asking questions - conditionals objectives learning the types of questions c++ can ask. writing...
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Asking Questions - Conditionals
Objectives
Learning the types of questions c++ can ask.
Writing simple questions in c++.
logical operators
relational operators
logical expressions
If statement
if with true part
if with true and false parts
nested if
What Kinds of Questions Can I Ask?
Is the sky blue? What is the meaning of Life?
Unfortunately..It's not that easy!
Then WHAT?
1. Are two values
= = equal (BE CAREFUL!)
!= not equal
> greater
>= greater than or equal
< less than
<= less than or equal
Examples: a > 4 , b = = 3 + d
2. Using those for more complicated
&& and
| | or
Examples:
( a > b ) | | ( c <=d)
(a > 3) && ( a < 12) means 3<a<12
RELATIONAL
They're the easiest to learn .. NATURAL.
expression RELATIONAL_OP expression
3 > a
2.3 + b = = c is ok but (2.3 + b) = = c is better
( b * 2 + d) ! = (23 - x)
Result is either
1 (true)
or
0 (false)
andA logical operator written &&.
Also returns 1(true) or 0(false).
BUT it expects inputs to be 1(true) or 0(false).
Both inputs must be true for the result to be true.
ENGLISH
Are you awake and in cs230?
Answer may be FALSE!
Are you alive and in cs230?
Answer is TRUE!
Truth Table for && (and) Fig. 2.28, pg 107
expression1 expression2 expression1 && expression 2
false (0) false (0) false (0)
false (0) true (1) false (0)
true (1) false (0) false (0)
true (1) true (1) true (1)
or
A logical operator written | |.
Also returns 1(true) or 0(false).
It expects inputs to be 1(true) or 0(false).
If either input or both are true the result is true.
ENGLISH
Are you a male or female?
Answer TRUE for everyone!
Are you in Richmond or in Newport News?
Answer is TRUE!
Truth Table for | | (or) Fig. 2.29, pg 107
expression1 expression2 expression1 || expression 2
false (0) false (0) false (0)
false (0) true (1) true (1)
true (1) false (0) true (1)
true (1) true (1) true (1)
Some Examples of Logical Expressions
w x y z
10 12 4 34
logical expression RESULT
w > 4 1 (t)
w > (y * 2) 1 (t)
(y - z) > (11 - w) 0 (f)
(w > 1) && ( x < 1) 0 (f)
(x > w) | | ( y > z) 1 (t)
Be careful about and/or/= =. Try this:
answer true for all 21 and 22 year olds
(age = 21) and (age = 22)
should be
(age = = 21) || (age = = 22)
English can be misleading. We associate reasonable interpretations.
1.Wrong = / ==2. and instead of or3. English instead of ||
if Statements
One of the numerous places logical expressions are used.
A fundamental program building-block.
"if" allows us to ask questions.
if (logical expression)
do_this_when_true;
Examples:
if (radius < 0.0 )
cout << "Illegal value for radius";
// more than one statement in true part
if(radius < 0.0 ) {
cout << "Illegal value for radius";
radius = 0.0;
}
// p.80 example
if (result == 1)
passes = passes +1;
else
failures = failures +1;
Nested ifUsed in mutually exclusive (only one) case
Examples: either freshman, soph, junior, senior
either grade is A, B, C, D, F, I.
either chevy, ford, chrysler
which income tax bracket
c++ Example: p. 65 (Chapter 2) book only has body of function
// function to return correct letter grade
// based on numerical test score
char letter_grade( float score)
{
if (score >=90)
return 'A';
else if (score >=80)
return 'B';
else if (score >=70)
return 'C';
else if (score >=60)
return 'D';
else
return 'F';
}
New type
Score(85.3)
Lettergrade (B)
Conclusion
Understanding the operation of if is not too bad.
Understanding how to use it will take practice.
Most programs will use them.
Labs will review them.
When I lecture on program design,
this will be addressed.
For Now:
Ignore switch and break statement.
Practice exercises to understand
how to read if
and
how to read/write logical expressions.