8.9 String Comparison

Since the C programming language does not define a built-in data type for strings, no operators are defined for use with strings. Instead, you must use the string functions defined in the standard library. For example, in Python, you could determine if two strings were identical using the == operator, but this can not be done in C.

The strcmp function is used to compare whether two strings are identical or to determine their relative lexicographical ordering. In Python, this could be done using the regular relational operators (==, <, >, etc).

The String Comparison Function

string.h

int strcmp(char str1[], char str2[]);

The function takes two strings and compares them lexicographically to determine their relative ordering or whether they are identical. The integer value returned indicates the result of the comparison:

A value of 0 indicates that the two strings are identical.
A negative value is returned if the first string str1 comes before the second string str2 in lexicographical order.
A positive value is returned if the first string str1 follows the second string str2 in lexicographical order.

Testing for Equality

Suppose the string array name has been declared and initialized. We can use the strcmp function to determine if the array contains the name "Smith":

if(strcmp(name, "Smith") == 0)
printf("The strings are the same.\n");
else
printf("The strings are not the same.\n");

Two strings are identical if they contain the exact same number of elements and the characters in corresponding elements are identical. Suppose name contains the string

when it is compared against the literal string "Smith", the strcmp function will return 0 indicating the strings are identical. Now suppose name contains "smith", what would the function return?

Since the characters in the first position of the two arrays are not equal, the two strings are not idential. Remember, the character 's' is not identical to the character 'S'. They have different ASCII code values: the code of 'S' is 83 and the code for 's' is 115. In this case, the function will return a positive integer value since the string "smith" follows the string "Smith" in relative order.

Testing Relative Order

Consider the string declarations

char name1[20] = "Mary";
char name2[20] = "Maria";

which result in the character arrays

Suppose we want to determine if the string in name1 precedes the string in name2. We will again need to use the strcmp function and compare the result against 0:

if(strcmp(name1, name2) < 0)
printf("The string '%s' precedes the string '%s'.\n", name1, name2);
else
printf("The string '%s' precedes the string '%s'.\n", name2, name1);

Likewise, if you want to determine whether the string in the first argument to strcmp follows the second, you would test if the result is greater than 0:

if(strcmp("xyz", "abc") > 0) {
printf("xyz follows abc alphabetically.\n");
}

Comparing Strings of Different Lengths

Consider the two strings

char name1[] = "Mary";
char name2[] = "Maria";

what value is assigned to variable result when the following statement is executed?

int result = strmcmp(name1, name2);

To determine this, we need to compare the two strings the way the strcmp function would. We know the strings are not identical, so 0 is not returned. Next, we look to see where the two strings first differ, which is at index position 3.

Examining the characters at that position, we quickly see that 'i' comes alphabetically before 'y'. Thus, the string "Maria" will precede the string "Mary" in relative order and the function will return a positive value.

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