String Reverse Program Python में — code, output और dry run

Problem statement

Approach 1: simple loop

Walk the string from the last character to the first and collect them one by one. len(s) - 1 is the last position, and the loop counts down to 0.

s = "python"
reversed_text = ""

for i in range(len(s) - 1, -1, -1):
    reversed_text = reversed_text + s[i]

print(reversed_text)
# Output: nohtyp

Each iteration adds one character to the front-runner string. Slow for huge inputs, but it shows exactly what "reversing" means.

Approach 2: reusable function

Put the loop in a function and return the result. A function makes the operation reusable and keeps the original string untouched — the caller stores the returned value.

def reverse_string(text):
    result = ""
    for ch in text:
        result = ch + result      # prepend: new char goes in FRONT
    return result

print(reverse_string("python"))   # nohtyp
print(reverse_string("a"))        # a
print(reverse_string(""))         # (empty string)

This version walks forwards but prepends each character, which reverses just as well — and handles the empty string with no extra code.

Approach 3: Pythonic alternative

Python's slice notation has a built-in step argument, and a step of -1 walks the whole string backwards in one expression — the idiomatic answer.

s = "python"
print(s[::-1])               # nohtyp
print("".join(reversed(s)))   # nohtyp — the explicit cousin

[::-1] reads "from start to end, stepping backwards". The reversed() + join() form spells the same idea out longhand and is easier to adapt when you need every second character or a partial reverse.

Dry run

Common errors

• s.reverse() call करना — string में reverse method होती ही नहीं (list में होती है)।
• String को in-place बदलने की कोशिश — strings immutable हैं।
• Countdown loop में range(len(s), 0, -1) लिखना — पहला character छूट जाएगा।
• reversed(s) को सीधे print करना — पहले join करना ज़रूरी है।
• Loop में पीछे की बजाय आगे जोड़ना — उल्टी की जगह वही string दोबारा बन जाएगी।

Practice on PyDebug

Concept समझने के बाद इन free problems को browser में solve करें:

• Range boundary practice
• Operator precedence practice
• Variables and assignment practice