Reading input

Earlier modules read one value from one line. Many CCC problems give you several values on a single line instead, such as two prices separated by a space. This lesson covers reading a line like that, and turning it into the values your program needs.

Splitting one line into several pieces

input() still reads one whole line as text, the same as before. A str value has methodsmethodA function called on a value using a dot, such as .split() called on a string.In the glossary, functions called on the value itself with a dot. .split() is one of them. Called with nothing inside its parentheses, it breaks a line into pieces wherever there is a run of spaces, and hands them back as a list of text.

parts = input().split()print(parts)

3 5

['3', '5']

The line 3 5 splits into a list of two pieces of text, '3' and '5'. Notice these are still text, not numbers, and notice they come back together inside one list named parts. .split() only cuts a line apart; it does not convert anything.

.split() also handles more than one space between values. A line with extra spaces between its numbers splits the same way a line with a single space would. The pieces are what is between the runs of spaces, not the spaces themselves.

Converting every piece at once with map

Each piece from .split() still needs int() to become a number, the way a single line did in the module on standard input. Converting several pieces one by one would need one int() call per piece, written out separately. The map(int, ...) call does all of them in a single step instead. It applies int() to every piece that .split() produced, and returns the converted results together.

a, b = map(int, input().split())print(a + b)

3 5

8

map(int, input().split()) reads the line, splits it into pieces, and converts every piece with int(), all in one call. The result feeds straight into the assignment below.

Assigning several names at once

a, b = map(int, input().split()) does two things on one line. It runs the split-and-convert step above, then assigns the two results to a and b at once. This is multiple assignmentmultiple assignmentAssigning several values to several names in one line, matched left to right by position.In the glossary: as many names on the left as there are values on the right, matched in order, left to right.

1a, b = input().split()2print(a, b)

3 5 2

Traceback (most recent call last):
  File "unpack_mismatch.py", line 1, in <module>
    a, b = input().split()
ValueError: too many values to unpack (expected 2)

The line 3 5 2 splits into three pieces of text, but only two names, a and b, wait on the left. Python cannot match three pieces to two names, so it raises a ValueError rather than quietly dropping one. The number of names on the left must match the number of pieces the line actually holds, every time.

Too few names on the left raises the same kind of error, the other way around: a ValueError saying not enough values were given instead of too many. Either way, the fix is the same: count how many values the problem's input actually gives you on that line, and write exactly that many names.

Removing extra whitespace with .strip()

input() already drops the newline at the end of a line for you. It leaves everything else untouched, including a space a person might type by accident before or after the text that matters. Note that .split() already ignores spaces at both ends of a line while breaking it apart. .strip() matters most for a line you read whole, with no .split() in between.

name = input()print(name + "!")

Maya 

Maya !

The input line has a space after the name. input() keeps that space, so name still carries it. Printing name + "!" next to it makes the extra space visible, as the gap that lands right before the !.

name = input().strip()print(name + "!")

Maya 

Maya!

.strip() removes whitespace from both ends of a string: spaces, tabs, and any newline still attached, though input() has already removed that part. Calling it here removes exactly the trailing space the first example kept, so the ! now sits right against the name with no gap.

Some judges overlook a trailing space, but others do not. The safest habit is to never rely on which kind you are facing, and .strip() removes the risk whenever a line you read might carry whitespace you did not ask for.

Reading two values, with a trace

(nothing printed yet)

Both presets read a single line with two numbers on it, then add them the same way, whatever the two numbers are. Watch the three steps run in order: parts holds the split pieces, then a and b appear together once map and the assignment convert them.

A common mistake is calling .split() on a value that has already gone through int(). The reverse mistake is calling int() on a whole unsplit line. Remember what each one accepts: .split() only works on text, and int() only works on one piece of text at a time, never on a whole list at once.

When one line holds a number of values that is not fixed

Every example so far has read a line with exactly two values, so a, b = ... always had exactly two names waiting on the left. Some problems instead give you a line whose count of values changes from one test case to the next. That count is decided by the test data, not by how you wrote the code.

input().split() still handles a line like that the same way, no matter how many pieces it holds. It returns a list with exactly as many pieces as the line has, whatever that count turns out to be. The trouble comes right after: multiple assignment needs to know in advance how many names to write, so it cannot handle a line whose value count changes between test cases. Reading every piece a line like that holds, one at a time, needs a loop instead of a fixed list of names. That waits for the module on for loops, which covers exactly this shape of problem.

Recap

This lesson covered splitting a line into pieces with .split(), converting every piece at once with map(int, ...), assigning matched values to several names at once, and removing extra whitespace with .strip().