C Program Looping Incorrectly - c

Question

I'm just a beginner and I'm trying to use whatever I know to make a simple program that:
Asks the user to input the letter 'S' or 's'. The program loops if 's' is not input. If the user does input 's', the program then
Asks the user to input a number, 1 or 2. The program loops if the incorrect number is input.
The problem I'm having is that after 's' is successfully input and the user is asked to enter a number, if an incorrect number is input (not 1 or 2) the program asks the user to input a letter again from the beginning which is incorrect. The program loops from the very beginning and doesn't work anymore. Can anyone help me fix this please?
#include <stdio.h>
#include <ctype.h>
int function(int num);
int main()
{
char input,ch,temp,c[64],exit;
int i,invalid,num,index,flag,day;
invalid = 0;
num = 0;
size_t length = 0;
index = 0;
flag = 0;
do
{
puts("Enter the letter S to start the program:");
scanf("%c", &input);
while( input!='\n' && (ch=getchar())!='\n' && ch!= EOF);
{
if(isalpha(input)==0)
{
printf("Invalid input. Please input something.\n");
continue;
}
if(input == 'S' || input == 's')
{
printf("\nProgram start.");
while( sscanf(c, "%d", &num) != 1)
{
length = 0;
flag = 0;
num = 0;
printf("\nEnter 1 for Module A. Enter 2 for Module B. Enter here: ");
fgets(c, 63, stdin);
length = strlen(c);
for(index = 0; index < length; ++index)
{
if(c[index] < '0' || c[index] > '9')
{
flag = 1;
break;
}
}
if( flag)
{
printf("\nInvalid character\n");
continue;
}
if( sscanf(c, "%d", &num) != 1)
{
printf("\nNo input detected.");
continue;
}
if(num == 1)
{
printf("\nModule A Selected.\n");
return(0);
}
if(num == 2)
{
printf("\nModule B Selected.\n");
return(0);
}
}
}
else
{
printf("\nInvalid input.");
continue;
}
}
}
while(1);
}

Answer 1

Make the scanf into like this.
scanf(" %c",&input);
Then While getting the input from the user using fgets It will place the new line character into that buffer. So this will lead to fails this condition.
if(c[index] < '0' || c[index] > '9')
{
flag = 1;
break;
}
So make the this condition into like this.
length=strlen(c)-1;// to skip the new line character
Or else to like this.
length=strlen(c);
if ( c[length] == '\n' )
c[length]='\0';
Output After placing this,
Enter the letter S to start the program:
S
Program start.
Enter 1 for Module A. Enter 2 for Module B. Enter here: 1
Module A Selected.
Make this in you code.
if(num == 1)
{
printf("\nModule A Selected.\n");
return(0);
}
else if(num == 2)
{
printf("\nModule B Selected.\n");
return(0);
}
else
{
printf("\nInvalid option\n");
c[0]='\0'; // It is for satisfy the second while loop condition.
continue;
}

Answer 2

Note that the loop:
while( input!='\n' && (ch=getchar())!='\n' && ch!= EOF);
is limited to the one line by the semicolon at the end. The following code is not the body of the loop, despite indentation trying to pretend that it is.
Also note that getchar() returns an int, not a char; you cannot reliably assign the result to a char and then test it for EOF. Depending on the platform, you will either never detect EOF at all or you will misdetect EOF when some other character (often ÿ, y-umlaut, U+00FF, LATIN SMALL LETTER Y WITH DIAERESIS) is typed. You must use int ch;.

Answer 3

Here. I fixed the problem using the following code. This way the code does the following:
Scans letters 'S' or 's'. Keeps looping if these are not entered.
Scans either number 1 or 2. Keeps looping until either number is entered and then exits.
The program does not loop from the very beginning (by outputting "Enter 'S' to start program), if any number other than 1 or 2 in entered in part 2 of the program. This was the problem originally.
The following is the correct code:
#include <stdio.h>
#include <ctype.h>
int function();
char input,temp,c[64],ch,exit;
int i,invalid,num,index,flag,start;
start = 0;
invalid = 0;
num = 0;
size_t length = 0;
index = 0;
flag = 0;
int main()
{
do
{
puts("Enter the letter S to start the program: ");
scanf("%c", &input);
while( input!='\n' && (ch=getchar())!='\n' && ch!= EOF);
{
if(isalpha(input)==0)
{
printf("Invalid input. Please input something.\n");
continue;
}
if(input == 'S' || input == 's')
{
printf("\nProgram start.");
start = 1;
if(start == 1)
{
function();
return(0);
}
}
else
{
printf("\nInvalid input.");
continue;
}
}
}
while(1);
}
int function()
{
while( sscanf(c, "%d", &num) != 1)
{
length = 0;
flag = 0;
num = 0;
printf("\nEnter 1 for Module A. Enter 2 for Module B. Enter here: ");
fgets(c, 63, stdin);
length = strlen(c);
length --;
for(index = 0; index < length; ++index)
{
if(c[index] < '0' || c[index] > '9')
{
flag = 1;
break;
}
}
if( flag)
{
printf("\nInvalid character\n");
continue;
}
if( sscanf(c, "%d", &num) != 1)
{
printf("\nNo input detected.");
continue;
}
if(num == 1)
{
printf("\nModule A Selected.\n");
return(0);
}
else if(num == 2)
{
printf("\nModule B Selected.\n");
return(0);
}
else
{
printf("\nInvalid option\n");
c[0]='\0'; // It is for satisfy the second while loop condition.
continue;
}
}
}