Use scanf(), gets() or fgets() function?

Question

I am trying to encrypt a string by Caesar encryption, but it only works for one word (no spaces between characters). I would like to encrypt whole sentence. I have read here about getting white spaces from keyboard into a string by gets() or fgets(), but I cannot make it functional.

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

int main(int argc, char** argv) {
    printf("Enter the shift: ");
    int shift,index,length;
    scanf("%d", &shift);

    printf("Enter the string to encrypt: " );
    char string[1000];
    //gets(string);
    scanf("%s",string);
    //fgets(string,1000,stdin);

    char encrypted[strlen(string) + 1];

    for(index = 0, length = strlen(string); index < length; index++ ){
        if (string[index]!=' ')
            encrypted[index] = 'A' + (string[index] -'A' + shift) % 26;
        else
            encrypted[index]=' ';
    }
    encrypted[ strlen(string)] = '\0';

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

    return (EXIT_SUCCESS);
}

How are your attempts "not functional"? What do you expect to happen, and what actually happens? — jwodder, Jun 26 '14 at 18:23
Replace the `scanf` with either a loop or a call to `fgets` -- lookup documentation using `man fgets` — Soren, Jun 26 '14 at 18:24
Please forget you ever heard about `gets`: It's just about impossible to use that safely. — Deduplicator, Jun 26 '14 at 18:24
Never ever use `gets()`, even in practice programs. [It is a horrible security hole](http://stackoverflow.com/questions/1694036). — Tom Zych, Jun 26 '14 at 18:25
for example: for input `string` "MY NAME" with shift 1 I expect variable `encrypted` to be"NZ OBNF", but for this code it would be only "NZ" — Dounchan, Jun 26 '14 at 18:29
`scanf(" %999[^\n]", string);`, though not bad, does not retain leading spaces. — chux - Reinstate Monica, Jun 26 '14 at 19:04
It is true, it ignores leading spaces, but they are not needed so far. — Dounchan, Jun 26 '14 at 19:19

David Ranieri · Answer 1 · 2014-06-27T07:31:56.447

In

char string[1000];
scanf("%s",string);

the scan terminates at whitespace.

To get the whole string (excluding the trailing newline) use:

char string[1000], *p;
fgets(string, sizeof(string), stdin);
if ((p = strchr(string, '\n')) != NULL) { 
    *p = '\0'; /* remove newline */
}

As pointed out by @chux you need to consume \n in scanf, I suggest:

int shift, index, length;
char string[1000], *p;

fgets(string, sizeof(string), stdin);
shift = atoi(string);
printf("Enter the string to encrypt: " );
fgets(string, sizeof(string), stdin);
if ((p = strchr(string, '\n')) != NULL) { 
    *p = '\0'; /* remove newline */
}

And you get:

Enter the shift: 1
Enter the string to encrypt: MY NAME
NZ OBNF

Likely has same problem as @Tom. `scanf("%d", &shift);` leaves a `'\n'` in `stdin`. — chux - Reinstate Monica, Jun 26 '14 at 18:45

score 0 · Answer 2 · answered Jun 26 '14 at 18:30

0

You're using scanf("%s",string);. That stops as soon as it hits whitespace. Instead, use

fgets(string, sizeof(string), stdin);

(Note that it's not a great idea to allocate a large string with something like char string[1000]; -- in most implementations, it goes on the stack. malloc is better.)

answered Jun 26 '14 at 18:30

Tom Zych

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I have already tried this, but program does NOT wait for my input. It allways put only '@' character as input string and then nothing happens. – Dounchan Jun 26 '14 at 18:37
1

@Dounchan change `scanf("%d", &shift);` to `fgets(string, sizeof(string), stdin); sscanf(string, "%d", &shift);` – chux - Reinstate Monica Jun 26 '14 at 18:38

BLUEPIXY · Answer 3 · 2014-06-26T19:41:01.870

0

The remaining line breaks without being consumed in the part of scanf("%d", &shift).
So, it appears that fgets and gets you to enter a new line up to not function.(Because they are given a new line remaining)

while(getchar()!='\n');//get rid of the newline up
scanf("%999[^\n]", string);//It corresponds to the `gets()`

edited Jun 26 '14 at 19:41

answered Jun 26 '14 at 19:33

BLUEPIXY

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Use scanf(), gets() or fgets() function?

3 Answers3