Programming Tutorials

Looping with the keyword goto in C++

By: Lakshmi in C++ Tutorials on 2007-09-09  

In the primitive days of early computer science, programs were nasty, brutish, and short. Loops consisted of a label, some statements, and a jump.

In C++, a label is just a name followed by a colon (:). The label is placed to the left of a legal C++ statement, and a jump is accomplished by writing goto followed by the label name. The following C++ program illustrates this.

Looping with the keyword goto.

1:    // 
2:    // Looping with goto
3:
4:    #include <iostream.h>
5:
6:    int main()
7:    {
8:           int counter = 0;      // initialize counter
9:    loop:  counter ++;           // top of the loop
10:            cout << "counter: " << counter << "\n";
11:           if (counter < 5)            // test the value
12:               goto loop;                 // jump to the top
13:
14:           cout << "Complete. Counter: " << counter << ".\n";
15:       return 0;
16: }
Output: counter: 1
counter: 2
counter: 3
counter: 4
counter: 5
Complete. Counter: 5.

Analysis: On line 8, counter is initialized to 0. The label loop is on line 9, marking the top of the loop. Counter is incremented and its new value is printed. The value of counter is tested on line 11. If it is less than 5, the if statement is true and the goto statement is executed. This causes program execution to jump back to line 9. The program continues looping until counter is equal to 5, at which time it "falls through" the loop and the final output is printed.

Why goto Is Shunned

goto has received some rotten press lately, and it's well deserved. goto statements can cause a jump to any location in your source code, backward or forward. The indiscriminate use of goto statements has caused tangled, miserable, impossible-to-read programs known as "spaghetti code." Because of this, computer science teachers have spent the past 20 years drumming one lesson into the heads of their students: "Never, ever, ever use goto! It is evil!"

To avoid the use of goto, more sophisticated, tightly controlled looping commands have been introduced: for, while, and do...while. Using these makes programs that are more easily understood, and goto is generally avoided, but one might argue that the case has been a bit overstated. Like any tool, carefully used and in the right hands, goto can be a useful construct, and the ANSI committee decided to keep it in the language because it has its legitimate uses. But as they say, kids, don't try this at home.

The goto Statement

To use the goto statement, you write goto followed by a label name. This causes an unconditioned jump to the label. Example

if (value > 10)     goto end;if (value < 10)     goto end;cout << "value is Â10!";end:cout << "done";

 

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WARNING: Use of goto is almost always a sign of bad design. The best advice is to avoid using it. In 10 years of programming, I've needed it only once.

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