Today Walmart, America’s and the world’s biggest retailer, unveils its first-quarter results. Analysts are expecting another dip in profits, as sales growth continues to stagnate. And although Target, one of Walmart’s rivals, is expected to reveal better figures tomorrow, these are difficult times for American grocers. Last week the Census Bureau reported that retail sales in April were flat. Even if the economy does start to boom again, both Walmart and Target may continue to struggle. Since the recession, ordinary Americans have apparently been forking over their extra income to dine out rather than buying more food to eat at home. In the past few months, as Mark Perry of the University of Michigan points out, they have been spending more in restaurants and bars than in grocery stores for the first time on record. Good news for American consumers won’t necessarily be good for Walmart.