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Chapter 4 - Assignment & Logical compares
Throughout this chapter, references are given to
various ranges of variables. Your compiler may use a
different range for some of the variables since the proposed
ANSI standard does not define specific limits for all data
types. Consult the documentation for your compiler for the
exact range for each of the variable types.
INTEGER ASSIGNMENT STATEMENTS
Load the file INTASIGN.C and display it for an example
of assignment statements. Three variables are defined for
use in the program and the rest of the program is merely a
series of illustrations of various assignments. The first
two lines of the assignment statements assign numerical
values to "a" and "b", and the next five lines illustrate
the five basic arithmetic functions and how to use them.
The fifth is the modulo operator and gives the remainder if
the two variables were divided. It can only be applied to
"int" or "char" type variables, and of course "int"
extensions such as "long", "short", etc. Following these,
there are two lines illustrating how to combine some of the
variables in some complex math expressions. All of the
above examples should require no comment except to say that
none of the equations are meant to be particularly useful
except as illustrations. The "char" type variable will be
defined in the description of the next example program.
The expressions in lines 17 and 18 are perfectly
acceptable as given, but we will see later in this chapter
that there is another way to write these for more compact
code.
This leaves us with the last two lines which may appear
to you as being very strange. The C compiler scans the
assignment statement from right to left, (which may seem a
bit odd since we do not read that way), resulting in a very
useful construct, namely the one given here. The compiler
finds the value 20, assigns it to "c", then continues to the
left finding that the latest result of a calculation should
be assigned to "b". Thinking that the latest calculation
resulted in a 20, it assigns it to "b" also, and continues
the leftward scan assigning the value 20 to "a" also. This
is a very useful construct when you are initializing a group
of variables. The last statement illustrates that it is
possible to actually do some calculations to arrive at the
value which will be assigned to all three variables. In
fact, the rightmost expression can contain variables, even
"a", "b", & "c".
The program has no output, so compiling and executing
this program will be very uninteresting. Since you have
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Chapter 4 - Assignment & Logical compares
already learned how to display some integer results using
the "printf" function, it would be to your advantage to add
some output statements to this program to see if the various
statements do what you think they should do.
This would be a good time for a preliminary definition
of a rule to be followed in C. The data definitions are
always given before any executable statements in any program
block. This is why the variables are defined first in this
program and in every C program. If you try to define a new
variable after executing some statements, your compiler will
issue an error.
ADDITIONAL DATA TYPES
Loading and editing MORTYPES.C will illustrate how some
additional data types can be used. Once again we have
defined a few integer type variables which you should be
fairly familiar with by now, but we have added two new
types, the "char", and the "float".
The "char" type of data is nearly the same as the
integer except that it can only be assigned numerical values
between -128 to 127 on most implementations of C, since it
is stored in only one byte of memory. The "char" type of
data is usually used for ASCII data, more commonly known as
text. The text you are reading was originally written on a
computer with a word processor that stored the words in the
computer one character per byte. In contrast, the integer
data type is stored in two bytes of computer memory on
nearly all microcomputers.
DATA TYPE MIXING
It would be profitable at this time to discuss the way
C handles the two types "char" and "int". Most functions in
C that are designed to operate with integer type variables
will work equally well with character type variables because
they are a form of an integer variable. Those functions,
when called on to use a "char" type variable, will actually
promote the "char" data into integer data before using it.
For this reason, it is possible to mix "char" and "int" type
variables in nearly any way you desire. The compiler will
not get confused, but you might. It is good not to rely on
this too much, but to carefully use only the proper types of
data where they should be used.
The second new data type is the "float" type of data,
commonly called floating point data. This is a data type
which usually has a very large range, a large number of
significant digits, and a large number of computer words are
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Chapter 4 - Assignment & Logical compares
required to store it. The "float" data type has a decimal
point associated with it and has an allowable range of from
3.4E-38 to 3.4E+38 when using most C compilers on
microcomputers, and is composed of about 7 significant
digits.
HOW TO USE THE NEW DATA TYPES
The first three lines of the program assign values to
all nine of the defined variables so we can manipulate some
of the data between the different types.
Since, as mentioned above, a "char" data type is in
reality an "integer" data type, no special considerations
need be taken to promote a "char" to an "int", and a "char"
type data field can be assigned to an "int" variable. When
going the other way, there is no standard, so you may simply
get garbage if the value of the integer variable is outside
the range of the "char" type variable. It will translate
correctly if the value is within the range of -128 to 127.
The third line illustrates the simplicity of
translating an integer into a "float", simply assign it the
new value and the system will do the proper conversion.
When going the other way however, there is an added
complication. Since there may be a fractional part of the
floating point number, the system must decide what to do
with it. By definitions , it will truncate it.
