for(n) Tcl Built-In Commands for(n)

```

```

# [NAME](#NAME)

for - 'For' loop

# [SYNOPSIS](#SYNOPSIS)

**for start test next body**

```

```

# [DESCRIPTION](#DESCRIPTION)

**for** is a looping command, similar in structure to the C **for** statement. The *start*, *next*, and *body* arguments must be Tcl command strings, and *test* is an expression string. The **for** command first invokes the Tcl interpreter to execute *start*. Then it repeatedly evaluates *test* as an expression; if the result is non-zero it invokes the Tcl interpreter on *body*, then invokes the Tcl interpreter on *next*, then repeats the loop. The command terminates when *test* evaluates to 0. If a **continue** command is invoked within *body* then any remaining commands in the current execution of *body* are skipped; processing continues by invoking the Tcl interpreter on *next*, then evaluating *test*, and so on. If a **break** command is invoked within *body* or *next*, then the **for** command will return immediately. The operation of **break** and **continue** are similar to the corresponding statements in C. **For** returns an empty string.

Note that *test* should almost always be enclosed in braces. If not, variable substitutions will be made before the **for** command starts executing, which means that variable changes made by the loop body will not be considered in the expression. This is likely to result in an infinite loop. If *test* is enclosed in braces, variable substitutions are delayed until the expression is evaluated (before each loop iteration), so changes in the variables will be visible. See below for an example:

# [EXAMPLES](#EXAMPLES)

Print a line for each of the integers from 0 to 9:

```
for {set x 0} {$x<10} {incr x} {


    puts "x is $x"
}
```

Either loop infinitely or not at all because the expression being evaluated is actually the constant, or even generate an error! The actual behaviour will depend on whether the variable *x* exists before the **for** command is run and whether its value is a value that is less than or greater than/equal to ten, and this is because the expression will be substituted before the **for** command is executed.

```
for {set x 0} $x<10 {incr x} {


    puts "x is $x"
}
```

Print out the powers of two from 1 to 1024:

```
for {set x 1} {$x<=1024} {set x [expr {$x * 2}]} {


    puts "x is $x"
}
```

# [SEE ALSO](#SEE_ALSO)

[break(n)](/man/os/tcl/break.n), [continue(n)](/man/os/tcl/continue.n), [foreach(n)](/man/os/tcl/foreach.n), [while(n)](/man/os/tcl/while.n)

# [KEYWORDS](#KEYWORDS)

boolean, for, iteration, loop

Tcl

---

Document: for(n) from tcl 9.1.0-r1 (Chainguard OS)
Source: https://downloads.sourceforge.net/project/tcl/Tcl/9.1.0/tcl9.1.0-src.tar.gz