit]
      C documentation forscanf,fscanf,sscanf
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