How to do it…

1. Create a string called str to input your sentence. As usual, the last character will be a null character:

char str[255];

2. Enter a sentence of your choice:

printf("Enter a sentence: ");

3. Execute the gets function to accept the sentence with blank spaces between the words, and initialize the i variable to 0, since each character of the sentence will be accessed through i:

gets(str);
i=0

4. Execute a while loop to access each letter of the sentence one by one, until the null character in the sentence is reached:

while(str[i]!='\0')
{
    { …
    }
}
i++;

5. Check each letter to verify whether it is a lowercase vowel. If the accessed character is a lowercase vowel, then the value 32 is subtracted from the ASCII value of that vowel to convert it to uppercase:

if(str[i]=='a' ||str[i]=='e' ||str[i]=='i' ||str[i]=='o' ||str[i]=='u')
    str[i]=str[i]-32;

6. When all of the letters of the sentence have been accessed, then simply display the entire sentence.

The convertvowels.c program for converting the lowercase vowels in a sentence to uppercase is as follows:

#include <stdio.h>
void main()
{
    char str[255];
    int  i;
    printf("Enter a sentence: ");
    gets(str); 
    i=0;
    while(str[i]!='\0')
    {
        if(str[i]=='a' ||str[i]=='e' ||str[i]=='i' ||str[i]=='o' 
        ||str[i]=='u')
            str [i] = str [i] -32;
        i++;
    }
    printf("The sentence after converting vowels into uppercase 
    is:\n");
    puts(str);
}

Now, let's go behind the scenes to understand the code better.