Loops

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For comparison, here are some code examples of loops in C, Java and Python.

The running example is code that outputs the powers of 2 from 0 to 8.

The while loop is a general looping structure. Below is a code fragment in C for a while loop where Condition as a Boolean condition and Statements are a block of statements.

while (Condition) {
	Statements
	}

Here is the associated code fragment whileloopgoto using goto statements for code fragment whileloop.

L10:
if ( ! (Condition) ) goto L20;
Statements
goto L10;
L20:

The while loop in C and Java have the same syntax. Here is the Python code fragment for the while loop.

while Condition:
	Statements

Here are the syntax diagrams and grammar for the while loop in C (or Java). Syntax diagram for EBNF

WhileLoop = "while" Condition "{" { Statement } "}" .

Note:

The curly braces in quotes, "{" and "}", indicate terminal symbols in the file.
The curly braces without quotes indicate repetition of the enclosed parts, in this case, the variable/nonterminal Statement.

For comparison, here are some code examples of while loops in C, Java and Python.

Here are some style comments.

I end many of my variable names with a digit.
I often separate declarations from initialization.
Most of my code examples are automatically and dynamically generated and run and included from a higher level text specification and text formatter system to look like they were hand-written. But sometimes it may be obvious that the code was generated and not hand-written.

Here is the C code [#3]

#include <stdio.h>
int main() {
	int pos1;
	int len1;
	int x1;
	len1 = 8;
	pos1 = 0;
	x1 = 1;
	printf("Powers of 2 from 0 to %d\n",len1);
	while (pos1 <= len1) {
		printf("\t2**%d is %d\n",pos1,x1);
		x1 *= 2;
		pos1++;
		}
	return 0;
	}

Here is the output of the C code.

Powers of 2 from 0 to 8
	2**0 is 1
	2**1 is 2
	2**2 is 4
	2**3 is 8
	2**4 is 16
	2**5 is 32
	2**6 is 64
	2**7 is 128
	2**8 is 256

Here is the Java code [#1]

public class loops31b_01 {
	public loops31b_01() {
		int pos1;
		int len1;
		int x1;
		len1 = 8;
		pos1 = 0;
		x1 = 1;
		System.out.print(String.format("Powers of 2 from 0 to %d\n",len1));
		while (pos1 <= len1) {
			System.out.print(String.format("\t2**%d is %d\n",pos1,x1));
			x1 *= 2;
			pos1++;
			}
		}
	public static void main(String [] args) {
		new loops31b_01();
		}
	}

Here is the output of the Java code.

Powers of 2 from 0 to 8
	2**0 is 1
	2**1 is 2
	2**2 is 4
	2**3 is 8
	2**4 is 16
	2**5 is 32
	2**6 is 64
	2**7 is 128
	2**8 is 256

Python is dynamically typed and does not require type declarations as does C or Java.

Unlike C or Java, Python uses indentation and not curly braces to determine blocks (of statements, methods, etc.).

Here is the Python code [#2]

len1 = 8
pos1 = 0
x1 = 1
print("Powers of 2 from 0 to {0:d}".format(len1))
while pos1 <= len1:
	print("\t2**{0:d} is {1:d}".format(pos1,x1))
	x1 *= 2
	pos1 += 1

Here is the output of the Python code.

Powers of 2 from 0 to 8
	2**0 is 1
	2**1 is 2
	2**2 is 4
	2**3 is 8
	2**4 is 16
	2**5 is 32
	2**6 is 64
	2**7 is 128
	2**8 is 256

For comparison, here are some code examples of for loops in C, Java and Python.

The for loop is more complicated than the while loop, is less general, and has subtle semantics that vary from language to language. It is best to only use simple for loops that iterate from start to stop by increment 1.

Here is the C code [#4]

#include <stdio.h>
int main() {
	int len1;
	int x1;
	len1 = 8;
	x1 = 1;
	printf("Powers of 2 from 0 to %d\n",len1);
	for (int pos1=0; pos1 <= len1; pos1++) {
		printf("\t2**%d is %d\n",pos1,x1);
		x1 *= 2;
		}
	return 0;
	}

Here is the output of the C code.

Powers of 2 from 0 to 8
	2**0 is 1
	2**1 is 2
	2**2 is 4
	2**3 is 8
	2**4 is 16
	2**5 is 32
	2**6 is 64
	2**7 is 128
	2**8 is 256

Here is the Java code [#2]

public class loops31b_02 {
	public loops31b_02() {
		int len1;
		int x1;
		len1 = 8;
		x1 = 1;
		System.out.print(String.format("Powers of 2 from 0 to %d\n",len1));
		for (int pos1=0; pos1 <= len1; pos1++) {
			System.out.print(String.format("\t2**%d is %d\n",pos1,x1));
			x1 *= 2;
			}
		}
	public static void main(String [] args) {
		new loops31b_02();
		}
	}

Here is the output of the Java code.

Powers of 2 from 0 to 8
	2**0 is 1
	2**1 is 2
	2**2 is 4
	2**3 is 8
	2**4 is 16
	2**5 is 32
	2**6 is 64
	2**7 is 128
	2**8 is 256

The Python for loop is general iterator structure and therefore takes a range object which goes from the start value up to, but not including, the ending value (for positive ranges).

Here is the Python code [#3]

len1 = 8
x1 = 1
print("Powers of 2 from 0 to {0:d}".format(len1))
for pos1 in range(0,len1+1):
	print("\t2**{0:d} is {1:d}".format(pos1,x1))
	x1 *= 2

Here is the output of the Python code.

Powers of 2 from 0 to 8
	2**0 is 1
	2**1 is 2
	2**2 is 4
	2**3 is 8
	2**4 is 16
	2**5 is 32
	2**6 is 64
	2**7 is 128
	2**8 is 256