Control flow

The for loop

A for loop allows the same block of code to be executed multiple times

for item in iterable:
    code to be executed

Iteration

Many of the data structures we've reviewed are defined as iterables in Python. Suppose we have a list of scan information where each item in the list contains a dict of scan properties

scans = [
    {
        'num': 1,
        'type': 'LOCALIZER',
    },
    {
        'num': 2,
        'type': 'MEMPRAGE'
    },
    {
        'num': 3,
        'type': 'BOLD'
    },
    {
        'num': 4,
        'type': 'BOLD'
    }
]

If you want to retrieve the scan numbers for all BOLD scans, you could use a for loop to iterate over scans and call the same block of code to populate a list of scan numbers

numbers = []

for scan in scans:
    if scan['type'] == 'BOLD':
        numbers.append(scan['num'])

print(numbers)

This will print the result [3, 4].

Iterating over a dictionary

It's also common to iterate over a dict, however there are some subtle differences. By default, when you iterate over a dict you'll only receive its keys

a = {
    'key1': 1,
    'key2': 2,
    'key3': 3
}

for key in a:
    value = a[key]
    print(value)

If you want to iterate over the dict returning both the key and value as a pair, you would use the following syntax

for key,value in a.items():
    print('key =', key, 'value =', value)

The .items() method on a dictionary will return all key/value pairs as a list of tuples. The for loop shown above is unpacking each tuple into the separate variables for key and value.