Encryption with swapping letters in C

Question

I need to write a program that reads from the input a sentence terminated by a new line. The program should write an encryption of the sentence in a way that the letter 'a' is swapped with 'z', 'b' with 'y' and so on. I am not supposed to use arrays, that is why I can't find a solution for this problem.

This is my code:

#include<stdio.h>

int main() {
    char alphabet, temp;
    while( scanf("%c", &alphabet) != '\n') {
        for(char i ='a', j='z'; i <= 'z', j>='a'; i++, j--) {
            if(alphabet == i) {
                temp = j;     
            }     
        }   
        printf("%c", temp);  
    }
    return 0;
}

When I input: abc as an output I get zyxxxxxxxxxxxxxxxxxxxxxxxxx(an infinite number of x).

So to sum up: Input:abc. Output: zyxxxxx(infinite number of x).

`scanf` returns the number of items successfully scanned. In your case it will be `0` or `1`. — Klas Lindbäck, Sep 15 '17 at 09:01
using a loop to swap letters? don't do that. use `temp = 'z' - (alphabet - 'a');` for instance — Jean-François Fabre, Sep 15 '17 at 09:02

Gerhardh · Answer 1 · 2017-09-15T09:28:18.433

3

You messed up the return value of scanf:

while( scanf("%c", &alphabet) != '\n'){

You want to stop the loop when the entered character (in alphabet) is a line break. But you test the return value of scanf which will never be '\n' as it returns the number of converted fields.

You could try like this:

while( scanf("%c", &alphabet) == 1 && alphabet != '\n'){

edited Sep 15 '17 at 09:28

answered Sep 15 '17 at 09:02

Gerhardh

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But why do you check == 1? – Marija Lyka Sep 15 '17 at 09:47
After reading the man page for `scanf`, which value would you suggest to fit better? – Gerhardh Sep 15 '17 at 09:48
I have no idea, that is why I'm asking :/ – Marija Lyka Sep 15 '17 at 09:49
It sais: "These functions return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.". You want to scan 1 field... – Gerhardh Sep 15 '17 at 09:50
Now I get it. Thanks! – Marija Lyka Sep 15 '17 at 09:51

gsamaras · Accepted Answer · 2017-09-15T09:16:21.363

2

Compile your code with warnings enabled (e.g. Wall flag)! This:

for(char i ='a', j='z'; i <= 'z', j>='a'; i++, j--)

should give:

warning: left-hand operand of comma expression has no effect [-Wunused-value]

You need to use the logical AND operator && and not a comma between the two comparissons.

Moreover, use getchar() instead, like this:

#include<stdio.h>

int main() {
    char temp;
    int ch;
    while ((ch = getchar()) != '\n' && ch != EOF) {
        for(char i ='a', j='z'; i <= 'z'&& j>='a'; i++, j--){
            if(ch == i){
                temp = j;     
            }     
        }
        if(ch <= 'z'&& ch >= 'a') // do not print if 'ch' is a digit for example
            printf("%c", temp);  
    }
    return 0;
}

Output:

zyx

If you have to use scanf(), then try reading into alphabet, check the return value of scanf, and compare to newline, like this:

while(scanf("%c", &alphabet) != 0 && alphabet != '\n')

edited Sep 15 '17 at 09:16

answered Sep 15 '17 at 09:02

gsamaras

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`printf("%c", temp);` : It is also used when the entered characters are not alphabetic lowercase letters. It may be an uninitialized value. or it may be the most recent value. – BLUEPIXY Sep 15 '17 at 09:11
`scanf` can return `EOF` which can be non-zero. Test succesful matches `== 1` instead of doing `!= 0`. – user694733 Sep 15 '17 at 09:27
1

But what if i input "abc cba". Then I get "zyxxxyz". Where does the space go? – Marija Lyka Sep 15 '17 at 09:27

Encryption with swapping letters in C

2 Answers2