c - 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 ("
Enter a new nucleotide: 
");
  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 ("
Bad 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 ("

List is empty");
    return;
  }
  loc=start;
  printf("

List is : ");
  while(loc!=NULL){
    printf ("%c", loc->nucleotide);
    loc=loc->point;
  }
  printf ("
");
}

/* Finds and displays percentage of the chain made up of each nucleotide. */
void Percentage(int size){
  struct node *loc;
  if(start == NULL){
    printf ("

List is empty");
    return;
  }
  loc=start;
  printf("

List 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 ("
"); 

  /* 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("
A: %f", Adouble);
  printf("
G: %f", Gdouble);
  printf("
T: %f", Tdouble);
  printf("
C: %f", Cdouble); 
}

/* There be dragons beyond here */
int main(){
  int navigate, size =0;
  char newChar = 'q';
  do{ /* Menu */
    printf("
 1. Create / Extend Sequence
");
    printf("
 2. Display Sequence
");
    printf("
 3. Count 
");
    printf("
 0. Exit 
");
    printf("
Please select an option (0 to 3)
");
    scanf("%d",&navigate);  
    switch (navigate){
      case 0: /* Exit */
        break;
      case 1: /* Add nucleotides */
        do{
          printf ("
Enter a new nucleotide: 
");
          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 ("
Bad Element");
          }
        }while(newChar != 'x');
        break;
      case 2:
        Display();
        break;
      case 3:
        Percentage(size);
        break;
      default:
        printf ("

Bad choice. Please select another.
");
    }
  } while (navigate !=0); 
  return 0 ;
}

Answer 1

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 == '
')
    continue;