Working with Strings

This page outlines a variety of techniques that are available within C for working with strings. Discussion of techniques falls into several categories:

• Characters and Substrings
  • Extraction of a character
  • Extraction of a substring
  • Breaking a string into pieces
• Conversion to numbers

All examples that follow may be found in the C-program, string-processing.c

Characters and Substrings

Extraction of a character

In C, a string is an array of characters, and each character can be accessed directly through in index within the array.

Example:

  char myArray[17] = "computer science";
  int i;  for (i = 0; i < strlen(myArray); i++)
    printf ("\tmyArray[%d] = %c\n", i, myArray[i]);

Extraction of a substring

Given a string, a common task is to extract a substring from a string. For example, in Scheme, one might use the substring function:

   (substring str start end)

According to the Scheme R5 documentation, "`Substring' returns a newly allocated string formed from the characters of string beginning with index start (inclusive) and ending with index end (exclusive)."

In C, there is not a function with the name substring, but the function strncpy accomplishes the same result; one copies end-start characters from str, starting with the offset start. If the extracted string does not go to the end of str, then one must remember to add a null character at the end.

The header of strncpy reads:

   char *strncpy(char *result, const char *original, int numberCharacters);

In addition to copying the characters from original to result, the function returns a pointer to result.

Several examples follow:

  /* extracting substrings from a string */
  /* the example uses myArray from above as the starting point */
  printf ("extracting substrings from a string\n");
  char subArray[20];  /* for a string copy, remember to have allocated space! */
  strncpy (subArray, myArray, 8);
  subArray[8] = 0; /* null terminate array */
  printf ("\t extract the first 8 characters (plus the null): %s\n", subArray);

  strncpy (subArray, myArray+3, strlen(myArray)); /* copying past end of myArray fills will nulls */
  printf ("\t extract all characters after the first 3: %s\n", subArray);
  
  int start = strlen(myArray) - 8;
  strncpy (subArray, myArray+start, 8);
  subArray[strlen(myArray)-8] = 0; /* null terminate array */
  printf ("\t extract the last 8 characters (plus the null): %s\n", subArray);

Breaking a string into pieces

Some applications require a string to be separated into pieces. For example, consider the string

"January,February,March,April,May,June,July,August,September,October,November,December"

We might want to break it into a sequence of substrings, separated by a given separator or delimiter. This task is accomplished by the following function:

   char *strtok(char *strptr, const char *delimiter);

The first call to strtok returns the first string before the given delimiter. For subsequent calls, a null string is used for the strptr parameter, and these calls return the next elements in the sequence. When no more elements are present, strtok returns NULL.

Warnings:

• Calls to strtok can change the first parameter.
• Since strtok changes the parameter, standard practice is to copy the original string first and use the copy when using strtok

The following example places the months given above into an array of 12 string points:

  char year[] = "January,February,March,April,May,June,July,August,September,October,November,December" ;
  /* copy original array */  
  char * yearCopy = malloc(sizeof(char)*(strlen(year) + 1));
  strcpy (yearCopy, year);
  /* array to receive the string pieces, including space for NULL strtok at end */
  char * months[13];
  /* loop puts strings into successive elements of months array */
  months[0] = strtok(yearCopy, ",");
  i = 1;
  while (months[i] = strtok(NULL, ",")) /* continue as long as token not NULL */
    {
      i++;
    }
  printf ("result of using strtok to break up a string of months\n");
  for (i = 0; i < 12; i++)
    printf ("\t month %d: %s\n", i, months[i]);
  printf ("original year array:  %s\n", year);

Conversion to numbers

Input from a command line (and input from scanf using %s format) is considered a string. The stdlib.h library contains several functions to convert strings to numbers:

• convert a character string to an int

   
      int atoi(const char *strptr);  /* converts a string to an int */
  

• convert a character string to a double

   
      double atof(const char *strptr);  /* converts a string to a double */
  

Examples:
	Function call	Result	Comments
	atoi("1234")	1234	int type returned
	atoi("3.14")	3	int type returned; digits after the decimal point ignored
	atof("1234")	1234.0	double type returned
	atof("3.14")	3.14	double type returned

    http://www.math.grin.edu/~walker/courses/161.sp12/string-processing.shtml