为什么我的C代码跳过下一个用户输入？

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;
}

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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