File I/O Includes
#include <fstream>
File I/O Data types
ifstream
ofstream
Opening Files
Closing Files
cin.good() is equivalent to cin
!cin is equivalent to cin.fail()
cin.good is true if there is no error condition (unrecoverable
or otherwise) and the end-of-file flag is not set.
cin.bad is true if there is an unrecoverable error
cin.fail is true if there is an unrecoverable error or an
"expected" condition, such as a conversion error, or if the file
is not found. Processing can often resume after a call to clear
with a zero argument.
Comparison operators
<, <=, >, >=, !=, ==
bool data type
It has only two values – true and false
The value of a Boolean expression is true or false
bool v = 3 < 5;
bool w = 5 < 3
The syntax of one kind of Boolean expression is
left expression <comparison operator> right expression
“fred” != 17
3.0 != 3
By default the data type of floating point constant is double.
One can qualify the data type with a suffix – F for float and D
for double
3.0F
3.0D
More Boolean expressions
5
5
3
3
5
5
5
< 3
<= 3
<= 5
>=5
>= 3
!= 3
== 3
(false)
(false)
(true)
(false)
(true)
(true)
(false)
This expression is not a safe Boolean expression. C++ evaluates
the expression but describes it as an unsafe expression
10 <= 15 <= 20
The correction version of the expression is
15 >= 10 &&15 <= 20
The meaning of the expression is that 15 is between 10 and 20.
In general a value is between two other value if the value is
greater than or equal to a lower bound and less than or equal to
an upper bound
The and operator, &&, combines several Boolean expressions into
a compound Boolean expression. The compound Boolean expression
is true if all the component expressions are true
For example,
int a = 3, b = 4, c = 10
a <= b && b <= c is true because both components are true
a <= c && c <= b is false because the second component is false
The or operator, ||,
is not as restrictive as and operator, &&
15 >= 10 || 15 <= 20 is true because at least one component
istrue
15 <= 10 || 15 <= 20 is true because the second component is
true
15 <= 10 || 20 <= 15 is false because both components are false
5 <= 10 || 5 >= 20
is true because the first component is
true
5 <= 10 && 5 >= 20 is false because the second component is
false
10 <= 5 || 10 >= 20 is false because both components is false
10 <= 5 && 10 >= 20 is false because one of the components is
false
An
An
An
An
And expression is true if all components are true
Or expression is true if at least one component is true
And expression is false if at least one component is false
Or expression is false if both components are false
Simple if statement
If age < 16 then
write ‘you have your sweet 16 party to look forward to’
If 13 <= age and age <= 19 then
write ‘you are a teenager now’
Two way if statement
If age < 16 then
write ‘you still have your sweet 16 to look forward to’
else
write ‘you already had your sweet 16’
end if
if 13 <= age and age <= 19 then
write ‘you are a teenager now’
else
write ‘you are not a teenager’
end if
Multi-way if statement
If age < 13 then
write ‘you are not a teenager yet’
else if age < 20 then
write ‘you are a teenager now’
else
write ‘you are not a teenager any more’
end if
Multi-way if statement with mutually exclusive conditions
If age < 13 then
write ‘you are not a teenager yet’
else if 13 <= age and age <= 19 then
write ‘you are a teenager now’
else
write ‘you are not a teenager any more’
end if
The following is an example of a multi-way if statement. It is
called a multi-branch if statement because it has several
conditions.
If age < 16 then
come back when 16
else if age < 18
apply for restricted license
else if age < 85 then
apply for regular license
else
apply for restricted license
We can write the same conditions using a compound condition.
Try to convince yourself that the two if statements produce the
same results
if age < 16 then
come back at 16
else if age < 18 or age >= 85 then
apply for restricted license
else
apply for normal license
end If
Or conditions let the programmer select one of several paths
If state is Alaska or state is Hawaii or state is Puerto Rico
then
increase taxes by 50%
else if state is California then
decrease taxes by 50%
else
increase taxes by 25%
end If
if statements
one-way if statement
If(boolean expression)
statement
e.g.
if(3 > 5)
cout << “3 > 5” << endl;
If the Boolean expression is true then the program executes the
statement. If the expression is false then the program does
nothing.
two way if statement
if(boolean expression)
t-statement
else
e-statement
If the Boolean expression is true then the program executes the
t-statement. If the Boolean expression is false then the program
executes the e-statement.
if(3 > 5)
cout << “3 > 5”;
else
cout << “5 > 3”;
The program displays “5 > 3”
operator precedence
Arithmetic statements have a higher precedence than relational
operators. Relational operators have a higher precedence than
logical operators. Logical operators have a higher precedence
than the assignment operator. Why? We must evaluate the
expression before we can compare it. Hence we do the
computations before we compare the results of the computations
and we have to evaluate the simple Boolean expressions before we
can join them into more complex expressions.
bool whatever = state == Alaska || state == Hawaii || state ==
PuertoRico
C++ evaluates the relational operators’ expressions first
because the relational operators must operate on values; then
the relational operators because the logical operators must
operate on Boolean values; finally the logical operators produce
a Boolean value – true or false
Arithmetic operator precedence
The unary negation operator has a very high precedence because
the other operators must operate on a number.