This program produces no output, and we haven't covered
a way to print out "char" and "float" type variables, so you
can't really get in to this program and play with the
results, but the next program will cover this for you.
Compile and run this program.
LOTS OF VARIABLE TYPES
Load the file LOTTYPES.C and display it on your screen.
This file contains nearly every standard simple data type
available in the programming language C. There are other
types, but they are the compound types (ie - arrays and
structures) that we will cover in due time.
Observe the file. First we define a simple "int",
followed by a "long int" which has a range of -2147483648 to
2147483647 with most C compilers, and a "short int" which
has a range that is identical to that for the "int"
variable, namely -32768 to 32767. The "unsigned" is next and
is defined as the same size as the "int" but with no sign.
The "unsigned" then will cover a range of 0 to 65535. It
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should be pointed out that when the "long", "short", or
"unsigned" is desired, the "int" is optional and is left out
by most experienced programmers. We have already covered
the "char" and the "float", which leaves only the "double".
The "double" covers a greater range than the "float" and has
more significant digits for more precise calculations. It
also requires more memory to store a value than the simple
"float". The "double" in most C compilers covers a range of
1.7E-308 to 1.7E+308.
Note that other compunding of types can be done such as
"long unsigned int", "unsigned char", etc. Check your
documentation for a complete list of variable types.
Another diversion is in order at this point. Your
compiler has no provisions for floating point math, but only
double floating point math. It will promote a "float" to a
"double" before doing calculations and therefore only one
math library will be needed. Of course, this is totally
transparent to you, so you don't need to worry about it.
Because of this, you may think that it would be best to
simply define every floating point variable as double, since
they are promoted before use in any calculations, but that
may not be a good idea. A "float" variable requires 4 bytes
of storage and a "double" requires 8 bytes of storage, so if
you have a large volume of floating point data to store, the
"double" will obviously require much more memory.
After defining the data types in the program under
consideration, a numerical value is assigned to each of the
defined variables in order to demonstrate the means of
outputting each to the monitor.
THE CONVERSION CHARACTERS
Following is a list of some of the conversion
characters and the way they are used in the "printf"
statement. A complete list of all of the conversion
characters should be included with the documentation for
your compiler.
d decimal notation
o octal notation
x hexadecimal notation
u unsigned notation
c character notation
s string notation
f floating point notation
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Each of these is used following a percent sign to
indicate the type of output conversion, and between those
two characters, the following fields may be added.
- left justification in its field
(n) a number specifying minimum field width
. to separate n from m
(m) significant fractional digits for a float
l to indicate a "long"
These are all used in the examples which are included
in the program presently displayed on your monitor, with the
exception of the string notation which will be covered later
in this tutorial. Note especially the variable field width
specification demonstrated in lines 33 to 36. This is not
part of the original definition of C, but it is included in
the proposed ANSI standard and will become part of the C
language. Compile and run this program to see what effect
the various fields have on the output.
You now have the ability to display any of the data
fields in the previous programs and it would be to your
advantage to go back and see if you can display some of the
fields anyway you desire.
LOGICAL COMPARES
Load and view the file named COMPARES.C for many
examples of compare statements in C. We begin by defining
and initializing nine variables to use in the following
compare statements. This initialization is new to you and
can be used to initialize variables while they are defined.
The first group of compare statements represents the
simplest kinds of compares since they simply compare two
variables. Either variable could be replaced with a
constant and still be a valid compare, but two variables is
the general case. The first compare checks to see if "x" is
equal to "y" and it uses the double equal sign for the
comparison. A single equal sign could be used here but it
would have a different meaning as we will see shortly. The
second comparison checks to see if "x" is greater than "z".
The third compare introduces the "NOT" operator, the
exclamation, which can be used to invert the result of any
logical compare. The fourth checks for "b" less than or
equal to "c", and the last checks for "r" not equal to "s".
As we learned in the last chapter, if the result of the
compare is true, the statement following the "if" clause
will be executed and the results are given in the comments.
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Note that "less than" and "greater than or equal to" are
also available, but are not illustrated here.
It would be well to mention the different format used
for the "if" statement in this example program. A carriage
return is not required as a statement separator and by
putting the conditional clause on the same line as the "if",
it adds to the readability of the overall program.
MORE COMPARES
The compares in the second group are a bit more
involved. Starting with the first compare, we find a rather
strange looking set of conditions in the parentheses. To
understand this we must understand just what a "true" or
"false" is in the C language. A "false" is defined as a
value of zero, and "true" is defined as a non-zero value.
Any integer or char type of variable can be used for the
result of a true/false test, or the result can be an implied
integer or char.
Look at the first compare of the second group of
compare statements. The expression "r != s" will evaluate
as a "true" since "r" was set to 0.0 above, so the result
will be a non-zero value. With most C compilers, it would
always be set to a 1, but you could get in trouble if you
wrote a program that depended on it being 1 in all cases.
