将char数组元素转换为大写或小写

Question

For my homework assignment we are asked to get a string from the user and:

1. convert it all to uppercase = completed
2. convert it all to lowercase = completed
3. convert the each character if it is upper to lower and vice versa. (problem)

I do not know if my logic is wrong or if it's a simple fix. Any suggestions are appreciated.

#include <stdio.h>


int main()
{
int i;
char sentense [30];
printf("Please enter a sentence\n");
fgets(sentense, 30, stdin);

for(i=0; sentense[i] != '\0'; i++)
{
    putchar(toupper(sentense[i]));
}

for(i=0; sentense[i] != '\0'; i++)
{
    putchar(tolower(sentense[i]));
}

for(i=0; sentense[i] != '\0'; i++)
{
    if(sentense[i] >='65' && sentense[i] <='90')
    {
        putchar(tolower(sentense[i]));
    }


    else if(sentense[i] >= '97' && sentense[i] <='122')
    {
        putchar(tolower(sentense[i]));
    }
    else
    {

    }
}



return 0;
}

Answer 1

You need to change

if(sentense[i] >='65' && sentense[i] <='90')

to

if(sentense[i] >= 65 && sentense[i] <= 90)

and rest case(s) as we want to compare the integer value here.

Enable the compiler warning and your compiler should be warning you about the mistake.

Alternatively, you can also make use of isupper() / islower() library functions, too.

Answer 2

• you should use isupper and islower to prevent the code to depend on character codes.
• you should use toupper instead of tolower to convert letters to uppercase.
• you may have to print characters other than English alphabets.

Fixed code:

#include <stdio.h>
#include <ctype.h>


int main()
{
    int i;
    char sentense [30];
    printf("Please enter a sentence\n");
    fgets(sentense, 30, stdin);

    for(i=0; sentense[i] != '\0'; i++)
    {
        putchar(toupper(sentense[i]));
    }

    for(i=0; sentense[i] != '\0'; i++)
    {
        putchar(tolower(sentense[i]));
    }

    for(i=0; sentense[i] != '\0'; i++)
    {
        if(isupper(sentense[i]))
        {
            putchar(tolower(sentense[i]));
        }


        else if(islower(sentense[i]))
        {
            putchar(toupper(sentense[i]));
        }
        else
        {
            putchar(sentense[i]);
        }
    }
    return 0;
}

Answer 3

Easy to understand, you should compare directly. Let's use:

if(sentense[i] >='A' && sentense[i] <='Z') 

to check the up-case. And use:

if(sentense[i] >='a' && sentense[i] <='z') 

to check the low-case.