Operations on Strings

ESC101: Fundamentals of Computing

Some common operations on strings

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• Compute the length of a string.
• Copy one string into another string variable
• Concatenate one string with another.
• Search for a substring in a given string.
• Reverse a string
• Find first/last/k-th occurrence of a character in a string
• … and more
• Case sensitive/insensitive versions

ESC101: Fundamentals of Computing

Computing the length of a string

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char str[10] = “Hello”;

int i = 0;

while(str[i] != ’\0’)

i++;

printf(“Length of %s is %d”,str,i);

ESC101: Fundamentals of Computing

Read a string and also compute its length

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int main() {

char str[100];

char ch;

int i = 0;

ch = getchar();

while(1){

if(ch==‘\n’) break;

str[i] = ch;

++i;

ch = getchar();

}

str[i] = ‘\0’;

printf("Length of %s is %d",str,i);

return 0;

}

Read the first character

If found a newline, break

Read the next character

Let’s put ‘\0’ in the end to mark the end of string

Not a newline. Store the read character at index i of str

Will store the length

ESC101: Fundamentals of Computing

Copying a string

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• We cannot copy content of one string variable to other using assignment operator

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• This is true for any array variable.
• Error because array initializer must be a list (comma separated values in {}) or a string.
• We need to do element-wise copying

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Array type is not assignable.

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C Pointers needed (will see this later)!

ESC101: Fundamentals of Computing

Copying a string element-by-element

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• Goal: Copy contents of string src into string dest.
• Declare dest with size at least as large as src.
• Use a loop to copy elements one-by-one
• Note the use of ‘\0’ for loop termination

char src[100] = “Hello World”;

char dest[100];

int i;

for (i = 0; src[i] != '\0’; i++)

dest[i] = src[i];

dest[i] = '\0';

Part of the loop

Not part of the loop

ESC101: Fundamentals of Computing

string.h

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• Many string operations are already implemented in string.h

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• It is a header file (“h” for header) with various functions on Strings

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• strlen(s): returns length of string s (without ‘\0’)

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• strcpy(d, s): copies s into d

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• strcat(d, s): appends s at the end of d (‘\0’ is moved to the end of result)

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ESC101: Fundamentals of Computing

string.h

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• strcmp(s1, s2): return an integer less than, equal to, or greater than zero if s1 is found, respectively, to be less than, to match, or be greater than s2.

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• Example:

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• Prints the value ‘l’-’p’ which is -4.

char str1[] = "Hello", str2[] = "Helpo";

int i = strcmp(str1,str2);

printf("%d", i);

ESC101: Fundamentals of Computing

string.h

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• strncpy(d, s, n)
• strncat(d, s, n)
• strncmp(d, s, n)
• restrict the function to “n” characters at most (argument n is an integer)
• first two functions-- Truncate the string s to the first “n” characters.
• third function-- Truncate the strings d, s to the first “n” characters.

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char str1[] = "Hello", str2[] = "Helpo";

printf("%d",strncmp(str1,str2,3));

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ESC101: Fundamentals of Computing

string.h

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• strcasecmp, strncasecmp:

case insensitive comparison.

• Example:

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• strcmp gives -32 because ‘E’ < ‘e’ .
• ‘E’-‘e’ = -32 .
• strcasecmp finds the first 3 characters the same (ignoring case) gives -4 because ‘l’ - ‘p’ = -4

-32 -4

char str1[] = "HELLO", str2[] = "Helpo";

int i = strcmp(str1,str2);

int j = strcasecmp(str1,str2);

printf("%d %d", i, j);

ESC101: Fundamentals of Computing

string.h

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• Many more utility functions.

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• strupr(s) : converts lower to upper case.
• strlwr(s) : converts upper to lower case.
• strstr(S,s) : searches string s in string S (example: strstr(“Hello”,”ll”);). Returns a pointer(memory address) to the first occurrence.

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• All functions depend on ‘\0’ as the end-of-string marker.

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ESC101: Fundamentals of Computing

Another useful string function

• atoi(str): converts a string into integer, e.g. atoi(“123”) will return 123 (integer). So we can write int a = atoi(“123”);
• atoi(str) will keep reading the string str until it finds a non-digit character in str and will return the integer containing the read digit characters in str
• atoi(“12abc3”) will return 12
• If no digit symbols found in str, atoi(str) will return 0
• If the read integer is larger than the range of integers, garbage will be returned
• Other function atof(str), atol(str) – return float and long
• Can also convert int/long to string (itoa, ltoa, etc)

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ESC101: Fundamentals of Computing

Strings: Summary

• Strings are character arrays
• The last character is ‘\0’ (null character) and marks end of string
• Many direct operations (e.g., assignment) not possible for strings. Have to be done element-wise (e.g., using a loop)
• string.h contains many useful functions (so you don’t need to write functions for basic operations, such as finding the length of string, copying one string into another, etc)

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