Humans are, by nature, repetitive beings. Watch the Star Trek: TNG episode “Cause and Effect”, and you will quickly understand what a loop is as the Enterprise is caught in a causality time loop. There are also times when programs must do a task more than once. If the repetition is limited to some countable number of recurrences with a foreseeable end then it is called a loop. And if, at least in the perception of the individual who is experiencing a certain situation, there is no end in sight then one is talking about endless or infinite loops. All of the above can be desirable or not. Here is a visual illustration:

The code in the program moves linearly from statement A, until it encounters a loop. At the start of the loop, the loop control tests some condition. If the condition is true, then the loop activates the piece of code (made up of one or more statements) in what is called the loop body. When that code is done the loop returns to the loop control, and the process starts all over. If the condition in the loop control becomes false, nothing happens, and the program ends the loop, and continues on to statement B. There is also a possibility that the first time into the loop, the condition becomes false, and in that case the loop is bypassed.

Loops are structures that allow for repetition – repeating an activity a number of times. There are different kinds of repetition:

1. Loops where the number of repetitions is known before the loop activates.
2. Loops where the number of repetitions isn’t known, and is controlled by some condition.
3. Loops that are infinite, i.e. could continue to infinity and beyond.

The best example of the first case encountered in most languages is the for loop. Here we use a variable called a loop index to control the loop. Each time a loop loops it is called an iteration. So if the loop index is x, and it repeats 100 times, then x will begin with a value of 1, and increment by 1 each iteration, until x reaches 100. In Julia this is easily achieved in the following manner (in this case the loop simply prints out each value of x):

for x = 1:100
   println(x)
end

In Ada it takes a similar format (the printing part is a little different, but just ignore that bit):

for x in 1..100 loop
    put(x); 
    new_line;
end loop;

Ada uses the keyword loop to signify it as a loop. Fortran also uses a similar construct, but has never used the for loop, opting instead for the keyword do. Same concept, different name.

do x = 1, 100
   write(*,*) x
end do

C also uses a for loop, although it can be somewhat more convoluted:

for (x=1; x<=100; x=x+1)
   printf("%d\n",x);

All have their varied syntax, but achieve the same goal – a known number of iterations.

Humans are, by nature, repetitive beings. Watch the Star Trek: TNG episode “Cause and Effect”, and you will quickly understand what a loop is as the Enterprise is caught in a causality time loop. There are also times when programs must do a task more than once. If the repetition is limited to some countable number of recurrences with a foreseeable end then it is called a loop. And if, at least in the perception of the individual who is experiencing a certain situation, there is no end in sight then one is talking about endless or infinite loops. All of the above can be desirable or not. Here is a visual illustration:

The code in the program moves linearly from statement A, until it encounters a loop. At the start of the loop, the loop control tests some condition. If the condition is true, then the loop activates the piece of code (made up of one or more statements) in what is called the loop body. When that code is done the loop returns to the loop control, and the process starts all over. If the condition in the loop control becomes false, nothing happens, and the program ends the loop, and continues on to statement B. There is also a possibility that the first time into the loop, the condition becomes false, and in that case the loop is bypassed.

Loops are structures that allow for repetition – repeating an activity a number of times. There are different kinds of repetition:

1. Loops where the number of repetitions is known before the loop activates.
2. Loops where the number of repetitions isn’t known, and is controlled by some condition.
3. Loops that are infinite, i.e. could continue to infinity and beyond.

The best example of the first case encountered in most languages is the for loop. Here we use a variable called a loop index to control the loop. Each time a loop loops it is called an iteration. So if the loop index is x, and it repeats 100 times, then x will begin with a value of 1, and increment by 1 each iteration, until x reaches 100. In Julia this is easily achieved in the following manner (in this case the loop simply prints out each value of x):

for x = 1:100
   println(x)
end

In Ada it takes a similar format (the printing part is a little different, but just ignore that bit):

for x in 1..100 loop
    put(x); 
    new_line;
end loop;

Ada uses the keyword loop to signify it as a loop. Fortran also uses a similar construct, but has never used the for loop, opting instead for the keyword do. Same concept, different name.

do x = 1, 100
   write(*,*) x
end do

C also uses a for loop, although it can be somewhat more convoluted:

for (x=1; x<=100; x=x+1)
   printf("%d\n",x);

All have their varied syntax, but achieve the same goal – a known number of iterations.