Lably
Skip to content

Label (computer science)

  • 9 min read
  • 3 chapters
  • 1 lab on Lably

Learn it on Lably

Lab · Class 6–12Data Visualization Lab

C

In C a label identifies a statement in the code. A single statement can have multiple labels. Labels just indicate locations in the code and reaching a label has no effect on the actual execution.

Function labels

Function labels consist of an identifier, followed by a colon. Each such label points to a statement in a function and its identifier must be unique within that function. Other functions may use the same name for a label. Label identifiers occupy their own namespace – one can have variables and functions with the same name as a label.

<span id="mwLQ"></span><span class="kt" id="mwLg">void</span><span class="w" id="mwLw"> </span><span class="nf" id="mwMA">foo</span><span class="p" id="mwMQ">(</span><span class="kt" id="mwMg">int</span><span class="w" id="mwMw"> </span><span class="n" id="mwNA">number</span><span class="p" id="mwNQ">)</span> <span class="p" id="mwNg">{</span> <span class="w" id="mwNw"> </span><span class="k" id="mwOA">if</span><span class="w" id="mwOQ"> </span><span class="p" id="mwOg">(</span><span class="n" id="mwOw">number</span><span class="w" id="mwPA"> </span><span class="o" id="mwPQ"><</span><span class="w" id="mwPg"> </span><span class="mi" id="mwPw">0</span><span class="p" id="mwQA">)</span> <span class="w" id="mwQQ"> </span><span class="k" id="mwQg">goto</span><span class="w" id="mwQw"> </span><span class="n" id="mwRA">error</span><span class="p" id="mwRQ">;</span> <span class="w" id="mwRg"> </span><span class="n" id="mwRw">bar</span><span class="p" id="mwSA">(</span><span class="n" id="mwSQ">number</span><span class="p" id="mwSg">);</span> <span class="w" id="mwSw"> </span><span class="k" id="mwTA">return</span><span class="p" id="mwTQ">;</span> <span class="nl" id="mwTg">error</span><span class="p" id="mwTw">:</span> <span class="w" id="mwUA"> </span><span class="n" id="mwUQ">fprintf</span><span class="p" id="mwUg">(</span><span class="n" id="mwUw">stderr</span><span class="p" id="mwVA">,</span><span class="w" id="mwVQ"> </span><span class="s" id="mwVg">"Invalid number!</span><span class="se" id="mwVw">\n</span><span class="s" id="mwWA">"</span><span class="p" id="mwWQ">);</span> <span class="p" id="mwWg">}</span>

Here error is the label. The statement goto can be used to jump to a labeled statement in the code. After a goto, program execution continues with the statement after the label.

Switch labels

Two types of labels can be put in a switch statement. A case label consists of the keyword case, followed by an expression that evaluates to integer constant. A default label consists of the keyword default. Case labels are used to associate an integer value with a statement in the code. When a switch statement is reached, program execution continues with the statement after the case label with value that matches the value in the parentheses of the switch. If there is no such case label, but there is a default label, program execution continues with the statement after the default label. If there is no default label, program execution continues after the switch.

<span id="mwZw"></span><span class="k" id="mwaA">switch</span><span class="w" id="mwaQ"> </span><span class="p" id="mwag">(</span><span class="n" id="mwaw">die</span><span class="p" id="mwbA">)</span> <span class="p" id="mwbQ">{</span> <span class="k" id="mwbg">default</span><span class="o" id="mwbw">:</span> <span class="w" id="mwcA"> </span><span class="n" id="mwcQ">printf</span><span class="p" id="mwcg">(</span><span class="s" id="mwcw">"invalid</span><span class="se" id="mwdA">\n</span><span class="s" id="mwdQ">"</span><span class="p" id="mwdg">);</span> <span class="w" id="mwdw"> </span><span class="k" id="mweA">break</span><span class="p" id="mweQ">;</span> <span class="k" id="mweg">case</span><span class="w" id="mwew"> </span><span class="mi" id="mwfA">1</span><span class="p" id="mwfQ">:</span> <span class="k" id="mwfg">case</span><span class="w" id="mwfw"> </span><span class="mi" id="mwgA">3</span><span class="p" id="mwgQ">:</span> <span class="k" id="mwgg">case</span><span class="w" id="mwgw"> </span><span class="mi" id="mwhA">5</span><span class="p" id="mwhQ">:</span> <span class="w" id="mwhg"> </span><span class="n" id="mwhw">printf</span><span class="p" id="mwiA">(</span><span class="s" id="mwiQ">"odd</span><span class="se" id="mwig">\n</span><span class="s" id="mwiw">"</span><span class="p" id="mwjA">);</span> <span class="w" id="mwjQ"> </span><span class="k" id="mwjg">break</span><span class="p" id="mwjw">;</span> <span class="k" id="mwkA">case</span><span class="w" id="mwkQ"> </span><span class="mi" id="mwkg">2</span><span class="p" id="mwkw">:</span> <span class="k" id="mwlA">case</span><span class="w" id="mwlQ"> </span><span class="mi" id="mwlg">4</span><span class="p" id="mwlw">:</span> <span class="k" id="mwmA">case</span><span class="w" id="mwmQ"> </span><span class="mi" id="mwmg">6</span><span class="p" id="mwmw">:</span> <span class="w" id="mwnA"> </span><span class="n" id="mwnQ">printf</span><span class="p" id="mwng">(</span><span class="s" id="mwnw">"even</span><span class="se" id="mwoA">\n</span><span class="s" id="mwoQ">"</span><span class="p" id="mwog">);</span> <span class="w" id="mwow"> </span><span class="k" id="mwpA">break</span><span class="p" id="mwpQ">;</span> <span class="p" id="mwpg">}</span>

Within a single switch statement, the integer constant associated with each case label must be unique. There may or may not be a default statement. There is no restriction on the order of the labels within a switch. The requirement that case labels values evaluate to integer constants gives the compiler more room for optimizations.

Sign in to keep reading

2 more chapters are free with a Lably account.

More in Computing and information technology

Artificial intelligenceIntelligence in machinesComputer scienceStudy of computationOperating systemSoftware that manages computer hardware resourcesMicrosoft WindowsComputer operating systemComputerProgrammable machine that processes dataLinuxFamily of Unix-like operating systems that use the Linux kernel