Look at the expression
X = -17 + 3
If the addition operator has a higher precedence than the
negation operator then the value of the expression would be -20
instead of -14.
Y = 35 – 17 + 3
C++ evaluate 8 + 7 then 4 + 21 and finally 15 < 25
8 + 7 < 4 + 21
15 < 4 + 21
15 < 25
We will write the algorithm 2 ways. The first way tests for a
bad number; the second way tests for a good number
version 1
prompt to enter number between 0 and 35
if number < 0 or number > 35 then
write error message
else if number <= 9 then
write number
else
compute letter from number
write number and letter
end if
version 2
prompt to enter number between 0 and 35
if number >= 0 and number <= 35 then
if number > 9 then
compute letter from number
write number and letter
else
write number
end if
else
write error message
end if
Look at the 2 versions’ first Boolean expressions
number < 0 or number > 35
number >= 0 and number <= 35
The first condition is true whenever the second condition is
false
The first condition is false whenever the second condition is
true
That means the first version’s then statement is the second
version’s else statement and the first version’s else statement
is the second version’s then statement.
C++ stores characters as numbers
A is 65
Z is 90
number is between 10 and 35
number – 10 is between 0 and 25
‘A’
‘A’
‘A’
‘A’
+
+
+
+
0 is ‘A’
1 is ‘B’
24 is ‘Y’
25 is ‘Z’
statement blocks
Sometimes we want an if statement to control more than one
statement.
if(boolean expression)
{
t-statement-1
…
t-statement-n
}
If(boolean expression)
{
t-statement-1
…
t-statement-n
}
else
{
e-statement-1
…
e-statement-m
}
Programs to write
Write a program that reads 3 numbers and then outputs the
numbers in descending order
Algorithm
read n1
read n2
read n3
if n1 <
swap
end if
if n1 <
swap
end if
if n2 <
swap
end if
n2 then
n1 and n2
n3 then
n1 and n3
n3 then
n2 and n3
How do you swap values?
Swap x and y
t = x
x = y
y = t
Write a program that generates a random number between 0 and 10
and asks the user the number. The program reads the user’s guess
and then tells the user if the guess is too high, too low or
equal to the random number. How do you generate random numbers?
#include
#include
#include
#include
#include
<cmath>
<iostream>
<stdio.h>
<stdlib.h>
<time.h>
using namespace std;
int main(int argc, char* argv[])
{
srand(unsigned(time(NULL)));
int secret_number = rand() % 11;
cout << secret_number << endl;
return 0;
}
“time(NULL)” is some form of the current clock time on the
computer. It is something like the number of seconds since
midnight January 1, 1970. I’m not sure what it is exactly but we
don’t really need to know.
“srand” initializes the random number generator function. If you
want to always generate different sequences of random numbers
then make sure srand’s parameter is always different.
“time(NULL)” goes a look way to insuring that the program
generates different sequences of random numbers. Sometimes you
want the program to generate the same sequence of random
numbers. Use srand a particular number as its parameter. You
will always generate the same sequence of random numbers.
Problems
Write a program that determines if a meeting room is in
violation of fire regulations for the maximum room capacity. The
program will read in the maximum room capacity and the number of
people to attend the meeting. If the number of people is less
than or equal to the maximum room capacity, the program
announces that it is legal to hold the meeting and tells how
many additional people may legally attend the meeting. If the
number of people exceeds the maximum room capacity, the program
announces that the meeting cannot be held as planned due to fire
regulations and tells how many people must be excluded in order
to meet the fire regulations and thereby allow the meeting to
take place.
Write a program that computes the cost of postage on a firstclass letter according to the following rate scale: 30 cents for
the first ounce or faction of an ounce, 11 cents for each
additional half ounce, plus a 5-dollar service charge if the
customer desires special delivery.
An hourly employee is paid at a rate of $9.73 per hour for
regular hours worked per week. Any hours over that are paid at
the overtime rate of one and one-half times that. From the
worker’s gross pay, 6% is withheld for social security tax, 14%
is withheld for federal income tax, 5% is withheld for state
income tax, and $6 per week is withheld for union dues. If the
worker has three or more covered dependents, then an additional
$10 is withheld to cover the extra cost of health insurance
beyond what the employer pays. Write a program that will take
the number of hours worked in a week and the number of
dependents as input and then output the worker’s gross pay, each
withholding amount, and the net take-home pay for the week.