I was taught this expression and pattern way back in the day. Sure, the name comes from old pumps that needed to be filled with water before they could pump water, but who cares? We're talking about code here.

Some really good examples and an explanation of what the pattern accomplishes would be welcome. How is this pattern regarded today?

Priming can sometimes get a defective loop working but at the cost of DRY. So it may be a brief stop on the way to a better design. Is this considered an anti pattern? Are there alternatives?


This metaphor almost certainly refers to the practice of establishing the first conditional check in a while loop. If you don't do this, the loop won't work. It is a well-established pattern, and it hasn't changed since the while loop was invented. The requirement for setting the initial condition in a while loop is not a defect.

int i = 0; // prime the pump
while (i < 10)
    Console.Write("While statement ");
    i++; // set condition again

The primer can be a read statement, or whatever properly sets the initial condition. Setting the initial condition using a read statement is called a "Priming Read."

string line;

using (StreamReader file = new StreamReader("c:\\test.txt"))
    line = file.ReadLine(); // Priming read.
    while(line != null)
        Console.WriteLine (line);
        line = file.ReadLine(); // Subsequent reads.   

In C#, the two Readline() calls can be combined into a single statement within the conditional:

while ((line = r.ReadLine()) != null)
    Console.WriteLine (line);
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    Nice. There are times when you need to do A and B in order with a test T exactly between them. The usual A T(B A) priming pattern does this but duplicates A. Any general alternatives to this general pattern? – candied_orange Apr 19 '16 at 18:25
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    See the last bit of code in my answer. – Robert Harvey Apr 19 '16 at 18:25
  • That one's a classic. Would you ever suggest using a break? – candied_orange Apr 19 '16 at 18:27
  • I'm not opposed to early exits from a loop if doing so simplifies the code. Sometimes you can just return. – Robert Harvey Apr 19 '16 at 18:28
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    An example would be looking for the first occurrence of a string in each line of a text file. You would still need the while loop, but once you found the string in the line you just read, you could either break or return. – Robert Harvey Apr 19 '16 at 18:37

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