ES101 - Lesson 3 : scanf

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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

You should setup Hercules to make sure you can provide input to the board correctly :

  • 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.
  • Right click on the serial window and make sure "CR/LF" and "Local Echo" are checked.
  • 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 otherwise you might miss the output if you open the COM port too late.

Note for future labs :
Normally, your future labs will build on previous labs. In case you want to preserve your old code, just rename the old main() function to something earlier such as lab3 and write a new main function. Note that there can only be ONE main() function per program.


We will improve the percentage calculator by allowing a user to input values for us:

  1. Declare the following float variables :
    • Maximum exam score, user's exam score, and percentage.
  2. Ask the user to input data into your variables, such as:
    • "What is the max score of your exam: "
    • "What was your score: "
  3. Calculate the percentage and print it out with output similar to:
    • "You got 45/50 and your percentage is: 90.0%"
  4. Display "Program finished, press any key to quit ..."
    Test to be sure that the program quits only when a user presses a key.
    Print out the key user pressed: "You pressed Q to quit the program"


Questions

  • After completing the assignment, try removing & symbol for one of the scanf statements.
    What is the program output? Does it work as expected?