How to understand the difference between the for and foreach loop structures in PHP for traversing arrays

Traversing an array is the most common thing in programming. Here we discuss the two methods of for and foreach:

First, we prepare two arrays for traversal:

$arr1=array(1=>'a', 3=>22, 5=>'b', 4=>'c', 8=>'d');

$arr2=array('a'=>'aaa', 'b'=>'bbb', 'c'=>'ccc', 'd'=>'ddd', 'e'=> ;'eee');

1: for loop structure

Loop 1:

for($i=0,$num=count($arr1);$i<$num;$i++){
    echo $arr1[$i].&#39; &#39;;
}

Output result: a 22 c

Loop 2:

for($i=0,$num=count($arr2);$i<$num;$i++){
    echo $arr2[$i].&#39; &#39;;
}

This statement has no output

Analysis:

Loop 1 only prints out the first two units of array $arr1, while $arr2 in loop 2 prints nothing. come out.

The reason is that the for loop increments according to the number, so for can only access the index array. For example, loop 1 incrementally accesses the $arr1 array from $i=0 to $i=4. The keys in the $arr1 array are from 0 to 4 cells, but the keys in the array are: 1,3,5,4,8. Array units with key values ​​exceeding 4 (5=>'b',8=>'d') will not be accessed, because count($arr1)=5, so $i<5; therefore the final output result Only: a 22 c; for all keys in $arr2 are characters, not numbers, so there is no output in loop 2.

2: foreach loop structure

Loop 3:

foreach($arr1 as $key=>$value){
    echo $key.'=>'.$value.' ';
}

Output result: 1=>a 3=>22 5=>b 4=>c 8=>d

Loop 4:

foreach($arr2 as $key=>$value){
    echo $key.'=>'.$value.' ';
}

Output result: a=>aaa b=>bbb c=>ccc d=>ddd e=>eee

Analysis:

The foreach loop structure loops according to the pointer inside the array. When foreach starts execution, the pointer inside the array will automatically point to Unit 1. Therefore, the next unit will be obtained in the next loop, and there is no need to traverse the entire array according to the array key. This is also the difference between foreach and for. Of course, foreach can only be used on arrays and objects, and since foreach relies on an internal array pointer, modifying its value in a loop may cause unexpected behavior.

Note: Each loop of for operates on the value under the corresponding index, and changes to each value will also be reflected in the traversed object. Each time foreach operates a unit, it takes its index and value into variables respectively, or only takes out the value into one variable, and then operates the variables with the index and value separately, without affecting the traversed object itself. . If you want to modify the value in the object during the traversal process, you need to add an & symbol before the variable in the declaration. For example: foreach($array as &$value).

Conclusion: If an array uses consecutive numbers as the keys of the array units, then a for loop structure can be used. If you are not sure whether the keys of the array or the keys of the array contain characters, you should use the foreach loop structure. .

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