Python for GATE: Types & Slicing

GATE DA does not test C. The programming questions are in Python, and the signature format is predict the output: you are shown a short snippet and must say exactly what it prints. No coding from scratch — just careful reading. This lesson builds the two skills that win those marks: knowing the types and reading slices. Both pay off well past the exam — every pandas and NumPy data pipeline you will ever write rests on this exact slicing and int-vs-float intuition.

Types, truthiness, and the two divisions

Every value has a type. The four you meet constantly:

• int — whole numbers: 42, -7.
• float — decimals: 3.14, 2.0 (note the dot — 2.0 is a float, 2 is an int).
• str — text in quotes: "GATE", 'da'.
• bool — True or False.

Truthiness: when a value is used as a condition, Python asks “is this truthy?” Falsy values are 0, 0.0, "" (empty string), [] (empty list), and None. Everything else is truthy — including -1 and "False" (a non-empty string).

The exam-favourite operators:

• / is true division and always returns a float: 7 / 2 = 3.5, and even 4 / 2 = 2.0.
• // is floor division: it divides then floors toward negative infinity. 7 // 2 = 3. (Watch negatives: -7 // 2 = -4, because floor of -3.5 is -4.)
• ** is exponentiation: 2 ** 10 = 1024. The caret ^ is not power in Python — it is bitwise XOR, a classic trap for ex-C programmers.
• len(x) gives the number of items; range(n) yields 0, 1, ..., n-1.

Indexing and slicing

A string or list is a sequence you can index. Positions count from 0 on the left and from -1 on the right.

A slice s[start:stop:step] returns a new sub-sequence:

• It includes start but stops before stop (stop is exclusive).
• step defaults to 1; omit a part and the default fills in (s[:3] is from the start, s[3:] is to the end, s[:] is a full copy).
• A negative step walks backward, so s[::-1] reverses the whole sequence.
• s[::2] takes every second item starting at index 0.

Play with it — change the slice and watch the output:

How GATE asks this

The pattern is predict the output, posed as an MCQ (four candidate outputs) or a NAT (give the printed integer, or the length of a slice). Typical wording: “What is the output of the following code?” with a one- or two-line snippet using a slice, //, or **. The test is whether you apply exclusive stop, negative indices, and float vs int division correctly under time pressure. There is no library to recall — just precise evaluation.

Worked example

Trace this line one slice at a time, against the index diagram above:

s = "GATE2026"
print(s[1:4], s[-2:], s[::-1], s[::2])

• s[1:4] — start at index 1 (A), stop before index 4. Indices 1, 2, 3 give A, T, E → ATE.
• s[-2:] — start two from the end (index -2, the second 2), go to the end. Characters at -2 and -1 → 26.
• s[::-1] — no start/stop, step -1, so walk the whole string backward → 6202ETAG.
• s[::2] — every second character from index 0: indices 0, 2, 4, 6 → G, T, 2, 2 → GT22.

So print (which joins its arguments with spaces) shows:

ATE 26 6202ETAG GT22

The fourth slice is the one that trips people up: index 6 of "GATE2026" is the second 2, not the 6 — so it is GT22, not GT26.

Quick check