Hi there👋can anybody help me to explain this code(especially this lines (10-22)thanks(:

Question

1)#include<stdio.h>
2)int main()
3){
4)char message[100], ch;
5)int i, key;
6)printf("Enter a message to encrypt: ");
7)gets(message);
8)printf("Enter key: ");
9)scanf("%d", &key);
10)for(i = 0; message[i] != '\0'; ++i){
11)ch = message[i];
12)if(ch >= 'a' && ch <= 'z'){
13)ch = ch + key;
14)if(ch > 'z'){
15)ch = ch - 'z' + 'a' - 1;
16)}
17)message[i] = ch;
18)}
19)else if(ch >= 'A' && ch <= 'Z'){
20)ch = ch + key;
21)if(ch > 'Z'){
22)ch = ch - 'Z' + 'A' - 1;
23)}
24)message[i] = ch;
25)}
26)}
27)printf("Encrypted message: %s", message);
28)return 0;
29)}

Answer 1

gets(message); //dont use gets , use fgets or scanf..

it accepts a string of input, and a intiger key value that is used for encryption
like for example if input is "abcdz" and key is 1, then encryption happens like:
ch='a' and ch+key => 'a'+1= 'b' and next charecter from string is 'c'
ch='b' and ch+key => 'b'+1= 'c'
ch='c' and ch+key => 'c'+1= 'd'
ch='d' and ch+key => 'd'+1= 'e'
ch='z' and ch+key => 'z'+1 is a special symbol
by code :
for(i = 0; message[i] != '\0'; ++i){
ch = message[i];
if(ch >= 'a' && ch <= 'z'){
ch = ch + key;

but now ch >'z' so
if(ch > 'z'){
ch = ch - 'z' + 'a' - 1;
=>
'z'+1 - 'z'+'a'-1 => 'a' (like turning around from 'a' again)
so now ch='a';

//this code works same but for capital letters
else if(ch >= 'A' && ch <= 'Z'){
ch = ch + key;
if(ch > 'Z'){
ch = ch - 'Z' + 'A' - 1;
}
message[i] = ch;
}
}
now at end encrypted message for input "abcdz" is "bcdea"

it just making a charecter farward of key position from alphabet
read about ASCII values of char
hope it helps.

Answer 2

Prefer using code Playground to post codes.