Why is my C code skipping the next user input?

Question

I'm trying to convert my Caesar Cipher code from taking a user given argument to using a user given input, but it's not going the way I've intended at all. I have this code, and it asks the first input for the ROT number, but then it skips the input for the rest of the code. Now, if I wanted to rotate by 2 and use the string bB, the output should be dD, and it is, but only if, when aksed for the input, you put "2 bB". I don't know why this is, and I've looked at other threads saying to just put scanf("%c", &blah);, but I don't know how to do this in my situation. Any help is thankful.

Edit: Changed char to int, as I did in my code just before I posted this.

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

int main(){
/**********************************************************************************************************************************/
    int bytesRead;
    int nbytes = 255;
    char *encryptString;
    encryptString = (char *) malloc (nbytes + 1);
    //char encryptString[256];
    char finalChar;
    char finalString[256];
    int rotNum;
/**********************************************************************************************************************************/    
puts("Please enter the ROT (rotate) number you wish to encrypt by: ");
scanf("%d", &rotNum);

printf("Please enter the phrase you'd like to encrypt: \n");
fgets(encryptString, sizeof(encryptString), stdin);
printf("The string entered is: %s\n", encryptString);
printf("The encrypted version is: ");

int n = strlen(encryptString) - 1;

int i;
for(i = 0; i < n; i++){     //For loop to go through the entire string entered
    if(isupper(encryptString[i])){
        finalChar = (((encryptString[i] - 65) + rotNum) % 26) + 65;
        finalString[i] = toupper(finalChar);
        //printf("%c\n", finalChar);
    }

    else if(islower(encryptString[i])){
        finalChar = (((encryptString[i] - 97) + rotNum) % 26) + 97;
        finalString[i] = tolower(finalChar);
        //printf("%c\n", finalChar);
    }

    else{
        finalChar = ' ';
        finalString[i] = finalChar;
    }

    printf("%c", finalString[i]);
}
printf("\n");
return 0;
}

it would help to know what it prints instead of the right answer ;) — Azael
– Azael, Commented Oct 14, 2014 at 6:31
amongst other things, this line: scanf("%d", &rotNum); is not reading the trailing '\n' from the first entry. Perhaps your code could be written as: scanf("%d\n", &rotNum); — user3629249
– user3629249, Commented Oct 15, 2014 at 8:44

P.P · Accepted Answer · 2014-10-14 06:44:27Z

2

You have few problems in your code:

1)

scanf("%d", &rotNum);

Here you are passing a char * to scanf(). Declare rotNum as int.

2) After reading the input rotNum, scanf() leaves a '\n' in the input buffer.fgets(); stops reading input once encounters a \n. So fgets() doesn't read at all. Use getchar(); after scanf() call to consume the newline char. Or better, read the rotNum using fgets() and parse it using sscanf().

3) Your second argument to fgets() is wrong.

fgets(encryptString, sizeof(encryptString), stdin);

Here, encryptString is a pointer. So this will give you the size of pointer on your platform, not the number of bytes (256) that it points to. Change it to:

fgets(encryptString, 256, stdin); // or whatever the bytes you allocate

Additioanlly,

1) Use a proper return type for main() such as int main(void) or int main(int argc, char **argv) or equivalent.
2) Check the return value of malloc() for NULL to see if it's failed.
3) Casting the malloc() return is unnecessary and error-prone.

answered Oct 14, 2014 at 6:44

P.P

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3 Comments

Voz Over a year ago

I'm reading the man page for sscanf, but it doesn't make a lot of sense to me. How do I parse it into sscanf?

P.P Over a year ago

For example:

char temp[256]; fgets(temp, sizeof(temp), stdin); /*input a number*/ if (sscanf(temp, "%d", &rotNum) != 1){ /* input failure, handle error */}

Voz Over a year ago

THANK YOU. Now I understand how fgets and sscanf work exactly. This "fun" project just turned into a two hour project to get those two lines of code. Thank you a ton.

Ashwani · Accepted Answer · 2014-10-14 06:47:12Z

0

I made three changes to your code and it worked without any problem:

1. Use  "int rotNum" instead of "char rotNum"
2. Use  "scanf("%s", encryptString)" instead of "fgets(encryptString, sizeof(encryptString), stdin)"
3. Either use "int n = strlen(encryptString)" or "for(i = 0; i <= n; i++)"

You have to make some other changes to make it work for negative rotNum.

answered Oct 14, 2014 at 6:47

Ashwani

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Comments

arya · Accepted Answer · 2014-10-14 07:10:56Z

0

as the man page of fgets says ,

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline.

So when you enter rotate number & then hit enter,the input buffer will contain number\n. While number will be stored in rotNum , \n will remain in stdin. So fgets will read it & returns without waiting for input.

Use scanf instead of fgets. If you are using linux machine, here is the man page

edited Oct 14, 2014 at 7:10

answered Oct 14, 2014 at 6:39

arya

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1 Comment

hyde Over a year ago

But if you suggest using scanf, you should include the format string to read entire line of text with it.

Azael · Accepted Answer · 2014-10-14 06:44:16Z

-1

Try fflush(stdin) before using fgets. This should clear the \n from stdin

answered Oct 14, 2014 at 6:44

Azael

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1 Comment

hyde Over a year ago

Behaviour of fflush(stdin) is undefined, don't use it.

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