如何使用R中嵌套的for循环将数据输入到数据框中

df <- data.frame(col1 = numeric(), col2 = character(), stringsAsFactors = FALSE)

vec1 <- c(1, 2, 3)
vec2 <- c("A", "B", "C")

for (i in 1:length(vec1)) {
  for (j in 1:length(vec2)) {
    row <- data.frame(col1 = vec1[i], col2 = vec2[j], stringsAsFactors = FALSE)
    df <- rbind(df, row)
  }
}

print(df)

df <- data.frame(col1 = numeric(), col2 = character(), stringsAsFactors = FALSE)

vec1 <- c(1, 2, 3)
vec2 <- c("A", "B", "C")

for (i in 1:length(vec1)) {
  for (j in 1:length(vec2)) {
    row <- data.frame(col1 = vec1[i], col2 = vec2[j], stringsAsFactors = FALSE)
    df <- rbind(df, row)
  }
}

print(df)