C/C++ Coding Exercisse – How to Implement ToLowerCase Function?

Implement function ToLowerCase() that has a string parameter str, and returns the same string in lowercase.

Allocate a string and assign input string to it. Change uppercase letters to lowercase by adding 32 to its ASCII code.

class Solution {
public:
    string toLowerCase(string str) {
        auto r = str;
        for (auto i = 0; i < str.length(); ++ i) {
            if (r[i] >= 'A' && r[i] <= 'Z') {
                r[i] += 32;
            } 
        }
        return r;
    }
};

Alternatively, you can modify the string inplace without allocating a new string, and return it.

class Solution {
public:
    string toLowerCase(string str) {
        for (auto i = 0; i < str.length(); ++ i) {
            if (str[i] >= 'A' && str[i] <= 'Z') {
                str[i] += 32;
            } 
        }
        return str;
    }
};

The C solution:

char* toLowerCase(char* str) {
    char* s = str;
    while (*str != '\0') { // not the end of the string
        if (*str >= 'A' && *str <= 'Z') {
            *str = *str + 32;
        }
        ++ str;
    }
    return s;
}

The string is an array of characters, which can be accessed by moving the pointer.

Using std::transform in C++

We can use the std::transform in C++ to transform a vector to another using a customize transform function, which we will use the std::tolower function to convert to a lower character.

class Solution {
public:
    string toLowerCase(string str) {
        std::transform(begin(str), end(str), begin(str), [](auto &ch) { 
            return std::tolower(ch); 
        });
        return str;
    }
};

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