Loops

6.1. Loops#

6.1.1. Repeating code using loops#

for <ele> in <sequence>:
    <body>

  • The loop index variable ele takes on each successive value in the sequence, and the statements in the body of the loop are executed once for each value.

  • For loops have a limitation – you have to know how many times you are looping – it is a definite loop. The number of iterations is determined when the loop starts. If you do not know how many times you will be looping, use a while loop, which is an indefinite loop that will continue to loop until its condition is no longer true.

while <condition>
    <body>

6.1.2. Controlling Loops#

  • break - Used to break a loop

  • continue - Used to skip the current loop and go to the next value

6.1.3. Range#

range(start,stop,step) # [start:step:stop)

6.1.4. Examples#

6.1.4.1. Example 1#

values = [4, 10, 3, 8, -6]
for i in range(len(values)):
    print(i)

6.1.4.2. Example 2#

values = [4, 10, 3, 8, -6]
for i in range(len(values)):
    print(i, values[i])

6.1.4.3. Example 3#

values = [4, 10, 3, 8, -6]
for i in range(len(values)):
    print(i, values[i])

6.1.4.4. Example 3#

values = [4, 10, 3, 8, -6]
for index, value in enumerate(values):
    print(index, value)

6.1.4.5. Example 4#

values = [4, 10, 3, 8, -6]
for i in range(len(values)):
    values[i] = values[i] * 2

6.1.4.6. Example 5#

metals = ['Li', 'Na', 'K']
weights = [6.941, 22.98976928, 39.0983]
for i in range(len(metals)):
    print(metals[i], weights[i])

6.1.4.7. Example 6#

metals = ['Li', 'Na', 'K']
weights = [6.941, 22.98976928, 39.0983]
for metal, weight in zip(metals, weights):
    print(metal, weight)

6.1.4.8. Example 7#

elements = [['Li', 'Na', 'K'], ['F', 'Cl', 'Br']]
for inner_list in elements:
    for item in inner_list:
        print(item)

6.1.4.9. Example 8#

info = [
    ['Isaac Newton', 1643, 1727],
    ['Charles Darwin', 1809, 1882],
    ['Alan Turing', 1912, 1954, 'alan@bletchley.uk']
]
for item in info:
    print(len(item))

6.1.5. How to sum numbers in an array?#

x = [1, 7, 3]

sum = 0
for ele in x:
    sum = sum *10 + ele

6.1.6. How to sort without sorting?#

string1 = '1134000234'
string2 = ''.join(sorted([ele for ele in string1]))
string3 = ''.join(sorted([ele for ele in string1], reverse=True))
print(string1, string2, string3)

store = [0] * 10

for char in string1:
    index = int(char)
    store[index] += 1

sorted_string = ''
for idx, ele in enumerate(store):
    sorted_string += str(idx) * ele
print(sorted_string)

6.1.7. How to sum an integer?#

number = 1234
sum = 0
while number:
    sum += number % 10
    number = number // 10 
print(sum)

6.1.8. File Handling#

with open(filename, 'r') as file:
    for line in file:
        if not line.strip(): # used for skipping empty lines!
            continue
        # do something with line

6.1.9. Skipping Bad values#

You can use a try/except blocks to skip bad values

try:
    value = float(line)
except:
    continue

6.1.10. Initial Values#

Sometimes before the for loop code you to have initialize a variable to 0 or an empty list.

  1. If you want to calculate the average or sum of a list, for example, you would need to initialize a variable to zero and update it every time you access a new element in a list.

total = 0
count = 0
for value in my_list:
   total += value
   count += 1
average = total / count

  1. If you want to keep track of some values as you loop over some list, you might need to initialize an empty list. One example is removing duplicates from a sequence.

unique_values = []
for ele in sequence:
    if ele not in unique_values:
        unique_values.append(ele)
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