Getting User Input

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Getting User Input
Jan. 29, 2008
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xkcd comic www.xkcd.com
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News

• Homepage mirror
• http//www.cs.indiana.edu/mewhiteh/i210
• First homework is assigned today
• Due this Thursday before lecture
• We'll look over submissions anonymously on
  Thursday
• Lecture notes will be posted before lecture from
  now on
• Password for week 3 sample code
• turtle

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QUIZ!

• What's the difference between f and g?
• What is an ASCII value?
• What is a string? How is it different than
  other variable types we've seen before?
• How would you declare a string of size 64 with
  the initial value of Go Giants! ?
• char myString password How big is
  myString?
• How would you change the string in 4 to No
  Giants! ?

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Hacker's Tip of the Day

• Alan Turing (1912-1954)?
• Some Key Interests
• Cryptography
• Artificial Intelligence

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Hacker's Tip of the Day

• How do we know if a computer has true artificial
  intelligence?

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Turing Test
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Getting User Input
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Warcraft Gold Conversion
int main( int argc, char argv )? float
costOneGold 0.069 int gold10Dollars
0 gold10Dollars 10.0 / costOneGold printf(
" 10 gets you 5d gold\n", gold10Dollars
) return( 0 )
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Warcraft Gold Conversion
int main( int argc, char argv )? float
costOneGold 0.069 int gold10Dollars
0 gold10Dollars 10.0 / costOneGold printf(
" 10 gets you 5d gold\n", gold10Dollars
) return( 0 )
This was the conversion rate...
...and this was the dollar amount to be converted
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Warcraft Gold Conversion

• What if you gave the program to a friend that
  doesn't know about programming?
• Would it be useful to her?
• How would she convert different dollar amounts
  into gold counts?

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Specific Vs. General Program

• The Warcraft gold conversion program is highly
  specific in purpose
• It's too specific...it only does 1 single
  conversion without recompiling
• General Version of the Program
• Allow different conversion rates to be used
• Allow different dollar amounts to be converted

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User Input

• The program user (not the programmer!) should set
  the conversion rate and dollar amount to be
  converted
• The program can then be reused for many different
  related conversion problems
• This makes the program better!

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Getting User Input

• A special function is used to get user input
  scanf
• scanf puts values from the user into variables
• Scanned variable values can be used normally
  after scanning
• scanf is from the stdio.h header file
• scanf/printf for basic I/O
• scanf for input
• printf for output

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scanf Example
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
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scanf Example
First, declare a new variable to store the user
input
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
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scanf Example
Its initial value doesn't really matter because
it will soon be replaced
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
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scanf Example
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
A normal printf message prompts the user to enter
a value
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scanf Example
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
scanf is the name of the new function that gets
user input
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scanf Example
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
d is for the type of variable you are scanning
for. In this case, an integer. (NOTICE there
is a space before the d!!!)?
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scanf Example
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
The variable to store the user input preceded by
an . It is very easy to forget the .
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scanf Example
int main( int argc, char argv )? int
userNumber 0 printf( "Please enter a number
from 1-10 " ) scanf( " d", userNumber
) printf( You entered the number d\n,
userNumber ) return( 0 )
Now we can print out the scanned value with a
normal printf statement.
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Steps to Using scanf

• Declare a variable that will be used to store
  the entered value
• Use printf to show a prompt that tells the user
  what kind of value to enter
• Use the scanf function with the appropriate
  d,f,... marker and in front of the variable
  name
• Use the variable with the scanned value normally

