CS 101

1
CS 101

• Lecture 4
• Repetition

2
pre and post increment and decrement

• If ii1 we may write i to mean the same
  thing, and is called the postincrement operator.
  i also means ii1 and is called the
  preincrement operator. --i and i-- are called the
  preincrement and postincrement operators and each
  means ii-1.
• The difference shows up in expressions and
  commands. If i 4 then coutltltxltlt ltltxwill
  print out 5 5 butcoutltltxltlt ltltx will print
  out 4 5.
• Similarly, if x5 and y3 thencoutltltxltlt ltltylt
  ltltxy--ltlt ltltxltlt ltlty will print5 3 18 6 2
  to the screen

3
Shortcut assignment and arithmetic operators

• We can write x3 to mean xx3, similarly
  we can write x4 x/5 x-2 x3if x,y are
  integer variables, x/y is truncated, i.e., 7/3 is
  2. If at least the numerator or denominator of
  x/y is a double or a float, then the result is an
  untruncated double or float. 7.0/3 is 2.333 We
  can also writedouble(7)/3 or 7/double(3) to get
  the same result, but not double(7/3), since this
  gives 2.0. If x,y are integer variables, xy
  gives the remainder when x is divided by y. 73
  is 1. 135 is 3. -135 is 3. In math, -135 is
  2.

4
The while loop

• If i is an integer variable containing 0, then
  the statementwhile(ilt5) coutltltiiltlt
  iwill print 0 1 4 9 to the screen. The
  pair of statements coutltltiiltlt i gets
  repeated 5 times, each time with a different
  value of i. The five values of i are 0,1,2,3,4.
  Before the pair of statements is executed, the
  test ilt5 is performed. If it is true, then both
  statements are executed. If not(as when 5lt5 is
  false), then the next statement after the while
  loop is executed.

5
while loop program

• While loops are usually used when it is not known
  in advance how many times the loop will be
  executed. The following program
  includeltiostream.hgtmain( )double sum0int
  i1while(1.0/(2ii2i1)gt.0001)sumsum1.0/(2
  ii2i1)icoutltltsumltltendlwill execute
  70 times and print .433617 to the screen. We have
  avoided solving the equation 2i2 2i 1 10000
  to find out in advance how many times we must
  loop.

6
for loops

• There are three parts to the for loop
  statement.for(int i1ilt5i)coutltltiltlt int
  i1 is called the initialization command, and is
  done first and only once. If there are several
  commands before the first then they must be
  separated by commas. ilt5 is called the test
  expression. If it is true then the body of the
  for loop, in this case, the statement coutltltiltlt
  is performed. If the body is performed then
  the final statement(s) is(are) carried out after
  the body. Then the test expression is evaluated
  again. The above for statement will print 1 2 3
  4 to the screen. The above for statement is
  equivalent tofor(int i1ilt5coutltltiltlt ,i)
  and also tofor(int i1ilt5)coutltltiltlt i
  and also to for(int i1 )if(ilt5)coutltltiltlt
  else break

7
break and continue

• If a break statement occurs in the body of a
  loop, the loop is exited. If a continue statement
  occurs in the body of a loop, the next iteration
  is performed. If the loop is a for loop then
  after the continue, the update portion of the for
  loop is executed.

8
Quiz 2 and solution

• Write a program which will ask the user to
  input 50 numbers(doubles), and print the average
  to a file called average.txtincludeltfstream.hgt
  main()coutltltPlease enter 50 numbers.\ndouble
  x, sum0for(int i0ilt50i)cingtgtxsumsum
  xofstream fout(average.txt)foutltltsum/50fo
  ut.close()

9
loops with a conditional

• Suppose we want to read 50 integers from the
  file input.txt and write the even integers to the
  file oute.txt and the odd integers to the file
  outo.txt.includeltfstream.hgtmain()ifstream
  fin(input.txt)ofstream foute(oute.txt),fouto
  (outo.txt)int xfor(int i0ilt50i)fingtgtx
  if(x21)foutoltltxltlt else fouteltltxltlt
  fin.close()fouto.close()foute.close()

10
Quiz 3 and solution

• Write a program which will ask the user to
  input 50 numbers(doubles), and print the numbers
  to three different files. If the number is
  divisible by 3, the number is printed to the file
  three0.txt if dividing the number by 3 yields a
  remainder of 1, the number is printed to the file
  three1.txt if dividing the number by 3 yields a
  remainder of 2, the number is printed to the file
  three2.txt
• includeltfstream.hgt
• main()int x
• ofstream out0(three0.txt),fout1(three1.txt),fo
  ut2(three2.txt)
• coutltltPlease enter 50 integrs.\n
• for(int i0ilt50i)cingtgtx if(x30)fout0ltltxltlt
  
• else if(x31)fout1ltltxltlt else fout2ltltxltlt
  
• fout1.close() fout1.close() fout2.close()