C: Iteration without a loop

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Here is the pseudo-code for an if-then-else statement.

If Then Else End If

Here is a shorter form used in examples.

If b Then s1 Else s2 End If

Here is the pseudo-code for the if-then statement where no else part is needed.

If Then End If

Here is a shorter form used in examples.

If b Then s End If

Iteration can be performed (a limited number of times) using conditional statements.

If b Then s End If

If b Then s If b Then s End If End If

If b Then s If b Then s If b Then s End If End If End If

Do you see the pattern? These are not separate if statements. They are a collection of nested if statements where b and s are exactly the same every place that they occur.

Below are some C examples for iteration using conditionals. In this case, the powers of 2 from 1 to the limit (up to 4) are output. Then, the final power is output.

The iteration body comments are to show how the code is organized. They would not be written in actual code. Here is the C code.

#include <stdio.h>

int main() {
	// declarations
	int count1;
	int limit1;
	int power1;
	// input
	printf("Enter a limit from 0 to 4\n");
	scanf("%d",&limit1);
	printf("You entered a limit of %d.\n",limit1);
	// process and output
	printf("\n");
	count1 = 0;
	power1 = 1;
	if (count1 != limit1) {
		count1 += 1;
		/// begin iteration body #1
		power1 *= 2;
		printf("\t2 to the power %d is %d\n",count1,power1);
		/// end iteration body #1
		if (count1 != limit1) {
			count1 += 1;
			/// begin iteration body #2
			power1 *= 2;
			printf("\t2 to the power %d is %d\n",count1,power1);
			/// end iteration body #2
			if (count1 != limit1) {
				count1 += 1;
				/// begin iteration body #3
				power1 *= 2;
				printf("\t2 to the power %d is %d\n",count1,power1);
				/// end iteration body #3
				if (count1 != limit1) {
					count1 += 1;
					/// begin iteration body #4
					power1 *= 2;
					printf("\t2 to the power %d is %d\n",count1,power1);
					/// end iteration body #4
					}
				}
			}
		}
	if (limit1 != 0) {
		printf("\n");
		}
	printf("Result: 2 to the power %d is %d.\n",limit1,power1);
	return 0;
	}

Here are some examples of input and output for the above program code.

Here is an example input.

4

For the above example input, here is the expected output.

Enter a limit from 0 to 4
You entered a limit of 4.

	2 to the power 1 is 2
	2 to the power 2 is 4
	2 to the power 3 is 8
	2 to the power 4 is 16

Result: 2 to the power 4 is 16.

Here is an example input.

3

For the above example input, here is the expected output.

Enter a limit from 0 to 4
You entered a limit of 3.

	2 to the power 1 is 2
	2 to the power 2 is 4
	2 to the power 3 is 8

Result: 2 to the power 3 is 8.

Here is an example input.

2

For the above example input, here is the expected output.

Enter a limit from 0 to 4
You entered a limit of 2.

	2 to the power 1 is 2
	2 to the power 2 is 4

Result: 2 to the power 2 is 4.

Here is an example input.

1

For the above example input, here is the expected output.

Enter a limit from 0 to 4
You entered a limit of 1.

	2 to the power 1 is 2

Result: 2 to the power 1 is 2.

Here is an example input.

0

For the above example input, here is the expected output.

Enter a limit from 0 to 4
You entered a limit of 0.

Result: 2 to the power 0 is 1.

In this program, we can check all possible input cases. In general, it is not possible to check all possible input cases.

Note the use of "+=" and "*=" in the above code.

The following forms are equivalent (as assignment statements, not in loops which have not yet been covered).

x1 = x1 + 1;
x1 += 1;
x1++;
++x1;

Adding 1 (one) to a variable is called an increment. In general, avoid "++" except in common patterns.

x1 = x1 - 1;
x1 -= 1;
x1--;
--x1;

Subtracting 1 (one) to a variable is called an decrement. In general, avoid "--" except in common patterns.

x1 = x1 * 2;
x1 *= 2;

If it makes sense for the problem being solved, the same short forms can be used with other operators such as multiplication, division, etc.

The term iteration means doing something more than once.

Iteration is the same as repetition.

Iteration is not the same thing as a loop. A loop is one way to do iteration, but one can do iteration by doing the same (or similar) thing more than one time.

Here is a C program that reads in a non-negative integer representing how many integers follow.

The comments for begin and end of iteration body are to show how the code is organized. One would not write the comments in the actual code.

Here is the best way to do iteration via loop unrolling. Here is the C code.

#include <stdio.h>

int main() {
	// declarations
	char valueUnits1[256];
	double value1;
	int valuesLen1;
	int valuesPos1;
	double valuesSum1;
	double valuesMean1;
	// input
	printf("Enter number of values and units.\n");
	scanf("%d %[^\n]s",&valuesLen1,&valueUnits1);
	printf("There are %d values with units %s.\n",valuesLen1,valueUnits1);
	printf("\n");
	if (valuesPos1 != valuesLen1) {
		valuesPos1 += 1;
		/// begin iteration body #1
		scanf("%lf",&value1);
		valuesSum1 += value1;
		printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
		/// end iteration body #1
		if (valuesPos1 != valuesLen1) {
			valuesPos1 += 1;
			/// begin iteration body #2
			scanf("%lf",&value1);
			valuesSum1 += value1;
			printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
			/// end iteration body #2
			if (valuesPos1 != valuesLen1) {
				valuesPos1 += 1;
				/// begin iteration body #3
				scanf("%lf",&value1);
				valuesSum1 += value1;
				printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
				/// end iteration body #3
				if (valuesPos1 != valuesLen1) {
					valuesPos1 += 1;
					/// begin iteration body #4
					scanf("%lf",&value1);
					valuesSum1 += value1;
					printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
					/// end iteration body #4
					}
				}
			}
		}
	// output
	if (valuesLen1 == 0) {
		printf("The mean is not defined for %d values.\n",valuesLen1);
		}
	else {
		valuesMean1 = valuesSum1 / valuesLen1;
		printf("\n");
		printf("The mean is %0.2lf %s.\n",valuesMean1,valueUnits1);
		}
	return 0;
	}

Here are some examples of input and output for the above program code.

