Switching case of each letter in a string - c

Question

#include <stdio.h>
int main() {
int i;
char arr[100];
for (i = 0; i < 100; i++)
scanf("%c", &arr[i]);
for (i = 0; i < 100; i++) {
if ('a' <= arr[i] && arr[i] <= 'z')
arr[i] =-32;
else
if ('A' <= arr[i] && arr[i] <= 'Z')
arr[i] =+32;
}
printf("%s", arr);
return 0;
}
There was a problem:
You have been given a String consisting of uppercase and lowercase English alphabets. You need to change the case of each alphabet in this String. That is, all the uppercase letters should be converted to lowercase and all the lowercase letters should be converted to uppercase. You need to then print the resultant String to output.
What is wrong with the above code? It is compiling successfully but there is a runtime error.

Answer 1

There are multiple problems in your code:
the main issue is the away you adjust the case: arr[i] =-32; does not decrement arr[i] by 32, but stores 32 into arr[i]. The combined assignment operator is spelled -=. You have the same problem for += in the other case.
Converting lower case to upper case by subtracting 32 works for ASCII, but is not portable to other character sets. Similarly, comparing to 'a' and 'z' works for ASCII, but not for EBCDIC. You should use the functions from <ctype.h>.
you read 100 characters with scanf("%c"...) but you do not check the return value, nor do you null terminate the array. Furthermore, you should read at most one less than the size of the array to leave space for the '\0' byte. As coded, your program invokes undefined behavior at printf("%s", arr); because arr is not null terminated.
Here is a corrected version:
#include <ctype.h>
#include <stdio.h>
int main(void) {
int c;
while ((c = getchar()) != EOF) {
if (isupper(c))
c = tolower(c);
else
if (islower(c))
c = toupper(c);
putchar(c);
}
return 0;
}

Answer 2

The most obvious problem is that you are not null-terminating the string, so when you call printf("%s", arr) the behavior will be unpredictable.

Answer 3

The problem with your code is that it never terminates the string. If you read 100 characters, and then you want to print them with %s in printf, you have to add a null terminator at the end, like this:
char arr[100+1]; // See +1
... // Reading code
arr[100] = '\0';
Note that library functions islower/isupper provide a portable, and a lot more readable, approach to testing character type. Similarly, tolower/toupper provide more information about your intentions to the reader of your code than simply adding and subtracting 32. Moreover, C code that uses these standard functions will work on systems with non-ASCII character encoding.

Answer 4

In order to printf("%s", arr), you need to terminate arr with a null-character.
One way to do it is by:
Declaring char arr[101]
Setting arr[100] = 0

Answer 5

Your code has serveral issues: not-null-terminated input string, unproper lower/uppercase conversion and confusion with =- / -= and =+ / += operators.
Next code is based on yours:
As an alternative to get a null-terminated string it uses fgets() instead of scanf(), just for the example.
Also uses C library functions for avoiding issues with different charsets, simplifing the conditions and for upper/lower case operations.
Edited to improve code quality, as #chqrlie suggested in a comment.
#include <stdio.h>
#include <ctype.h>
int main()
{
int i;
char arr[101];
printf("Enter string: ");
fgets(arr, sizeof(arr), stdin);;
for(i=0;i<strlen(arr); i++)
{
int value = (unsigned char) arr[i]; // to properly use int parameters, as expected by ctype.h next functions
if (isupper(value))
{
arr[i]=tolower(value);
}
else {
if (islower(value))
{
arr[i]=toupper(value);
}
}
}
printf("%s", arr);
return 0;
}
Test it here.