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For loop

PHP: For loop

With a while loop you can solve any element-iteration task, but it stands out for its verbosity. For while you need to introduce a counter in advance, set the stopping condition, and remember to change the counter inside the loop body. When there are only a few loops, this is fine, but in real code loops appear at every turn. So managing the counter by hand is tiresome, especially when the pattern is always the same.

For such situations PHP has the for loop. It gathers all the counter management in one place — inside the parentheses:

while:                      for:

$i = 0;                     for ($i = 0; $i < 5; $i += 1) {
while ($i < 5) {                ...
    ...                     }
    $i += 1;
}
│                           │
└── management scattered    └── all management on a single line

Let's look at what's inside the parentheses:

  1. The initial counter value — this code runs exactly once before the first iteration
  2. The predicate (the loop repetition condition) — runs on each iteration, just like in while
  3. The description of how the counter changes — this code runs at the end of each iteration

In every other respect the principle of operation is exactly the same as the while loop.

Let's see how to implement a string-reversal function using a for loop. Such a task comes up in interviews:

<?php

function reverseString(string $str): string
{
    // Initial value
    $result = '';

    for ($i = 0; $i < strlen($str); $i += 1) {
        $currentChar = $str[$i];
        // Concatenate in reverse order
        $result = "{$currentChar}{$result}";
    }

    // The loop ends when the whole string has been traversed
    return $result;
}

print_r(reverseString('go!')); // => !og

You can read this code like this:

The loop with index $i repeats while the condition $i < strlen($str) holds, and after each step $i increases by 1

Let's go through in detail how the loop in the example above works at each iteration step:

reverseString('go!')

before the loop: result = ''

iteration 1: currentChar = 'g'
result = currentChar . result
       = 'g' . ''
       = 'g'

iteration 2: currentChar = 'o'
result = currentChar . result
       = 'o' . 'g'
       = 'og'

iteration 3: currentChar = '!'
result = currentChar . result
       = '!' . 'og'
       = '!og'

Now let's count how many times a character appears in a string, ignoring case:

<?php

// $text - arbitrary text
// $char - the character to count
function charsCount(string $text, string $char): int
{
    // Since we're looking for a sum, the initial value is 0
    $result = 0;

    for ($i = 0; $i < strlen($text); $i += 1) {
        // We bring everything to lowercase
        // so as not to depend on the current case
        if (strtolower($text[$i]) === strtolower($char)) {
            $result += 1;
        }
    }

    return $result;
}

charsCount('hexlet!', 'e'); // 2
charsCount('hExlet!', 'e'); // 2
charsCount('hExlet!', 'E'); // 2
charsCount('hexlet!', 'a'); // 0

Instructions

Implement a function normalizeFilename() that prepares a file name for saving. The function takes a string and returns a new string where all spaces are replaced with the _ character.

Example call:

<?php

normalizeFilename('my photo.png');      // 'my_photo.png'
normalizeFilename('final report.pdf');  // 'final_report.pdf'
normalizeFilename('already_ready.txt'); // 'already_ready.txt'

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
Found a bug? Have something to add? Pull requests are welcome!