Write your own strcmp that ignores cases

• Difficulty Level : Medium
• Last Updated : 29 Oct, 2021

Write a modified strcmp function which ignores cases and returns -1 if s1 < s2, 0 if s1 = s2, else returns 1. For example, your strcmp should consider “GeeksforGeeks” and “geeksforgeeks” as same string.
Source: Microsoft Interview Set 5
Following solution assumes that characters are represented using ASCII representation, i.e., codes for ‘a’, ‘b’, ‘c’, … ‘z’ are 97, 98, 99, … 122 respectively. And codes for ‘A’, “B”, ‘C’, … ‘Z’ are 65, 66, … 90 respectively.
Following are the detailed steps.
1) Iterate through every character of both strings and do following for each character.
a) If str1[i] is same as str2[i], then continue.
b) If inverting the 6th least significant bit of str1[i] makes it same as str2[i], then continue. For example, if str1[i] is 65, then inverting the 6th bit will make it 97. And if str1[i] is 97, then inverting the 6th bit will make it 65.
c) If any of the above two conditions is not true, then break.
2) Compare the last (or first mismatching in case of not same) characters.

C++

 #include using namespace std;/* implementation of strcmp that ignores cases */int ic_strcmp(string s1, string s2){    int i;    for (i = 0; s1[i] && s2[i]; ++i)    {        /* If characters are same or inverting the        6th bit makes them same */        if (s1[i] == s2[i] || (s1[i] ^ 32) == s2[i])        continue;        else        break;    }     /* Compare the last (or first mismatching in    case of not same) characters */    if (s1[i] == s2[i])        return 0;     // Set the 6th bit in both, then compare    if ((s1[i] | 32) < (s2[i] | 32))        return -1;    return 1;} // Driver program to test above functionint main(){    cout<<"ret: "<

C

 #include  /* implementation of strcmp that ignores cases */int ic_strcmp(char *s1, char *s2){    int i;    for (i = 0; s1[i] && s2[i]; ++i)    {        /* If characters are same or inverting the           6th bit makes them same */        if (s1[i] == s2[i] || (s1[i] ^ 32) == s2[i])           continue;        else           break;    }     /* Compare the last (or first mismatching in       case of not same) characters */    if (s1[i] == s2[i])        return 0;     // Set the 6th bit in both, then compare    if ((s1[i] | 32) < (s2[i] | 32))        return -1;    return 1;} // Driver program to test above functionint main(void){    printf("ret: %d\n", ic_strcmp("Geeks", "apple"));    printf("ret: %d\n", ic_strcmp("", "ABCD"));    printf("ret: %d\n", ic_strcmp("ABCD", "z"));    printf("ret: %d\n", ic_strcmp("ABCD", "abcdEghe"));    printf("ret: %d\n", ic_strcmp("GeeksForGeeks", "gEEksFORGeEKs"));    printf("ret: %d\n", ic_strcmp("GeeksForGeeks", "geeksForGeeks"));    return 0;}

Output:

ret: 1
ret: -1
ret: -1
ret: -1
ret: 0
ret: 0

Time Complexity: O(min(|s1|, |s2|))

Auxiliary Space: O(1)