Error in storing char as variable from user input

Question

the idea is that i type in a sentence and store it as a string... then i can choose a letter and change it to a different letter

int main(int argc, char *argv[])
{

    char string[100];
    char newLetter;
    char oldLetter;
    int i = 0;

    printf("Please enter your sentence : ");
    gets(string);

    printf("\n\nWord is : %s" , string );

    printf("\n\nTarget : ");
    scanf("%s", &oldLetter);
    printf("Replace with : ");
    scanf("%s", &newLetter);

    for ( i = 0; i < sizeof(string); i++) 
    { 
        if (string[i] == oldLetter)
        { 
            string[i] = newLetter; 
            break;
        } 
    }
    printf("\n\nWord is : %s" , string );

    system("PAUSE");   
    return 0;
}

any help in where I've gone wrong is appreciated

eg. input could be - yellow lorry red lorry

then target - r

change to - t

output - yellow lotty ted lotty

Dangerous code. scanf() will keep scanning and the filling in data into memory until Uth finds a white-space character, but you gave it only a byte of memory. — reza.safiyat
– reza.safiyat, Commented Mar 5, 2014 at 21:16
I would recommend you use the loop construct like this: while(string[i++] != '\0') { if(string[i] == oldLetter) string[i] = newLetter; } — reza.safiyat
– reza.safiyat, Commented Mar 5, 2014 at 21:27

reza.safiyat · Accepted Answer · 2014-03-05 21:18:01Z

1

Change the %s in the scanning of the two letters to %c and the code will run flawlessly.

answered Mar 5, 2014 at 21:18

reza.safiyat

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2 Comments

Utkan Gezer Over a year ago

it most probably still won't

Utkan Gezer Over a year ago

Basically because of the new-line character \n left inside the stream, which will get recognized as the value to be scanned into the newLetter, if not discarded beforehand.

Utkan Gezer · Accepted Answer · 2014-03-05 22:03:53Z

0

First things first, since you are aiming to store a character to char oldLetter and char newLetter via scanf, you should be using the format specifier %c instead of the %s.

However, that won't be enough, because of the following: When you use functions like scanf or gets, you prompt user to input characters to the stdin stream. stdin stream is a buffered stream. You may think of it as a:

Stream of river that you;
drop characters into
each character you drop remains inside the river
until something takes them out

When scanf comes, and you type, say, A and then press the enter key, you put the following characters in the stream:

'A' '\n'

Where \n is the new-line character. With the enter key-press, you also inform the scanf that you're done. scanf then starts issuing the stdin buffer, let's see... 'A', a proper value for %c character. It takes that one out, leaves \n behind.

Then the next scanf comes, seeks for a %c in stream, finds the \n ready, takes that out. This is what you wouldn't want. Two ways to prevent it:

use fflush( stdin ); after the scanf calls, or
use while( getchar( ) != '\n' ); after the scanf calls

to dismiss/discard the remaining characters in the buffer.

And get rid of that break; if you want to replace each occurrence and not just the first one.

edited Mar 5, 2014 at 22:03

answered Mar 5, 2014 at 21:40

Utkan Gezer

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20 Comments

this Over a year ago

No. fflush( stdin ) is undefined behavior.

Utkan Gezer Over a year ago

@self. Dude, OP uses scanf( "%c", ... ) to get a character from standard input, and you make fflush( stdin ) a problem? One cannot please everyone, but hey, I tried, and offered two alternatives there. Second one also isn't perfect, I'd say while ( stdin->_cnt ) getchar( ); would pretty much be perfect, but that would just be an overkill, for the reason at the beginning of this comment.

this Over a year ago

Also while( getchar( ) != '\n' ); should check for != EOF.

this Over a year ago

Because you don't check for EOF and might miss it, causing an inf loop.

Utkan Gezer Over a year ago

@self. That doesn't explain anything. Explain me why EOF should not be missed when each line is terminated by a '\n'.

|

M.M · Accepted Answer · 2014-03-07 07:53:10Z

0

You can do it right, or you can do it wrong and hope that the person doesn't enter the right series of keystrokes or pipe in a file that doesn't end in newline, to break it ... up to you

answered Mar 7, 2014 at 7:53

M.M

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