ES101 - Lesson 3 : scanf

scanf

The scanf function can be used to input information to your program variables. Here is an overview:

• Compile and load your program
• Open Hercules and go to Serial tab.
• Right click in the middle of the screen and double check the following items are checked:
  • CR/LF Enable
  • Local Echo
• Anything typed inside Hercules window will be transmitted to the board.

scanf syntax is similar to printf, but it scans for input. The specifier list is the same, and one or more inputs can be captured similarly to how you use printf. Let's start with a code example:

int age = 0;
float height = 0;

printf("What is your age? ");
scanf("%i", &age);

printf("What is your height? ");
scanf("%f", &height);

Notice the following differences and similarities of scanf vs.printf

• scanf uses same specifiers,%i,%f etc.
• scanf variables after the double quotes have & symbol.

The & (ampersand) symbol is the address operator. Instead of passing the variable's value, you pass the variable's address such that scanf can input the data into the memory location of the variable's address. If you forget to include the & operator, you will experience unexpected results because scanf would probably input the data at the wrong memory location.

scanf should be intuitive to understand except when you try to scanf a single character (%c). What happens is that a prior scanf does not consume the <Enter key> character (0x0A) and the next scanf picks up this character when you try to use: scanf("%c", &my_char);. To solve this problem a call to getchar() is made, which discards this unconsumed hanging character, and scanf will wait for a new character from Hercules terminal.

int age = 0;
char my_char = 0;

printf("What is your age? ");
scanf("%i", &age);

/** 
 * This code will completely skip over the prompt.
 * If you insert getchar(); *twice* before the next scanf() statement,
 * then it will work as expected as the 'dangling' <enter key> characters
 * will be discarded and scanf will be forced to look for new input.
 */
printf("Press any key: ");
scanf("%c", &my_char);

printf("You entered: %c which is the number %u\n", my_char, my_char);

Common Mistake

Your code is executed sequentially so do not try to assign variables before asking for input:

int weight_lb = 0;
int weight_kg = weight_lb * 1.6; // <-- Assigning this too early!

printf("What is your weight in lb? ");
scanf("%i", &weight_lb);

printf("Your weight in kg: %i\n", wieght_kg);

Assignment

To provide input to your Board from your computer's PC, you need to open Hercules Program at 38400bps by going to the Serial Tab. To enter input, you can simply type on the Hercules window and hit the Enter key to enter text from your keyboard, which will be transmitted to the Board. Also note that after you open the COM Port in Hercules, the reset button should be pressed on the Board to restart the program.

1. Declare four variables and choose their appropriate type, such as int, or float. Use a meaningful name based on the following uses:
   • A variable to store the age (in years) of the user
   • A variable to store the age in months.
   • A variable to store the height in inches of the user.
   • A variable to store the height in centimeters.
2. Ask the user to input their age in years and store it to your age variable.
3. Ask the user to input their height and store it to your height variable in inches.
4. Calculate the age in months and store it to the appropriate variable.
5. Calculate the height in centimeters (2.54 cm per inch) and store it to the appropriate variable.
6. Output the user's age in months and years, height in inches, and height in centimeters.
   • The height in centimeters should only display 2 decimal digits.
7. After displaying all output, create a prompt that inputs a char:

   Display "Program finished, press any key to quit ..."

   Test to be sure that the program quits only when a user presses a key.

Questions

• After completing the assignment, try removing & symbol for the scanf statement that asks for user's height.

  What is the program output? Does it work as expected?