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Grade 9 algorithmsClasa a IX-a

The grade 9 part of the collection, chapter by chapter. Read the idea, run the code against the examples and track what you've learned.

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87algorithms
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Grade 9Clasa a IX-a

Digits of a number

Cifrele unui număr · 15 algorithms

Number of occurrences of digit kNumărul de apariții ale cifrei kWe go through the digits of n and count how many are equal to k. With do-while, for n = 0 and k = 0 the answer is correctly 1.O(n)
Largest number formed from the digits of nCel mai mare număr format cu cifrele lui nWe count how many times each digit appears, in a frequency array with 10 positions (frecv[c] = how many times digit c appears). The largest number…O(n)
Check digit (digital root)Cifra de controlThe check digit is obtained by repeatedly replacing the number with the sum of its digits until a single digit remains (for example 99999 -> 45 ->…O(n)
Largest and smallest digitCifra maximă și cifra minimăWe start with the maximum at 0 (the smallest possible digit) and the minimum at 9 (the largest possible digit), then compare every digit with them.…O(n)
Checking for distinct digitsVerificarea cifrelor distincteChecks whether all digits of n are different. We remember which digits we have already seen (vazut[c] becomes true on the first occurrence of c); if…O(n)
Printing the digits in their order in the numberAfișarea cifrelor în ordinea din numărWith n % 10 the digits come out from right to left. To print them from left to right, we first compute p = the largest power of 10 not greater than…O(n)
Doubling the odd digitsDublarea cifrelor impareEvery odd digit of n appears twice in the result. We build the result from right to left with rez = rez + digit * p; an odd digit is added twice, so…O(n)
Removing the even digitsEliminarea cifrelor pareWe build a new number from the odd digits of n, keeping their order. The digits come from right to left, so every kept digit is placed in front of…O(n)
Replacing one digit with anotherÎnlocuirea unei cifre cu altaReplaces every occurrence of digit x in n with digit y. We rebuild the number from right to left with rez = rez + digit * p, putting y in place of…O(n)
Palindrome numberNumăr palindromA number is a palindrome if it reads the same from left to right and from right to left, i.e. it is equal to its reverse. We work on a copy, because…O(n)
Number of digits of a numberNumărul de cifre ale unui numărCounts the digits of n by cutting off the last digit at every step (n / 10) until the number becomes 0. We use do-while so that the number 0 is…O(n)
Reverse of a numberOglinditul unui numărBuilds the number made of the digits of n written in reverse order. At every step we take the last digit (n % 10), append it to the right of the…O(n)
First digit of a numberPrima cifră a unui numărThe first (leftmost) digit is what remains after we cut off all the other digits: we divide by 10 while the number has at least two digits (n >= 10).O(n)
Product of the digits of a numberProdusul cifrelor unui numărMultiplies the digits of n. The product starts from 1 (the identity element of multiplication), not from 0. With do-while, for n = 0 we correctly…O(n)
Sum of the digits of a numberSuma cifrelor unui numărAdds up the digits of n. The last digit is obtained with n % 10, then removed with n / 10. We repeat until n becomes 0.O(n)

Divisibility and prime numbers

Divizibilitate și numere prime · 17 algorithms

Number sequences read one at a time

Șiruri de numere citite pe rând · 13 algorithms

Number bases

Baze de numerație · 6 algorithms

One-dimensional arrays (vectors)

Tablouri unidimensionale (vectori) · 11 algorithms

Sorting and searching

Sortare și căutare · 10 algorithms

Frequency arrays, prefix sums and the Sieve of Eratosthenes

Frecvență, sume parțiale și ciurul lui Eratostene · 10 algorithms

Text files

Fișiere text · 5 algorithms