View source for 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.

<br/>
'''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:

<syntaxhighlight lang="c">
int age = 0;
float height = 0;

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

printf("What is your height? ");
scanf("%f", &height);
</syntaxhighlight>

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: <code>scanf("%c", &my_char);</code>.  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.


<syntaxhighlight lang="c">
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);
</syntaxhighlight>

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=== Common Mistake ===
Your code is executed sequentially so do not try to assign variables before asking for input:
<syntaxhighlight lang="c">
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", weight_kg);
</syntaxhighlight>


<br/>

== 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''' :
<BR/>
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.'''

<BR/>
We will improve the percentage calculator by allowing a user to input values for us:
#  Declare the following float variables :
#*  Maximum exam score, user's exam score, and percentage.
#  Ask the user to input data into your variables, such as: 
#*  "What is the max score of your exam: "
#*  "What was your score: "
# Calculate the percentage and print it out with output similar to:
#*  "You got 45/50 and your percentage is: 90.0%"
# 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"'''

<br/>
=== Questions ===
*  After completing the assignment, try removing & symbol for one of the scanf statements.  
*: What is the program output?  Does it work as expected?