- This happens because declaring variables inside a for loop wasn't valid C until C99(which is the standard of C published in 1999), you can either declare your counter outside the for as pointed out by others or use the -std=c99 flag to tell the compiler explicitly that you're using this standard and it should interpret it as such.
- /dev/loop. are loop devices making plain files accessible as block devices. They have nothing to do with RAM occupation. They are typically used for mounting disk images, in your case apparently for Ubuntu Snap. See this Wikipedia article for details.
Declaring a variable in the first part of a for loop was a feature added to C as part of the C99 standard. However, most compilers by default use the old C89 standard that don't support this construct.
Loops are used to repeat a block of code. Being able to have your programrepeatedly execute a block of code is one of the most basic but useful tasksin programming -- many programs or websites that produce extremely complexoutput (such as a message board) are really only executing a single task manytimes. (They may be executing a small number of tasks, but in principle, toproduce a list of messages only requires repeating the operation of reading insome data and displaying it.) Now, think about what this means: a loop lets you write a very simplestatement to produce a significantly greater result simply by repetition.
One Caveat: before going further, you should understand the concept ofC++'s true and false, because it will be necessary when working with loops(the conditions are the same as with if statements). There are three types ofloops: for, while, and do..while. Each of them has their specific uses. Theyare all outlined below.
FOR - for loops are the most useful type. The syntax for a for loop is
The variable initialization allows you to either declare a variable and give it a value or give a value to an already existing variable. Second, the condition tells the program that while the conditional expression is true the loop should continue to repeat itself. The variable update section is the easiest way for a for loop to handle changing of the variable. It is possible to do things like x++, x = x + 10, or even x = random ( 5 ), and if you really wanted to, you could call other functions that do nothing to the variable but still have a useful effect on the code. Notice that a semicolon separates each of these sections, that is important. Also note that every single one of the sections may be empty, though the semicolons still have to be there. If the condition is empty, it is evaluated as true and the loop will repeat until something else stops it.
Example: This program is a very simple example of a for loop. x is set to zero, while x is less than 10 it calls cout<< x <<endl; and it adds 1 to x until the condition is met. Keep in mind also that the variable is incremented after the code in the loop is run for the first time.
WHILE - WHILE loops are very simple. The basic structure is
Dev C++ 5.11
while ( condition ) { Code to execute while the condition is true} The true represents a boolean expression which could be x 1 or while ( x != 7 ) (x does not equal 7). It can be any combination of boolean statements that are legal. Even, (while x 5 || v 7) which says execute the code while x equals five or while v equals 7. Notice that a while loop is the same as a for loop without the initialization and update sections. However, an empty condition is not legal for a while loop as it is with a for loop.Example: This was another simple example, but it is longer than the above FOR loop. The easiest way to think of the loop is that when it reaches the brace at the end it jumps back up to the beginning of the loop, which checks the condition again and decides whether to repeat the block another time, or stop and move to the next statement after the block.
DO..WHILE - DO..WHILE loops are useful for things that want to loop at least once. The structure isNotice that the condition is tested at the end of the block instead of the beginning, so the block will be executed at least once. If the condition is true, we jump back to the beginning of the block and execute it again. A do..while loop is basically a reversed while loop. A while loop says 'Loop while the condition is true, and execute this block of code', a do..while loop says 'Execute this block of code, and loop while the condition is true'.
Example: Keep in mind that you must include a trailing semi-colon after the while in the above example. A common error is to forget that a do..while loop must be terminated with a semicolon (the other loops should not be terminated with a semicolon, adding to the confusion). Notice that this loop will execute once, because it automatically executes before checking the condition.
Quiz yourself
Previous: If Statements
C++ 17 For Loop
Next: Functions
Back to C++ Tutorial Index
A loop is used for executing a block of statements repeatedly until a particular condition is satisfied. For example, when you are displaying number from 1 to 100 you may want set the value of a variable to 1 and display it 100 times, increasing its value by 1 on each loop iteration.
In C++ we have three types of basic loops: for, while and do-while. In this tutorial we will learn how to use “for loop” in C++.
Syntax of for loop
Flow of Execution of the for Loop
As a program executes, the interpreter always keeps track of which statement is about to be executed. We call this the control flow, or the flow of execution of the program.
First step: In for loop, initialization happens first and only once, which means that the initialization part of for loop only executes once.
Second step: Condition in for loop is evaluated on each loop iteration, if the condition is true then the statements inside for for loop body gets executed. Once the condition returns false, the statements in for loop does not execute and the control gets transferred to the next statement in the program after for loop.
Third step: After every execution of for loop’s body, the increment/decrement part of for loop executes that updates the loop counter.
Fourth step: After third step, the control jumps to second step and condition is re-evaluated.
C For Loop
The steps from second to fourth repeats until the loop condition returns false.
Example of a Simple For loop in C++
Here in the loop initialization part I have set the value of variable i to 1, condition is i<=6 and on each loop iteration the value of i increments by 1.
Output:
Infinite for loop in C++
A loop is said to be infinite when it executes repeatedly and never stops. This usually happens by mistake. When you set the condition in for loop in such a way that it never return false, it becomes infinite loop.
For example:
This is an infinite loop as we are incrementing the value of i so it would always satisfy the condition i>=1, the condition would never return false.
Here is another example of infinite for loop:
Example: display elements of array using for loop
Output: