Généralités
L’utilisation massive de la call stack peut amoindrir les performances.
Factorielle
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private static int Factorielle(int i)
{
if (i == 1)
{
return 1;
}
else
{
return i * Factorielle(i - 1);
}
}
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i = 3
return 3 * Factorielle(2)
return 2 * Factorielle(1)
return 1
Invertion d'une chaine de caractères
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private string Reverse(string input)
{
if (input.Length == 1)
{
return input;
}
// version 1: on met le premier caractère à la fin
return Reverse(input.Substring(1)) + input[0];
// version 2: on met le dernier caractère en premier
return input.Last() + Reverse(input.Substring(0, input.Length - 1));
}
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v1
input = "abc"
return Reverse("bc") + "a"
return Reverse("c") + "b"
return "c"
v2
input = "abc"
return "c" + Reverse("ab")
return "b" + Reverse("a")
return "a"
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private Dictionary<char, Func<decimal, decimal, decimal>> operations =
new Dictionary<char, Func<decimal, decimal, decimal>>
{
['+'] = (current, next) => current + next,
['-'] = (current, next) => current - next,
['/'] = (current, next) => current / next,
['*'] = (current, next) => current * next
};
public decimal Evaluate(string expression)
{
foreach (var operation in operations)
{
if (expression.Contains(operation.Key))
{
var parts = expression.Split(operation.Key);
return parts.Skip(1).Aggregate(
Evaluate(parts[0]),
(current, next) => operation.Value(current, Evaluate(next)));
// équivalent à
var total = Evaluate(parts[0]);
for (int i = 1; i < parts.Length; i++)
{
total = operation.Value(total, Evaluate(parts[i]));
}
return total;
}
}
decimal.TryParse(expression, out decimal value);
return value;
}
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expression = "1 + 2 * 3"
split + : ["1", "2 * 3"]
Evaluate("1") → return 1
Evaluate("2 * 3")
split * : ["2", "3"]
Evaluate("2") → return 2
Evaluate("3") → return 3
operation * → 2 * 3 = 6
operation + → 1 + 6 = 7
expression = "2 * 3 + 1"
split + : ["2 * 3", "1"]
Evaluate("2 * 3")
split * : ["2", "3"]
Evaluate("2") → return 2
Evaluate("3") → return 3
operation * → 2 * 3 = 6
Evaluate("1") → return 1
operation + → 6 + 1 = 7
