Python for statement

Here is a C-like way to process/print out elements of a list from end to beginning.

Here is the Python code [#1]

list1 = [ "red" , "green" , "blue"]
for i1 in range(len(list1)-1, -1, -1):
	color1 = list1[i1]
	print("{0:s}".format(color1))

Here is the output of the Python code.

blue
green
red

Here is a more Pythonic way to process/print out elements of a list from end to beginning.

Here is the Python code [#2]

list1 = [ "red" , "green" , "blue"]
for color1 in reversed(list1):
	print("{0:s}".format(color1))

Here is the output of the Python code.

blue
green
red

Try writing a short Python program to search through a list looking for an element and determine whether that element is in the list and do something only if that element is found. For now, do not use any Python iterators. Use a while loop. Do not use any functions, is this code example is being used for a specific point. Here is the Python code [#3]

list1 = [ 2 , 3 , 5 , 7 , 9 , 13 ]
value0 = 7
i1 = 0
n1 = len(list1)
found1 = -1
print("Searching for value {0:d}.".format(value0))
while i1 != n1:
	value1 = list1[i1]
	if value1 == value0:
		found1 = i1
		break
	i1 += 1
if found1 == -1:
	print("Value not found.")
else:
	print("Value found at position {0:d}.".format(found1))

Here is the output of the Python code.

Searching for value 7.
Value found at position 3.

Here is the Python code [#4]

list1 = [ 2 , 3 , 5 , 7 , 9 , 13 ]
value0 = 7
i1 = 0
n1 = len(list1)
print("Searching for value {0:d}.".format(value0))
while i1 != n1:
	value1 = list1[i1]
	if value1 == value0:
		break
	i1 += 1
else:
	print("Value not found.")
print("i1={0:d}".format(i1))

Here is the output of the Python code.

Searching for value 7.
i1=3

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:

Notice how the generated code can be modified slightly to support the else (i.e., nobreak) construct of the while or for.

L10: if ( ! (Condition) ) goto L20; Statements if (Break) go to L30; goto L10; L20: // no break // loop completed normally L30:

if (Condition) {
	Statements-Then-Part
	}
else {
	Statements-Else-Part
	}

Here is the generated code. Compare the else of the while.

L10: if ( ! (Condition) ) goto L20; Statements-Then-Part goto L30; L20: Statements-Else-Part L30:

During the "goto" wars (started by Edgar Dijkstra), Donald Knuth studied the usage of goto statements in programmer code. From this (as in the above example), he recommended the addition of an else to the while and for since the patterns matched very closely.

Today, the else to the while or for might be better thought of as the nobreak part of the while or for.