Good programming practice would be to not use the resulting
1 in any calculations. Even though the two variables that
are compared are "float" variables, the result will be of
type "integer". There is no explicit variable to which it
will be assigned so the result of the compare is an implied
integer. Finally the resulting number, probably 1 in this
case, is assigned to the integer variable "x". If double
equal signs were used, the phantom value, namely 1, would be
compared to the value of "x", but since the single equal
sign is used, the value 1 is simply assigned to "x", as
though the statement were not in parentheses. Finally,
since the result of the assignment in the parentheses was
non-zero, the entire expression is evaluated as "true", and
"z" is assigned the value of 1000. Thus we accomplished two
things in this statement, we assigned "x" a new value,
probably 1, and we assigned "z" the value of 1000. We
covered a lot in this statement so you may wish to review it
before going on. The important things to remember are the
values that define "true" and "false", and the fact that
several things can be assigned in a conditional statement.
The value assigned to "x" was probably a 1, but remember
that the only requirement is that it is nonzero.
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The next example should help clear up some of the above
in your mind. In this example, "x" is assigned the value of
"y", and since the result is 11, the condition is non-zero,
which is true, and the variable "z" is assigned 222.
The third example, in the second group, compares "x" to
zero. If the result is true, meaning that if "x" is not
zero, then "z" is assigned the value of 333, which it will
be. The last example in this group illustrates the same
concept, since the result will be true if "x" is non-zero.
The compare to zero is not actually needed and the result of
the compare is true. The third and fourth examples of this
group are therefore identical.
ADDITIONAL COMPARE CONCEPTS
The third group of compares will introduce some
additional concepts, namely the logical "AND" and the
logical "OR". We assign the value of 77 to the three
integer variables simply to get started again with some
defined values. The first compare of the third group
contains the new control "&&", which is the logical "AND".
The entire statement reads, if "x" equals "y" AND if "x"
equals 77 then the result is "true". Since this is true,
the variable z is set equal to 33.
The next compare in this group introduces the "||"
operator which is the "OR". The statement reads, if "x" is
greater than "y" OR if "z" is greater than 12 then the
result is true. Since "z" is greater than 12, it doesn't
matter if "x" is greater than "y" or not, because only one
of the two conditions must be true for the result to be
true. The result is true, so therefore "z" will be assigned
the value of 22.
LOGICAL EVALUATION
When a compound expression is evaluated, the evaluation
proceeds from left to right and as soon as the result of the
outcome is assured, evaluation stops. Namely, in the case
of an "AND" evaluation, when one of the terms evaluates to
"false", evaluation is discontinued because additional true
terms cannot make the result ever become "true". In the
case of an "OR" evaluation, if any of the terms is found to
be "true", evaluation stops because it will be impossible
for additional terms to cause the result to be "false". In
the case of additionally nested terms, the above rules will
be applied to each of the nested levels.
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PRECEDENCE OF OPERATORS
The question will come up concerning the precedence of
operators. Which operators are evaluated first and which
last? There are many rules about this topic, but I would
suggest that you don't worry about it at this point.
Instead, use lots of parentheses to group variables,
constants, and operators in a way meaningful to you.
Parentheses always have the highest priority and will remove
any question of which operations will be done first in any
particular statements.
Going on to the next example in group three, we find
three simple variables used in the conditional part of the
compare. Since all three are non-zero, all three are
"true", and therefore the "AND" of the three variables are
true, leading to the result being "true", and "z" being
assigned the value of 11. Note that since the variables,
"r", "s", and "t" are "float" type variables, they could not
be used this way, but they could each be compared to zero
and the same type of expression could be used.
Continuing on to the fourth example of the third group
we find three assignment statements in the compare part of
the "if" statement. If you understood the above discussion,
you should have no difficulty understanding that the three
variables are assigned their respective new values, and the
result of all three are non-zero, leading to a resulting
value of "TRUE".
THIS IS A TRICK, BE CAREFUL
The last example of the third group contains a bit of a
trick, but since we have covered it above, it is nothing new
to you. Notice that the first part of the compare evaluates
to "FALSE". The remaining parts of the compare are not
evaluated, because it is an "AND" and it will definitely be
resolved as a "FALSE" because the first term is false. If
the program was dependent on the value of "y" being set to 3
in the next part of the compare, it will fail because
evaluation will cease following the "FALSE" found in the
first term. Likewise, "z" will not be set to 4, and the
variable "r" will not be changed.
