Introduction to C Programming CE003121

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Introduction to C ProgrammingCE00312-1

• Lecture 13
• Strings in C

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Strings in C

• A string is an array of characters terminated by
  a \0 character and is limited to the size of
  the array.
• e.g. char name10
• can represent a string of maximum 9 characters
• (10 including \0)
• Initialisation
• char word15 example
• // \0 in addition, rest undefined

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• An array name or string name is a reference to an
  array of elements (called a pointer in C).
• Using a string as an entity (ie with a \0), as
  opposed to an array of individual characters,
  allows us to use the string facilities of C
  without thinking about the \0

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Input and Output of Strings

• Output
• use s format in printf
• e.g. printf(s, word)
• Input
• Two forms
• 1) s format can be used in scanf
• eg scanf(s, name)
• // no required for a string
• reads all characters up to space or newline.

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• Input
• 2) gets reads a whole line as a string and then
  we can extract the data in the line individually
  using sscanf
• (note the extra s).
• gets returns NULL if there is no more data in the
  input (for the keyboard when control and d are
  pressed together).

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Example of using gets

• char line81 // maximum 80 chars \0
• char name21
• int age
• gets(line) // stores all characters
• // replaces \n by \0
• sscanf(line, sd, name, age)
• scans the string, line, stores a string
  (terminated by a space) in name, and an integer
  in age, thus for the following input
• Bailey 60
• string name contains Bailey and age contains
  60. Subsequent reading starts on the next line.

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Standard String Functions

• include string.h
• // required for using string functions
• strlen(s) // returns length of string, s -
  // not counting \0
• For example
• printf(string s is of length d,
• word, strlen(word))

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More String Functions

• strcpy(s1, s2) // copies string s2 to s1,
• // but s1 s2 is incorrect in C
• For example
• strcpy(word, name)
• strcat(s1,s2) // appends s2 on to end of s1,
• // but s1 must be big enough
• For example
• strcat(word, name)
• // word now contains exampleBailey

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• Note
• strcpy and strcat also return s1
• For example
• printf(concatenation of s and s is s,
• word, name, strcat(word, name))
• produces
• concatenation of example and Bailey is
  exampleBailey

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Comparing Strings

• strcmp(s1, s2)
• // returns value of 0 if s1 the same as s2,
• // returns -ve value if s1 less than s2,
• // returns ve value if s1more than s2,
• // we cant use , lt , gt
• For example
• if (strcmp(name, word) lt 0)
• printf(s is less than s, name, word)

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String example No. 1

• Input 10 strings (each of at most 20 characters)
  into an array, sort them into alphabetical order
  and output them.
• include stdio.h
• include string.h
• // prototypes
• void read_strings(char 21, int)
• void print_strings(char 21, int)
• void sort(char 21, int)

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Main Function

• Array of 10 strings, list, must be declared in
  main.
• Array name, list, and its size, 10, must be
  passed to each function that uses it.
• int main(void)
• char list1021 // 20 chars each \0
• read_strings(list, 10)
• print_strings(list, 10)
• sort(list, 10)
• print_strings(list, 10)
• return 0
• // end main

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read_strings function

• Array, s, is 2-dimensional array (only no. of
  columns, ie 21, declared) - each row is a string.
• Function, gets, reads a line (ie string) into row
  si.
• void read_strings(char s21, int n)
• int i
• printf(\ntype one string per line\n)
• for (i0 iltn i)
• // read each line into row si
• gets(si)
• // end read strings

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print_strings function

• Print each row of array, list, as a string using
  s format.
• void print_strings(char list21, int n)
• int i
• printf(\nlist of strings is\n\n)
• for (i0 iltn i)
• // print each row listi as string
• printf(s\n, listi)
• // end print strings

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void sort(char a21, int n) // Array, a,
of strings int i, j char temp21 // temp
string for (j n-1 j gt 0 j--) // each
pass of j comparisons for (i 0 i lt j
i) // each comparison if (strcmp(ai,
ai1) gt 0) // ai gt ai1 // swap
the 2 strings strcpy(temp, ai)
strcpy(ai, ai1)
strcpy(ai1, temp) // else no
swap // end of each pass // end of all
passes // end sort