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A - A Substring 解説 by en_translator

For beginners

Roughly, there are two approaches.

  1. Print the characters that are not removed in order.
  2. Construct a string to be printed, and print it.

We describe each approach.

1. Print the characters that are not removed in order

The characters removed from \(S\) are the \(1\)-st, \(2\)-nd, \(\ldots,A\)-th, \(N-B+1\)-th \(,N-B+2\)-th, \(\ldots\), and \(N\)-th characters.

In other words, the characters that are not removed are the \(A+1\)-th through \(N-B\)-th characters.

The problem can be solved by printing these characters in order. If the printing function of your language emits a space or a newline at the end by default, be careful not to print an excessive delimiter when printing the characters one by one.

The following is sample code.

Sample code in C++

#include <iostream>
#include <string>
using namespace std;

int main() {
    int N, A, B;
    cin >> N >> A >> B;
    string S;
    cin >> S;

    // Remove the first A characters and remove B characters
    // = in 0-based indexing, A-th and later characters, but before the B-th, remain.
    int start = A;
    int end = N - B;

    // Print the characters not removed
    for (int i = start; i < end; ++i) {
        cout << S[i];
    }
    cout << endl;
    return 0;
}

Python での実装例

N, A, B = map(int, input().split())

S = input()

# Remove the first A characters and remove B characters
# = in 0-based indexing, A-th and later characters, but before the B-th, remain.
start = A
end = N - B

# Print the characters not removed
for i in range(start, end):
    print(S[i], end='')

2. Construct a string to be printed and print it

In some languages, there is a method to extract a consecutive portion of a string to obtain a new string.

For example, in C++ this can be done by calling a member function substr on a string-type variable; in Python, by accessing a str-type variable as a slice.

The following is sample code.

Sample code in C++

The substr function accepts the position of the first character and the number of characters to extract, to obtain a consecutive portion of a string.

#include <iostream>
#include <string>
using namespace std;

int main() {
    int N, A, B;
    cin >> N >> A >> B;
    string S;
    cin >> S;

    // (in 0-based indexing) from the A-th character, extract a total of (N - A - B) characters
    cout << S.substr(A, N - A - B) << endl;
    return 0;
}

Sample code in Python

In Python, when you want to remove the last \(B\) characters of a string using the slice notation, if you apply [:-B] the behavior is not what you want when \(B=0\), so you need to tweak the method.

For example, one can append one more character to \(S\) so that \(B\) becomes positive, or access by \(N-B\) instead of \(-B\).

N, A, B = map(int, input().split())

S = input()
S += 'a' # to access with [:-B], append one excessive character
B += 1 # increment B as well

print(S[A:-B])

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