C: Variables

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A computer variable is a named storage location.

A mathematical variable is a placeholder for a value.

Variable names start with a letter and continue with a letter or digit with no spaces. An underscore character can be used but should not be used as the first character of a variable name.

A variable name identifies that variable. Such names are called identifier

Whatever method you use for naming variables, be consistent within the same program or group of programs.

There are many ways to create a variable for "total hours". Here are some ways.

totalhours
total_hours (underscore in place of space)
totalHours (camel-case)
TotalHours (camel-case)

Do not use TOTALHOURS as all uppercase is used (by tradition) for constants (named literals).

A name such as th (for "total hours") is not very meaningful and should be avoided.

The English word "declare" means to say something of importance. Example: The Declaration of Independence.

In many computer languages, computer variables must be declared and initialized before being used. (Being used means being used in an expression).

Some programming languages have implicit declarations.

C requires that all variables be declared (and initialized) before use.

Here is the C code.

#include <stdio.h>

int main() {
	// declarations
	int i1;
	double r1;
	// initialization
	i1 = 123;
	r1 = 4.56;
	// usage in (output) expression
	printf("i1 has value %d\n",i1);
	printf("r1 has value %5.4lf\n",r1);
	return 0;
	}

Here is the output of the C code.

i1 has value 123
r1 has value 4.5600

Note the use of the format specifier lf in the printf statement and not f.

Here is what happens with no variable declarations.

Here is the C code.

#include <stdio.h>

int main() {
	// declarations - omitted
	// initialization
	i1 = 123;
	r1 = 4.56;
	// usage in (output) expression
	printf("i1 has value %d\n",i1);
	printf("r1 has value %5.4lf\n",r1);
	return 0;
	}

Here is the error text of the C code.

D:\W\PROGRAM1.C\code17-02.c: In function 'main':
D:\W\PROGRAM1.C\code17-02.c:7:2: error: 'i1' undeclared (first use in this function)
  i1 = 123;
  ^~
D:\W\PROGRAM1.C\code17-02.c:7:2: note: each undeclared identifier is reported only once for each function it appears in
D:\W\PROGRAM1.C\code17-02.c:8:2: error: 'r1' undeclared (first use in this function); did you mean 'i1'?
  r1 = 4.56;
  ^~
  i1

Here is what happens with no variable initializations.

Here is the C code.

#include <stdio.h>

int main() {
	// declarations
	int i1;
	double r1;
	// initialization - omitted
	// usage in (output) expression
	printf("i1 has value %d\n",i1);
	printf("r1 has value %5.4lf\n",r1);
	return 0;
	}

Here is the output of the C code.

i1 has value 0
r1 has value 0.0000

C may not raise an exception if variables are not initialized. This can be the source of very hard-to-find programming errors.