How to encrypt a String in C - c

Question

I am kind of new to coding and in this particular situation I am trying to change each character in a string.
My first attempt is to use a pointer to access each character (since I think it is some sort of an array so I won´t need an index for loop) and change it to '%'
Else I want to change the current location of the pointer to be +5 letters in the alphabet.
At the ende the pointer should move one forward.
So this string = "This is confidential information"
should look like this = "Ymnx%nx%htsknijsynfq%nsktwrfynts"
There is an Error at line 22 at the "if" statement where i want change the empty char to %
#include <stdio.h>
#include <ctype.h>
void encryptString(char *sPtr);
int main(void)
{
char string[] = "This is confidential information"; //given string to encrypt
printf("The string before encrypting is: %s", string);
encryptString(string);
printf("\nThe string after encrypting is: %s\n", string);
}
void encryptString(char *sPtr)
{
while(*sPtr != '\0'){ //loop is running until end of string
if(*sPtr == ''){ // should convert space char into a '%')
char *sPtr = '%';
}
else{ //should shift the output of character +5 in the alphabet
char *sPtr =+ 5;
}
++sPtr; //make sPtr point to the next character
}
}

Answer 1

There were some issues in your code, especially the way your have written x =+ 5. Also, I don't understand why you added the char in front of the assignment char *sPtr =+ 5. As it is written C interprets this as a cast to char, but dereferencing a pointer to char already has the char type, so it is useless at least and dangerous at most.. It actually creates a new variable that shadows the outer sPtr, sets it to point to some weird low memory location, then doesn't dereference it
Lastly you can think about what happen when the alphabet is finished, for instance when there is a z character. I have here your code fixed with an idea on how to wrap around with a ASCII table. Please mind that the C standard does not HAVE TO use ASCII, so this implementation won't work everywhere, it's more of an idea for you.
#include <stdio.h>
#include <ctype.h>
#define START_ALPHABET_ASCII 65
#define ALPHABET_LENGTH_ASCII (123-START_ALPHABET_ASCII)
void encryptString(char *sPtr);
int main(void)
{
char string[] = "This is confidential information zzz"; //given string to encrypt
printf("The string before encrypting is: %s", string);
encryptString(string);
printf("\nThe string after encrypting is: %s\n", string);
}
void encryptString(char *sPtr)
{
while(*sPtr != '\0'){ //loop is running until end of string
if(*sPtr == ' '){ // should convert space char into a '%')
*sPtr = '%';
}
else{ //should shift the output of character +5 in the alphabet
*sPtr = ((*sPtr)+5 - START_ALPHABET_ASCII)%(ALPHABET_LENGTH_ASCII) + START_ALPHABET_ASCII ;
}
++sPtr; //make sPtr point to the next character
}
}
Which prints
The string before encrypting is: This is confidential information zzz
The string after encrypting is: Ymnx%nx%htsknijsynfq%nsktwrfynts%EEE