Q.Program 8-4: Write a program which reverses a string passed as parameter and stores the reversed string in a new string. Use a user defined function for reversing the string.

A function walks the parameter string backwards with negative indices, appending each character to a NEW string, and returns it (NCERT Program 8-4):

def reverseString(st):
    newStr = ''
    length = len(st)
    for i in range(-1, -length - 1, -1):
        newStr += st[i]
    return newStr

st = input("Enter a string: ")
st1 = reverseString(st)
print("The original string is:", st)
print("The reversed string is:", st1)

Enter a string: Python
The original string is: Python
The reversed string is: nohtyP
``` …

Idea. This program contrasts with Program 8-3: there we merely printed in reverse; here the reversed text must be stored in a new string. The function takes the string as a parameter, iterates the negative indices -1 down to -len(st), concatenates each character onto newStr, and returns the result — the original string stays intact (strings are immutable anyway).

def reverseString(st):
    """Return a new string that is the reverse of st."""
    newStr = ''
    length = len(st)
    for i in range(-1, -length - 1, -1):   # -1, -2, ..., -length
        newStr += st[i]
    return newStr

# main program
st = input("Enter a string: ")
st1 = reverseString(st)
print("The original string is:", st)
print("The reversed string is:", st1)

Key lines:

• range(-1, -length - 1, -1) — produces -1, -2, …, -length: last character first, first character last.
• newStr += st[i] — the accumulation step; this is what builds the new reversed string (the difference from Program 8-3).
• return newStr — the reversed string travels back to the caller and lands in st1.

Trace for st = "Python" (length 6):

| i | st[i] | newStr |

|---|-------|--------| …