POTENTIAL PROBLEM AREAS
The last group of compares illustrate three
possibilities for getting into a bit of trouble. All three
have the common result that "z" will not get set to the
desired value, but for different reasons. In the case of
the first one, the compare evaluates as "true", but the
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semicolon following the second parentheses terminates the
"if" clause, and the assignment statement involving "z" is
always executed as the next statement. The "if" therefore
has no effect because of the misplaced semicolon. The
second statement is much more straightforward because "x"
will always be equal to itself, therefore the inequality
will never be true, and the entire statement will never do a
thing, but is wasted effort. The last statement will always
assign 0 to "x" and the compare will therefore always be
"false", never executing the conditional part of the "if"
statement.
The conditional statement is extremely important and
must be thoroughly understood to write efficient C programs.
If any part of this discussion is unclear in your mind,
restudy it until you are confident that you understand it
thoroughly before proceeding onward.
Compile and run this program. Add some printout to see
the results of some of the operations.
THE CRYPTIC PART OF C
There are three constructs used in C that make no sense
at all when first encountered because they are not
intuitive, but they greatly increase the efficiency of the
compiled code and are used extensively by experienced C
programmers. You should therefore be exposed to them and
learn to use them because they will appear in most, if not
all, of the programs you see in the publications. Load and
examine the file named CRYPTIC.C for examples of the three
new constructs.
In this program, some variables are defined and
initialized in the same statements for use below. The first
executable statement simply adds 1 to the value of "x", and
should come as no surprise to you. The next two statements
also add one to the value of "x", but it is not intuitive
that this is what happens. It is simply by definition that
this is true. Therefore, by definition of the C language, a
double plus sign either before or after a variable
increments that variable by 1. Additionally, if the plus
signs are before the variable, the variable is incremented
before it is used, and if the plus signs are after the
variable, the variable is used, then incremented. In the
next statement, the value of "y" is assigned to the variable
"z", then "y" is incremented because the plus signs are
after the variable "y". In the last statement of the
incrementing group of example statements, the value of "y"
is incremented then its value is assigned to the variable
"z".
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The next group of statements illustrate decrementing a
variable by one. The definition works exactly the same way
for decrementing as it does for incrementing. If the minus
signs are before the variable, the variable is decremented,
then used, and if the minus signs are after the variable,
the variable is used, then decremented.
THE CRYPTIC ARITHMETIC OPERATOR
Another useful but cryptic operator is the arithmetic
operator. This operator is used to modify any variable by
some constant value. The first statement of the "arithmetic
operator" group of statements simply adds 12 to the value of
the variable "a". The second statement does the same, but
once again, it is not intuitive that they are the same. Any
of the four basic functions of arithmetic, "+", "-", "*", or
"/", can be handled in this way, by putting the function
desired in front of the equal sign and eliminating the
second reference to the variable name. It should be noted
that the expression on the right side of the arithmetic
operator can be any valid expression, the examples are kept
simple for your introduction to this new operator.
Just like the incrementing and decrementing operators,
the arithmetic operator is used extensively by experienced C
programmers and it would pay you well to understand it.
THE CONDITIONAL EXPRESSION
The conditional expression is just as cryptic as the
last two, but once again it can be very useful so it would
pay you to understand it. It consists of three expressions
within parentheses separated by a question mark and a colon.
The expression prior to the question mark is evaluated to
determine if it is "true" or "false". If it is true, the
expression between the question mark and the colon is
evaluated, and if it is not true, the expression following
the colon is evaluated. The result of the evaluation is
used for the assignment. The final result is identical to
that of an "if" statement with an "else" clause. This is
illustrated by the second example in this group. The
conditional expression has the added advantage of more
compact code that will compile to fewer machine instructions
in the final program.
The final two lines of this example program are given
to illustrate a very compact way to assign the greater of
two variables "a" or "b" to "c", and to assign the lessor of
the same two variables to "c". Notice how efficient the
code is in these two examples.
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TO BE CRYPTIC OR NOT TO BE CRYPTIC
Several students of C have stated that they didn't like
these three cryptic constructs and that they would simply
never use them. This will be fine if they never have to
read anybody else's program, or use any other programs
within their own. I have found many functions that I wished
to use within a program but needed a small modification to
use it, requiring me to understand another person's code.
It would therefore be to your advantage to learn these new
constructs, and use them. They will be used in the remainder
of this tutorial, so you will be constantly exposed to them.
This has been a long chapter but it contained important
material to get you started in using C. In the next
chapter, we will go on to the building blocks of C, the
functions. At that point, you will have enough of the basic
materials to allow you to begin writing meaningful programs.
PROGRAMMING EXERCISES
1. Write a program that will count from 1 to 12 and print
the count, and its square, for each count.
1 1
2 4
3 9 etc.
2. Write a program that counts from 1 to 12 and prints the
count and its inversion to 5 decimal places for
each count. This will require a floating point number.
1 1.00000
2 .50000
3 .33333
4 .25000
etc.
3. Write a program that will count from 1 to 100 and print
only those values between 32 and 39, one to a line.
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