CS105: Introduction to Python

The input command is necessary when we want to obtain input from a keyboard. So far, variables have been initialized or assigned values within a given Python script, with commands such as:

a=3
b=27
c=a+b

It is often the case that a program requires keyboard input from the user to perform its function. This is exactly what the 'input' command is for. This instruction can output a message, and the program will wait for the user to input a value. Try running this command in the Repl.it run window:

city=input('Enter the name of the city where you are located: ')
print(city)

You should be aware that the input function returns data of type str (that is, string data). If numerical data is required, an extra step must be taken to perform a type conversion using either the int or float commands. Copy and paste this set of commands into the run window and then run code using an input value of 34 when prompted for user input:

temperature_str=input('Enter the temperature: ')
print(temperature_str)
print(type(temperature_str))
print()
temperature_int=int(input('Enter the temperature: '))
print(temperature_int)
print(type(temperature_int))
print()
temperature_float=float(input('Enter the temperature: '))
print(temperature_float)
print(type(temperature_float))
print()

The variable names have been chosen to emphasize and distinguish between their data types. The input command offers the convenience of creating user-defined values within a program. At the same time, it is important to make sure the input value's data type matches the intended use of the variable.

Loops are necessary when we need to perform a set of operations several times. The first type of loop we will discuss is called the while loop. The while loop checks a logical condition like the if statement. If the condition is true, the code inside the while loop will execute until the condition being checked becomes false. Run this code in Repl.it:

avalue= int(input('Please enter the number 10: '))
while (avalue != 10):
    print('Your input value is not equal to 10')
    print('Please try again: ')
    avalue= int(input('Enter the number 10: '))
print('Thank you')
print('You entered a value of 10')

This while loop checks to see if the value input was equal to 10. If not, the while loop will continue to execute the indented code until a value of 10 is input. The syntax rules are similar to those of the if statements:

• The logical expression following the "while" must end with a colon (:) 
• The code to be executed if the logical condition is satisfied must be indented. 
• Indentation is how Python knows you have a group of commands inside the while statement Indentation can be accomplished in Repl.it using the "tab" key. In the Repl.it editor, this is equivalent to typing two spaces. Some Python editors prefer four spaces for indentation. 

Make sure you run the examples provided in Repl.it. When we execute the code within a loop over and over again, the process is known as iteration.

The for loop is useful when we know how many times a loop is to be executed. Rather than basing the number of iterations on a logical condition, the for loop controls the number of times a loop will iterate by counting or stepping through a series of values. Run this snippet of code in Repl.it:

for i in range(5):
    print(i)

The range(5) command creates a series of five values from 0 to 4 for the variable i to cycle through. The statement for i in range(5) effectively means use the variable i to count through five values starting at 0 and stopping at 4 (Python, by design, begins counting with the value 0). Also, once again, pay close attention to the placement of the colon (:) at the end of the for statement as well as the indentation. The syntax rules should look very familiar at this point. They are exactly the same as for if statements and while loops.

Consider this next snippet of code:

n=8
sumval=0;
for i in range(n):
    sumval = sumval+i
    print('Adding ', i,'to the previous sum = ',sumval)

This loop counts from 0 to n-1, where n has been set to a value of 8. An initial sum is set to zero and then successive values of the variable i are added to the sum each time an iteration of the loop takes place. In other words, the loop counter variable i can be used within the loop as it is being updated. As will we see, the loop variable is often used as part of some computation within the loop. Here is an example of how powerful Python can be when using a for loop to cycle through a series of values.

strval='forever'
e_count=0
for letter in strval:
    if letter =='e':
        e_count = e_count + 1
print('The letter e occurred ', e_count, 'times')

Notice how the "if" statement is "nested" (and, therefore, indented) within the "for" loop. The variable letter cycles through each character in the string strval, which has been initialized to the string forever. If an "e" is found while iterating through each character, the count variable is incremented. When the loop is finished, the number of occurrences is output to the screen.

It is also possible to nest loops within loops. We will discuss more of these types of loops as we go deeper into the course. For now, run this code in Repl.it:

m=4
n=3
for i in range(m):
    for j in range(n):
        print('i=',i,' j=',j)

This is called a nested "for" loop because the inner "for" loop depending on the variable j is nested within the outer "for" loop, which depends on the variable i. This should be clear from the indentation of the inner loop with respect to the outer loop. For each iteration of the variable i, the variable j cycles through all of its possible values which depend upon range(m). In order for ito be updated to its next value, the inner loop must iterate through all of its values. Make sure you can relate the program output shown the screen to how the nested loop iterates through its values.

After you watch the video and test the examples in Repl.it, revisit the file in the "while" loop section and read through the section on "for" loops. Make sure to practice the for loop examples in Repl.it.

Sometimes it is necessary to exit a loop abruptly when a certain condition is met. The "break" statement is sometimes useful for such an application. Consider this code:

strval='forever'
for letter in strval:
    if letter =='e':
        break
print('The letter e has been found')

In this case, the break statement terminates the loop if the character e is found in the strval string.

On the other hand, encountering a given condition within a loop may require skipping over a set of commands and effectively "fast-forward" to the next iteration. The "continue" statement is useful for these applications.

Finally, the "pass" statement within a loop is a null statement that does nothing. It is sometimes useful when placed in a program where code is intended but has not yet been written. Read this page and practice the examples.

Here are some supplementary videos that you can review if you'd like a bit more practice the concepts in this unit.

In this video, course designer Eric Sakk walks through the major topics we covered in Unit 3. As you watch, work through the exercises to try them out yourself.