Here is an example input.

4 seconds
14.1
71.7
23.3
85.6

For the above example input, here is the expected output.

Enter number of values and units.
There are 4 values with units seconds.

	Value 1 of 4 is 14.10 seconds.
	Value 2 of 4 is 71.70 seconds.
	Value 3 of 4 is 23.30 seconds.
	Value 4 of 4 is 85.60 seconds.

The mean is 48.67 seconds.

Here is an example input.

2 meters
75.2
21.5

For the above example input, here is the expected output.

Enter number of values and units.
There are 2 values with units meters.

	Value 1 of 2 is 75.20 meters.
	Value 2 of 2 is 21.50 meters.

The mean is 48.35 meters.

Here is an example input.

1 ounces
31.2

For the above example input, here is the expected output.

Enter number of values and units.
There are 1 values with units ounces.

	Value 1 of 1 is 31.20 ounces.

The mean is 31.20 ounces.

Here is an example input.

0 feet

For the above example input, here is the expected output.

Enter number of values and units.
There are 0 values with units feet.

The mean is not defined for 0 values.

Notice the redundancy in the above program and the limitations of the above program.

Computer science is the search for finite representations of (potentially) infinite objects.

In order to remove the above redundancies and remove the limitations of the above program, the concept of a loop will be soon introduced.

Here is another way to do the above. This way should not be used as it requires a check for every possibility along the way.

Do not use this example for any work! It is for comparison only.

Here is the C code.

#include <stdio.h>

int main() {
	// declarations
	char valueUnits1[256];
	double value1;
	int valuesLen1;
	int valuesPos1;
	double valuesSum1;
	double valuesMean1;
	// input
	printf("Enter number of values and units.\n");
	scanf("%d %[^\n]s",&valuesLen1,&valueUnits1);
	printf("There are %d values with units %s.\n",valuesLen1,valueUnits1);
	printf("\n");
	if (valuesPos1 != valuesLen1) {
		valuesPos1 += 1;
		/// begin iteration body #5
		scanf("%lf",&value1);
		valuesSum1 += value1;
		printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
		/// end iteration body #5
		}
	if (valuesPos1 != valuesLen1) {
		valuesPos1 += 1;
		/// begin iteration body #6
		scanf("%lf",&value1);
		valuesSum1 += value1;
		printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
		/// end iteration body #6
		}
	if (valuesPos1 != valuesLen1) {
		valuesPos1 += 1;
		/// begin iteration body #7
		scanf("%lf",&value1);
		valuesSum1 += value1;
		printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
		/// end iteration body #7
		}
	if (valuesPos1 != valuesLen1) {
		valuesPos1 += 1;
		/// begin iteration body #8
		scanf("%lf",&value1);
		valuesSum1 += value1;
		printf("\tValue %d of %d is %0.2lf %s.\n",valuesPos1,valuesLen1,value1,valueUnits1);
		/// end iteration body #8
		}
	// output
	if (valuesLen1 == 0) {
		printf("The mean is not defined for %d values.\n",valuesLen1);
		}
	else {
		valuesMean1 = valuesSum1 / valuesLen1;
		printf("\n");
		printf("The mean is %0.2lf %s.\n",valuesMean1,valueUnits1);
		}
	return 0;
	}

Here are some examples of input and output for the above program code.

Here is an example input.

4 seconds
14.1
71.7
23.3
85.6

For the above example input, here is the expected output.

Enter number of values and units.
There are 4 values with units seconds.

	Value 1 of 4 is 14.10 seconds.
	Value 2 of 4 is 71.70 seconds.
	Value 3 of 4 is 23.30 seconds.
	Value 4 of 4 is 85.60 seconds.

The mean is 48.67 seconds.

Here is an example input.

2 meters
75.2
21.5

For the above example input, here is the expected output.

Enter number of values and units.
There are 2 values with units meters.

	Value 1 of 2 is 75.20 meters.
	Value 2 of 2 is 21.50 meters.

The mean is 48.35 meters.

Here is an example input.

1 ounces
31.2

For the above example input, here is the expected output.

Enter number of values and units.
There are 1 values with units ounces.

	Value 1 of 1 is 31.20 ounces.

The mean is 31.20 ounces.

Here is an example input.

0 feet

For the above example input, here is the expected output.

Enter number of values and units.
There are 0 values with units feet.

The mean is not defined for 0 values.

Make no unnecessary checks to see of the limit has been reached. (Follow the example code below).

Note: At this time, we do not know enough to separate the repeated input, processing, and output.

This program does not use a loop. This method is known as loop unrolling and is used in special cases to speed up certain code.

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