For loops¶

A for loop executes commands once for each value in a collection.¶

  • Doing calculations on the values in a list one by one is as painful as working with pressure_001, pressure_002, etc.

  • A for loop tells Python to execute some statements once for each value in a list, a character string, or some other collection.

  • “for each thing in this group, do these operations”

for number in [2, 3, 5]:
    print(number)
  • This for loop is equivalent to:

print(2)
print(3)
print(5)
  • And the for loop’s output is:

A for loop is made up of a collection, a loop variable, and a body.¶

for number in [2, 3, 5]:
    print(number)

A for loop is made up of a collection, a loop variable, and a body.¶

  • The collection, [2, 3, 5], is what the loop is being run on.

  • The body, print(number), specifies what to do for each value in the collection.

  • The loop variable, number, is what changes for each iteration of the loop.

    • The “current thing”.

The first line of the for loop must end with a colon, and the body must be indented.¶

  • The colon at the end of the first line signals the start of a block of statements.

  • Python uses indentation rather than {} or begin/end to show nesting.

    • Any consistent indentation is legal, but almost everyone uses four spaces.

for number in [2, 3, 5]:
print(number)
  • Indentation is always meaningful in Python.

firstName = "Jon"
  lastName = "Smith"
  • This error can be fixed by removing the extra spaces at the beginning of the second line.

Loop variables can be called anything.¶

  • As with all variables, loop variables are:

    • Created on demand.

    • Meaningless: their names can be anything at all.

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