Loop Patterns

Go has one loop keyword. There is no while, no do, no foreach, and no comprehension. for covers all of it by dropping clauses:

for i := 0; i < n; i++ { }   // counter
for n != 1 { }               // while
for { }                      // forever, until break
for i, v := range items { }  // each element

break ends the innermost loop and continue skips to its next iteration. When you need to leave an outer loop from inside an inner one, label it and say which: break search. That label is not a goto; it names a loop, and it is the readable alternative to a found flag threaded through two conditions.

range over a slice gives you index and value. Take only the index with for i := range items, and drop the index with for _, v := range items. The value is a copy, so assigning to v changes nothing.

Task

Finish the six functions in main.go. Each one is a different shape of loop.

1. SumTo(n int) int adds 1 through n with the three-clause form. n below 1 sums to 0.
2. CollatzSteps(n int) int counts steps to reach 1: halve an even number, triple an odd one and add 1. CollatzSteps(1) is 0, CollatzSteps(27) is 111, and anything below 1 is -1.
3. OddSquares(n int) []int returns the squares of the odd numbers up to n, using continue to skip the even ones.
4. NextPowerOfTwo(n int) int returns the smallest power of two at least as large as n, with a for that has no clauses and a break.
5. FirstIndex(items []string, target string) int returns the index of the first match, or -1, and stops as soon as it finds one.
6. FindCell(grid [][]int, target int) (int, int) scans a grid row by row and returns the row and column of the first matching cell, or -1, -1. Rows may have different lengths, including zero.

Hints

• A for i := 1; i <= n; i++ never runs when n is 0, so the "n below 1" cases need no extra if. Check that before writing one.
• The Collatz loop needs its guard first: n < 1 returns -1 before the loop, because the loop would otherwise never reach 1.
• continue runs the post statement on its way round, so the counter still advances. A continue in a condition-only for that forgets to advance the variable itself is an infinite loop, and in the runner that is a timeout rather than an error message.
• Appending to a nil slice is fine, so var out []int is a complete initialisation. OddSquares(0) then returns nil, which has length zero and is what the tests expect.
• FirstIndex can just return i from inside the range; that is a break that also hands back the answer.
• FindCell is the one that wants a label. Write search: on the line above the outer for, then break search from the inner loop. Do it with a plain break instead and the outer loop keeps going, which quietly returns the last match rather than the first.
• Ranging over an empty or nil slice runs the body zero times, so the empty row in the test grid needs no special case.