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Warcraft Gold Conversion User Inputs
int main( int argc, char argv )? float
costOneGold 0.0 float dollarAmount
0.0 int numberGold 0 printf( Please enter
the cost for 1 piece of gold ) scanf( f,
costOneGold ) printf( Please enter the
dollar amount to convert ) scanf( f,
dollarAmount ) numberGold dollarAmount /
costOneGold printf( " .2f gets you d
gold\n", dollarAmount, numberGold ) return( 0
)
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Warcraft Gold Conversion User Inputs
int main( int argc, char argv )? float
costOneGold 0.0 float dollarAmount
0.0 int numberGold 0 printf( Please enter
the cost for 1 piece of gold ) scanf( f,
costOneGold ) printf( Please enter the
dollar amount to convert ) scanf( f,
dollarAmount ) numberGold dollarAmount /
costOneGold printf( " .2f gets you d
gold\n", dollarAmount, numberGold ) return( 0
)
First, prompt for the conversion rate.
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Warcraft Gold Conversion User Inputs
int main( int argc, char argv )? float
costOneGold 0.0 float dollarAmount
0.0 int numberGold 0 printf( Please enter
the cost for 1 piece of gold ) scanf( f,
costOneGold ) printf( Please enter the
dollar amount to convert ) scanf( f,
dollarAmount ) numberGold dollarAmount /
costOneGold printf( " .2f gets you d
gold\n", dollarAmount, numberGold ) return( 0
)
Next, use scanf to get the value from the user
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Warcraft Gold Conversion User Inputs
int main( int argc, char argv )? float
costOneGold 0.0 float dollarAmount
0.0 int numberGold 0 printf( Please enter
the cost for 1 piece of gold ) scanf( f,
costOneGold ) printf( Please enter the
dollar amount to convert ) scanf( f,
dollarAmount ) numberGold dollarAmount /
costOneGold printf( " .2f gets you d
gold\n", dollarAmount, numberGold ) return( 0
)
Also, prompt and scanf for the dollar amount to
be converted.
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Warcraft Gold Conversion User Inputs
int main( int argc, char argv )? float
costOneGold 0.0 float dollarAmount
0.0 int numberGold 0 printf( Please enter
the cost for 1 piece of gold ) scanf( f,
costOneGold ) printf( Please enter the
dollar amount to convert ) scanf( f,
dollarAmount ) numberGold dollarAmount /
costOneGold printf( " .2f gets you d
gold\n", dollarAmount, numberGold ) return( 0
)
Calculate the number of gold using the entered
values
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Warcraft Gold Conversion User Inputs
int main( int argc, char argv )? float
costOneGold 0.0 float dollarAmount
0.0 int numberGold 0 printf( Please enter
the cost for 1 piece of gold ) scanf( f,
costOneGold ) printf( Please enter the
dollar amount to convert ) scanf( f,
dollarAmount ) numberGold dollarAmount /
costOneGold printf( " .2f gets you d
gold\n", dollarAmount, numberGold ) return( 0
)
Print out the dollar amount entered and the
calculated number of gold after conversion
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scanf Ignoring Literal Values Used for
Formatting
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Scanning Literal Values

• Sometimes we want the user to enter a value in a
  certain format
• Sometimes the user expects to enter a value in a
  certain format
• We can use scanf to allow any kind of format for
  data entry
• scanf can ignore some symbols and concentrate
  only on the specific values needed

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Scanning Literal Values - Example

• People like to enter dates like this
• 01/29/2008 (today)?
• 04/29/2008 (the course final exam)?
• mm/dd/yyyy (general form)?
• dd/mm/yyyy (European style)?
• Numeric variables (integers and floats) would not
  be able to store these date values as entered

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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
Declare different variables for each part of the
date
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
Print a prompt that shows the exact format
expected by scanf
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
Use literal forward slashes, /, in the scanf.
This means the user must enter in this format, or
the program will not work.
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
The slashes are not stored in any variables.
They are just there for formatting purposes.
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
Put a space in front of each d.
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
This line scans the 3 parts of the date all at
once. We could have used a separate scanf for
each part.
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
List the variables in order. Each is preceded by
an .
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) return( 0 )
Print out the date using a different format.
Remember, just the numeric values were stored, so
the output format can be different than the input
format.
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scanf with Literals
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on d-d-d\n",
month, day, year ) printf( " European format
d-d-d\n", day, month, year ) return( 0 )
Remember, the order of the variables affects the
print out order
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Formatting with Leading Zeros
int main( int argc, char argv )? int month
0 int day 0 int year 0 printf( Please
enter your birthday like this mm/dd/yyyy
) scanf( d/ d/ d, month, day, year
) printf( " You were born on 02d-02d-d\n",
month, day, year ) return( 0 )
Use 02d to print out a 0 before any single digit
day or month. What do you think 04d would do?
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scanf with Strings
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scanf with Strings

• scanf works with any variable types
• integers, floats, characters, strings
• Scanning for strings works much the same way as
  for numeric types
• There are a few extra considerations

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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
Declare two string variables with size
MaxStringLength, which is defined as a constant
above (not shown)?
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
The initial values for these strings are empty.
scanf will give them values.
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
Print a prompt for the user to enter his home
city.
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
Use scanf with s to grab a string.
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
NOTICE a string variable does NOT have an in
front of it in a scanf!
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
Prompt and scan for the state value, too.
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
Print out the entered values for both city and
state. Separate them with a comma.
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
Try entering Los Angeles for your home city.
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scanf with Strings
int main( int argc, char argv )? char city
MaxStringLength char state
MaxStringLength printf( Please enter
your home city ) scanf( s, city
) printf( Please enter your home state
) scanf( s, state ) printf( " You hail
from s, s\n", city, state ) return( 0 )
Try setting a very small MaxStringLength and
entering a longer city name.