Problems with character input using scanf()

Question

I'm trying to input a character into a linked list, where the character can be 'A','a','G','g','T','t','C' or 'c'.

I'm not yet familiar with C and I know I've screwed something up here:

do{
  printf ("\nEnter a new nucleotide: \n");
  scanf("%c",&newChar);
          /* Checking */
  if(newChar == 'A' ||
     newChar == 'a' || 
     newChar == 'G' || 
     newChar == 'g' || 
     newChar == 'T' || 
     newChar == 't' || 
     newChar == 'C' || 
     newChar == 'c' )
  {
    AddToSequence(newChar);
    size++;
  } else {
    printf ("\nBad Element");
  }
}while(newChar != 'x');

newChar is initialized with a junk value, in this case 'q'.

Entering 'x' exits the loop, entering any acceptable value calls AddToSequence(), and any unacceptable value gets a warning.

For some reason, no matter what value is in newChar, it will jump to the else. It will also jump straight past the scanf without waiting for entry from the user and do two loops every time it loops. Can anyone tell me where I'm going wrong?

Full program:

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

/*Structure declaration for the node*/
struct node{
   char nucleotide;
   struct node *point;
}*start;

/* Adds a nucleotide to the chain. Creates a new linked list if no chain exists exists.*/
void AddToSequence(char nucleotide){
  struct node *loc, *first;
  //Dynamic memory is been allocated for a node
  first=(struct node*)malloc(sizeof(struct node));
  first->nucleotide=nucleotide;
  first->point=NULL;
  if(start==NULL){
    /*If list is empty*/
    start=first;
  }else{
    /*Element inserted at the end*/
    loc=start;
    while(loc->point!=NULL){
      loc=loc->point;
      loc->point=first;
    }
  }
}

/* Display elements */
void Display(){
  struct node *loc;
  if(start == NULL){
    printf ("\n\nList is empty");
    return;
  }
  loc=start;
  printf("\n\nList is : ");
  while(loc!=NULL){
    printf ("%c", loc->nucleotide);
    loc=loc->point;
  }
  printf ("\n");
}

/* Finds and displays percentage of the chain made up of each nucleotide. */
void Percentage(int size){
  struct node *loc;
  if(start == NULL){
    printf ("\n\nList is empty");
    return;
  }
  loc=start;
  printf("\n\nList is : ");
  int A = 0, G =0, T =0, C = 0;
  double Adouble = 0, Gdouble =0, Tdouble=0, Cdouble=0;
  while(loc!=NULL){
    if(loc->nucleotide=='A' || 'a'){A++;}
    if(loc->nucleotide=='G' || 'g'){G++;}
    if(loc->nucleotide=='T' || 't'){T++;}
    if(loc->nucleotide=='C' || 'c'){C++;}    
    loc=loc->point;   
  }
  printf ("\n"); 

  /* Convert to double for percentages as int loses precision */
  Adouble =A;
  Gdouble =G;
  Tdouble =T;
  Cdouble =C; 
  Adouble =(Adouble/size)*100;
  Gdouble =(Gdouble/size)*100;
  Tdouble =(Tdouble/size)*100;
  Cdouble =(Cdouble/size)*100; 
  printf("\nA: %f", Adouble);
  printf("\nG: %f", Gdouble);
  printf("\nT: %f", Tdouble);
  printf("\nC: %f", Cdouble); 
}

/* There be dragons beyond here */
int main(){
  int navigate, size =0;
  char newChar = 'q';
  do{ /* Menu */
    printf("\n 1. Create / Extend Sequence\n");
    printf("\n 2. Display Sequence\n");
    printf("\n 3. Count \n");
    printf("\n 0. Exit \n");
    printf("\nPlease select an option (0 to 3)\n");
    scanf("%d",&navigate);  
    switch (navigate){
      case 0: /* Exit */
        break;
      case 1: /* Add nucleotides */
        do{
          printf ("\nEnter a new nucleotide: \n");
          scanf("%c",&newChar);
          /* Some error checking */
          if(newChar == 'A' || newChar == 'a' || newChar == 'G' || newChar == 'g' || newChar == 'T' || newChar == 't' || newChar == 'C' || newChar == 'c' ){
            AddToSequence(newChar);
            size++;
          } else {
            printf ("\nBad Element");
          }
        }while(newChar != 'x');
        break;
      case 2:
        Display();
        break;
      case 3:
        Percentage(size);
        break;
      default:
        printf ("\n\nBad choice. Please select another.\n");
    }
  } while (navigate !=0); 
  return 0 ;
}

Answer 1

add space to "%c" to catch the newline character. the space charcter is used to catch space characters, tabulations, newline

scanf("%c ",&newChar);

Answer 2

You're leaving the '\n' on stdin:

scanf("%d",&navigate);  
getchar(); // consume the newline character
...
scanf("%c",&newChar);
getchar(); // consume the newline character

Or since you're already using scanf() you can tell scanf itself to take care of the newline character:

scanf("%d\n", &navigate);
....
scanf("%c\n",&newChar);

Even better you can leave it open by adding a space after the format specificer:

scanf("%d ", &navigate);
....
scanf("%c ",&newChar);

Just in case the user wants to do something like: 2<tab key><enter key>

Regardless of how you handle it, the point is you need to consume the newline character.

Answer 3

Use

newChar=getche();

This is a nonstandard function that gets a character from the keyboard, echoes to screen.

Answer 4

You don't handle the newline. The %c specifier doesn't skip blanks. Try:

scanf(" %c", &newChar);
    /* ^ <-- Makes `scanf` eat the newline. */

Or maybe add an explicit test.

scanf(...);
if (newChar == '\n')
    continue;