Is it hard to learn how to knit? I've thought about getting into knitting scarves and shawls for cancer patients. I have two or three friends that do this in their free time.
It isn't hard, really, especially in the early stages where you're just doing straight knit and purl stitches. Then you'll learn yarn-overs and yarn-togethers (to make lace or other patterns). Again, not really hard, you just have to pay attention. As you increase in skill you can get more complicated or not, as you like.
I learned to knit when I was 13 and my mom signed up for a knitting class at Sears for herself, me, and my two younger sisters. I was the only one who really enjoyed it and stayed with it.
I taught myself to crochet about 10 years ago and now I do probably 90% crochet projects, 10% knit.
Advantages of crochet: it's much easier, the fabrics typically are denser (great for scarves and hats, particularly), and it is far, far easier to correct a mistake. You just rip it back to where you made the mistake and carry on.
Personally I think the stitch patterns are more fun to make; fans and shells and lace and picots and varying the length of stitches, and many other methods, all are ways of introducing texture and pattern into your work.
It's much easier to make lacy, decorative patterns in crochet vs. knitting.
Advantages of knitting: you use less yarn for a given project (crochet uses maybe 2 to 3 times as much, if it's a solid fabric), fabrics are typically tighter and smoother.
It is harder (for me, anyway) to correct a mistake in knitting because all the stitches are, well, knit together. You sometimes have to go a row below where the mistake shows up in order to get it right. And then you have to re-load the stitches onto the needle.
Crochet is also far easier on the hands than knitting. My left hand tends to cramp up while knitting; I have never have any problem while crocheting.
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General comments: if you're just learning, buy yourself an inexpensive skein of Super Saver or Lion Brand Wool-Ease or something similar and make yourself a scarf or a few baby hats or things like that. A sweater is a very difficult project for a beginner to make, because gauge will be crucial, you will probably have to join seams to make the sleeves and maybe the sides, neckline might be tricky, etc., etc. There's just a lot of fiddling around to do. And you'll want better yarn, which is an expensive way to get started.
Scarves and hats and shawls make better starter projects; you can finish something fairly quickly and be proud of it, and gauge doesn't matter much, and inexpensive yarns work just fine.
Also lap robes (small afghans). People LOVE to get those as presents; for the maker, it's not as much of a time and money commitment as a full afghan, and for the receiver, it doesn't take up much room in the recipient's home, and it's a cozy thing to have handy.