C++ tolower()

#include <iostream>
#include <cctype>
using namespace std;

int main() {

  // convert 'A' to lowercase
  char ch = tolower('A');

  cout << ch;

  return 0;
}

// Output: a

tolower() Syntax

The syntax of the tolower() function is:

tolower(int ch);

tolower() Parameters

The tolower() function accepts the following parameter:

• ch - a character, casted to int type or EOF

tolower() Return Value

The tolower() function returns:

• For Alphabets - the ASCII code of the lowercase version of ch
• For Non-Alphabets - the ASCII code of ch

tolower() Prototype

The function prototype of tolower() as defined in the cctype header file is:

int tolower(int ch);

As we can see, the character argument ch is converted to int i.e. its ASCII code.

Since the return type is also int, tolower() returns the ASCII code of the converted character.

tolower() Undefined Behavior

The behaviour of tolower() is undefined if:

• the value of ch is not representable as unsigned char, or
• the value of ch is not equal to EOF.

Example 1: C++ tolower()

#include <cctype>
#include <iostream>
using namespace std;

int main() {
  char c1 = 'A', c2 = 'b', c3 = '9';

  cout << (char) tolower(c1) << endl;
  cout << (char) tolower(c2) << endl;
  cout << (char) tolower(c3);

  return 0;
}

Output

a
b
9

Here, we have converted the characters c1, c2, and c3 to lowercase using tolower().

Notice the code for printing the output:

cout << (char) tolower(c1) << endl;

Here, we have converted the return value of tolower(c1) to char using the code (char) tolower(c1).

Also notice that initially:

• c2 = 'b' and so tolower() returns the same value
• c3 = '9' and so tolower() returns the same value

Example 2: C++ tolower() without Type Conversion

#include <cctype>
#include <iostream>
using namespace std;

int main() {
  char c1 = 'A', c2 = 'b', c3 = '9';

  cout << tolower(c1) << endl;
  cout << tolower(c2) << endl;
  cout << tolower(c3);

  return 0;
}

Output

97
98
57

Here, we have converted the characters c1, c2, and c3 to lowercase using tolower().

However, we have not converted the returned values of tolower() to char. So, this program prints the ASCII values of the converted characters, which are:

• 97 - the ASCII code of 'a'
• 98 - the ASCII code of 'b'
• 57 - the ASCII code of '9'

Example 3: C++ tolower() with String

#include <cctype>
#include <iostream>
#include <cstring>
using namespace std;

int main() {
  char str[] = "John is from USA.";
  char ch;

  cout << "The lowercase version of \"" << str << "\" is " << endl;

  for (int i = 0; i < strlen(str); i++) {

    // convert str[i] to lowercase
    ch = tolower(str[i]);

    cout << ch;
  }

  return 0;
}

Output

The lowercase version of "John is from USA." is 
john is from usa.

Here, we have created a C-string str with the value "John is from USA.".

Then, we converted all the characters of str to lowercase using a for loop. The loop runs from i = 0 to i = strlen(str) - 1.

for (int i = 0; i < strlen(str); i++) {
...
}

In other words, the loop iterates through the whole string since strlen() gives the length of str.

In each iteration of the loop, we convert the string element str[i] (a single character of the string) to lowercase and store it in the char variable ch.

ch = tolower(str[i]);

We then print ch inside the loop. By the end of the loop, the entire string has been printed in lowercase.