C looping using EOF, how to read next line?

Question

I'm facing an issue while trying to resolve a particular problem in C. User input must be read in encrypted form and the output must be the decrypted message, that is, it is a replacement cipher.The problem is that when I read the first input, the program loops printing the decrypted message, it doesn't ask to read other inputs.
I'm really confused about this as I was told that I should use EOF, something I'm not very knowledgeable about. I could even solve it another way, but I need to implement the EOF, and because I don't know that, I'm having difficulty solving the exercise.

I am aware that the code I created is not the best for the resolution, however, it might work if someone helps me with this loop issue.

Source Code

#include <stdio.h>
#include <string.h>

void alterChar(int i, char phrase[])
{
    for (i = 0; phrase[i] != '\0'; i++)
    {
        if (phrase[i] == '@')
        {
            phrase[i] = 'a';
        }
                
        if (phrase[i] == '&')
        {
            phrase[i] = 'e';
        }
                
        if (phrase[i] == '!')
        {
            phrase[i] = 'i';
        }
                
        if (phrase[i] == '*')
        {
            phrase[i] = 'o';
        }
                
        if (phrase[i] == '#')
        {
            phrase[i] = 'u';
        }
                
        printf("%s\n", phrase);
    }
}

int main()
{
    int i;
    char phrase[256];
    
    while (scanf("%[^\n]s", phrase) != EOF)
    {
        ///scanf(" %[^\n]s", phrase);
    
        alterChar(i, phrase);
    }
    
    return 0;
}

Answer 1

The line

while (scanf("%[^\n]s", phrase) != EOF)

is wrong, for several reasons:

1. You seem to be undecided on whether to use the %s or the %[] conversion format specifier. The character s is not part of the %[] format specifier. Therefore, when using your format string, scanf will attempt to match a literal s. This is probably not what you want, so it should be removed from the format string.

2. If scanf is unable to match any characters because the line is empty, it will return 0 and will not touch the contents of phrase. However, you are incorrectly treating the return value of 0 as success.

3. If the user enters more than 255 characters, then you will be writing to the array phrase out of bounds, causing undefined behavior (i.e your program may crash). Therefore, you should limit the number of matched characters to 255 characters, so that the function won't write more than 256 characters (including the terminating null character of the string).

For the reasons stated above, you should change the line to the following:

while ( scanf( "%255[^\n]", phrase ) != 1 )

Also, this scanf statement will not consume the newline character from the input stream, so the next function call to scanf will fail. Therefore, you must call a function to consume the newline character from the input stream, for example by calling getchar, or by telling scanf to consume all whitespace input beforehand, like this:

while ( scanf( " %255[^\n]", phrase ) != 1 )

Alternatively, you could use the function fgets instead (which I recommend), as that function will also consume the newline character and write it to the string.

while ( fgets( phrase, sizeof phrase, stdin ) != NULL )

When using the function fgets, you should also verify that an entire line was read in, for example by writing the following code afterwards:

//verify that entire line was read in
if ( strchr( phrase, '\n' ) == NULL )
{
    printf( "line to long for input buffer!\n" );
    exit( EXIT_FAILURE );
}

Note that if you decide to use the function fgets, you will have an additional newline character at the end of the line, instead of only a '\0', so in the function alterChar, you will have to change the line

for (i = 0; phrase[i] != '\0'; i++)

to

for (i = 0; phrase[i] != '\n'; i++)

Another problem in your code is that the line

alterChar(i, phrase);

does not make sense. You are passing the value of an uninitialized variable to the function.

You should probably change the function prototype from

void alterChar(int i, char phrase[])

to

void alterChar( char phrase[] )

and maybe also give the function a better name, as it does not deal with single characters, but whole strings, and also prints it:

void alterAndPrintPhrase( char phrase[] )

Also, in that function, you should move the line

printf("%s\n", phrase);

outside the loop.

After applying all the fixes mentioned above, your code should look like this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void alterAndPrintPhrase( char phrase[] )
{
    for ( int i = 0; phrase[i] != '\n'; i++ )
    {
        if (phrase[i] == '@')
        {
            phrase[i] = 'a';
        }
                
        if (phrase[i] == '&')
        {
            phrase[i] = 'e';
        }
                
        if (phrase[i] == '!')
        {
            phrase[i] = 'i';
        }
                
        if (phrase[i] == '*')
        {
            phrase[i] = 'o';
        }
                
        if (phrase[i] == '#')
        {
            phrase[i] = 'u';
        }
    }

    printf( "%s\n", phrase );
}

int main()
{
    char phrase[256];
    
    while ( fgets( phrase, sizeof phrase, stdin ) != NULL )
    {
        //verify that entire line was read in
        if ( strchr( phrase, '\n' ) == NULL )
        {
            printf( "line to long for input buffer!\n" );
            exit( EXIT_FAILURE );
        }

        alterAndPrintPhrase( phrase );
    }
    
    return 0;
}

This program has the following behavior:

Input:

Th!s !s @ t&st.

Output:

This is a test.

Answer 2

i think just putting the printf outside of the loop help and use feof (fetch en of file) to know where you are.

#include <stdio.h>
#include <string.h>

void alterChar(char phrase[]) {
    for (int i = 0; phrase[i] != '\0'; i++) {
        if (phrase[i] == '@') {
            phrase[i] = 'a';
        }

        if (phrase[i] == '&') {
            phrase[i] = 'e';
        }

        if (phrase[i] == '!') {
            phrase[i] = 'i';
        }

        if (phrase[i] == '*') {
            phrase[i] = 'o';
        }

        if (phrase[i] == '#') {
            phrase[i] = 'u';
        }
    }
    printf("%s\n", phrase);
}

int main() {
    char phrase[256];

    while (!feof(stdin)) {
        scanf(" %[^\n]s", phrase);
        alterChar(phrase);
    }

    return 0;
}