CS161 Introduction to Computer Science

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CS161 Introduction to Computer Science

• Topic 4

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Today in CS161

• Assignments/Questions
• What is needed for Program 2?
• Using the Math Library
• Input/Output and Formatting
• Sample Algorithm

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To Raise to Power of...

• In C there is no operator that will raise some
  number by another
• For example, for x3 you cant type
• x3 ILLEGAL!
• Instead, you must use the pow function
• float answer
• answer pow(x, 3)

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To Raise to Power of...

• To use the power function, we must do two very
  important things...
• include the math library
• include ltmath.hgt
• compile our programs using a -lm flag
• g -lm prog2.cpp

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Next, to Format our Output

• We must learn about precision
• By default, real numbers are displayed with no
  more than 6 digits, plus the decimal point
• This means that 6 significant digits are
  displayed in addition to the decimal point and a
  sign if the number is negative

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Default Precision -- Examples

• float test
• cout ltlt Please enter a real number
• cin gtgt test
• cout ltlt test

Input Resulting Output
1.23456789 1.23457 10.23456789 10.2346 100.234
56789 100.235 1000.23456789 1000.23 100000.23456
789 100000
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To Change Precision

• float test
• cout ltlt Please enter a real number
• cout.precision(3) //3 instead of 6!!
• cin gtgt test cout ltlt test ltlt endl

Input Resulting Output
1.23456789 1.23 10.23456789
10.2 100.23456789 100 10000.23456789 1e04
(Exponential notation)
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Another way to do this...

• include ltiomanip.hgt
• float test
• cout ltlt Please enter a real number
• cin gtgt test
• cout ltltsetprecision(3) ltlt test ltlt endl

• setprecision is a manipulator
• To use it, we must include the iomanip.h header
  file
• There is no difference between
• cout.precision(3) and cout ltltsetprecision(3)

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What is width?

• The width of a field can be set with
• cout.width(size)
• If what you are displaying cannot fit, a larger
  width is used
• to prevent the loss of information
• Important
• Width is only in effect for the next output

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How does width work...

• float test
• cout.precision(4) cout.width(10)
• cin gtgttest cout ltlt test
• cout ltltendl ltlttest

Input Resulting Output
1.23456789 1.235 1.235

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Another way to do this...

• include ltiomanip.hgt
• float test
• cout.precision(4)
• cin gtgttest
• cout ltltsetw(10) ltlt test
• cout ltltendl ltlttest

Input Resulting Output
1.23456789 1.235 1.235
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Trailing Zeros

• For real numbers, trailing zeros are discarded
  when displayed
• To display trailing zeros we use
• cout.setf(iosshowpoint)

Input Resulting Output
1.2300 1.23 (for an precision of 3 or
greater)
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Displaying Trailing Zeros

• float test
• cout.precision(4)
• cout.setf(iosshowpoint)
• cin gtgttest cout ltlt test ltltendl
• cout.unsetf(iosshowpoint) //reset...
• cout ltlttest

Input Resulting Output
1.2300 1.230 1.23
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Displaying Dollars and Cents!

• There is another meaning to precision...
• if we put in our programs
• cout.setf(iosfixed,iosfloatfield)
• then, subsequent precision applies to the number
  of digits after the decimal point!
• cout.precision(2) cout ltlttest
• 1.2300 1.23
• 1.20 1.2
• To display trailing zeros we use
• cout.setf(iosshowpoint)

Input Resulting Output
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Displaying Dollars and Cents!

• Since we ALSO want trailing zero displayed...do
  all three
• cout.setf(iosfixed,iosfloatfield)
• cout.precision(2)
• cout.setf(iosshowpoint)
• cout ltlttest
• 1.2300 1.23
• 1.20 1.20
• To display trailing zeros we use
• cout.setf(iosshowpoint)

Input Resulting Output
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Now... the Algorithm...

• Your algorithm is not
• I sat down at the terminal
• I got into the editor
• I entered my program
• I tried to compile it but got errors
• I DONT WANT TO SEE THIS!!!!

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Your Algorithm...

• First define the major tasks
• Then break down these into subtasks
• For example, the major tasks might be
• Welcome the user
• Get the loan amount, interest rate, duration
• Calculate the monthly payment
• Display the results
• Sign off Message

Not Detailed Enough!
Not Detailed Enough!
But...a good start...