Lesson 7: The switch statement and logical
operators
This lesson will show you how to:
The switch statement is a construct that
is used when many conditions are being tested for. When
there are many conditions, it becomes too difficult and
complicated to use the if and else if constructs. Nested
if/else statements arise when there are multiple alternative
paths of execution based on some condition that is being
tested for. Here's an example, this is a simple calculator
that can be used to add, multiply, subtract, and divide.
If this program was using if else statements than this program
will work fine, but the if/else block is cumbersome. It
would be easy, particularly if there were more choices and
maybe sub choices involving more if/else's to end up with
program that doesn't perform the actions intended. Here's
the same program with a switch.
#include <stdio.h>
int main(void)
{
float numb1 = 0, numb2 = 0; /*
the two numbers to work on */
int menu = 1;
/* add or substract or divide or multiply */
float total = 0;
/* the result of the calculation */
char calType;
/* what type of calculation */
printf("Please enter in the
first of the two numbers\n\t");
scanf("%f", &numb1);
/* READ first number */
printf("\n\nPlease enter
the second of the two numbers\n\t");
scanf("%f", &numb2);
/*
READ second number */
printf("\n\nWhat would you
like to do?\n\n"); /* WRITE instructions
*/
printf("\t1 = add\n");
printf("\t2 = substract\n");
printf("\t3 = multiply\n");
printf("\t4 = divide\n");
printf("\n\nPleas make your
selection now:\n\t");
scanf("%d",&menu);
/*
READ calculation type */
switch (menu) /*
select the type of calculation */
{
case 1: total = numb1 + numb2;
calType
= '+'; /*
assign a char to symbolise calculation type */
break;
case 2: total = numb1 - numb2;
calType
= '-';
break;
case 3: total = numb1 * numb2;
calType
= '*';
break;
case 4: total = numb1 / numb2;
calType
= '/';
break;
default: printf("Invalid option
selected\n");
}
if (menu == 3 && numb2 ==
0) /*
cannot divide by 0 */
printf("\n\n\tYou
cannot divide by 0\n\n");
/*
display result to 2 decimal places */
printf("\n\n*************************");
printf("\n\n\t%.3f %c %.3f
= %.2f", numb1, calType, numb2, total);
printf("\n\n*************************\n\n");
return 0;
}
The keyword default is executed when none of the
conditions being tested for in the switch statement are
met or executed. The break statement must be used
after each condition because if it were not used than all
the conditions from the one met will be executed. For example
if case 2 was met, and there was no break statement at the
end of the case, case 3 and case 4 and even default would
all be executed.
The general form of a switch statement is:
switch (variable) {
case expression1:
do something 1;
break;
case expression2:
do something 2;
break;
....
default:
do default processing;
}
Logical Operators
In more realistic examples, it is probably necessary to
evaluate multiple conditions to determine what parts of
code should execute. For instance, if condition 1 is true
and condition 2 is true process one way, if condition 1
is true and condition two is false process another way.
C provides several logical operators that allow more complex
relational expressions to be formed and evaluated.
| Operator |
Description |
Example |
Evaluation |
| |
|
|
|
| && |
AND |
(5 > 1) && (3>10) |
False |
| && |
AND |
(2 >1) && (10 > 9) |
True |
| || |
OR |
(3> 1) || (10 > 11) |
True |
| || |
OR |
(4> 2) || (10 > 5) |
True |
| ! |
NOT |
!(5>1) |
False |
| ! |
NOT |
!(2 > 3) |
True |
| |
|
|
|
As can be seen in this table, && will return true
only if both expressions are true, while || will be true
if either expression is true. The operator "!"
provides logical negation. One very important consideration
when forming expressions is the order of precedence (what
is executed when) of the relational and logical operators.
Relational operators are of higher precedence (they are
executed first) than the logical and the order of evaluation
is from left to right. Here are some examples that illustrate
what this means.
if (myChoice == 'A' and myAge < 25) is evaluated as
if ((myChoice == 'A') and (myAge < 25))
Suppose x = 8, y = 49, z = 1.
if (x < 7 && y > 50 || z < 2) is evaluated
as
if (((x < 7) && (y > 50)) || (z < 2)) which
is TRUE, not as
if ((x < 7) && ((y > 50) || (z < 2)) which
is FALSE.
Now, here are a few final points to wrap up this lesson.
First, even if you are sure about the order of precedence
of an expression, use many parenthesis (just in case). This
serves to increase readability and will help avoid errors.
Second, there is such a thing as green tea ice cream and
I recommend that you not buy it.
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